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Isothermal mass transfer of aluminium onto molybdenum in liquid metals
Reactor Science and Technology (Journal of Nuclear Energy Parts A/B) 1962, Vol. 16, pp. 355 to 367. Pergamon Press Ltd. Printed in Northern Ireland
ISOTHERMAL MASS TRANSFER OF ALUMINIUM MOLYBDENUM IN LIQUID METALS
ONTO
A. K. COVINGTON,* J. D. BAIRD? and A. A. WOOLF Associated Electrical Industries, Aldermaston Court, Aldermaston, Berks. (Received 25 October 1961)
Abstract-The rate of growth has been studied of the alloy layer formed on molybdenum when it is immersed in solutions of aluminium in the liquid metals, bismuth, cadmium, indium, lead, thallium or tin at 500°C. The published work on the aluminium-molybdenum phase diagram is surveyed and from this and some additional studies the compounds in the alloy layer were identified as Al,Mo, A&MO and Al,,Mo. The only other well-established compound in the system AlMo,, was found to grow too slowly at this temperature to be observed in the layer. Considerable differences were found in the rates of growth and composition of the alloy layer in the different solvent metals. The solubilities and rates of solution of aluminium in the solvent metals were measured in order to elucidate the rate controlling step in the mass transfer and its dependence on the liquid metal. Provided that the liquid metal was saturated with aluminium in the vicinity of the molybdenum, diffusion in the solid state was the rate controlling step. However, this condition was not satisfied for those liquid metals
in which aluminium was sparingly soluble (e.g. lead) and under our test conditions the rate of solution was found to be of the same order as the rate of removal of aluminium by alloying. The implications of these results on the use of liquid metals in reactor technology is discussed. 1. INTRODUCTION
first two steps were expected to be rapid compared with the formation time of a measurable layer (IO-100 hr). Hence in the initial experiments it was assumed that the liquid would be saturated throughout the test period, provided sufficient aluminium were present. Later, differences in behaviour in the different liquid metals showed that this assumption was unjustified and that the first two steps could not be ignored. The latter were studied by investigating the effect on layer growth of varying the concentration of aluminium, and the position of aluminium relative to molybdenum, and also by measuring the solubility and rates ofsolution of aluminium at different temperatures.
phenomenon of isothermal mass transfer in liquid metals (TAYLOR and WARD, 1958; COVINGTON, GEACH and WOOLF, 1958) was discovered incidentally while investigating the corrosion of solid metals by liquid metals insupposedlyinert containers. BRASUNAS(1953) THE
found that a molybdenum specimen immersed in liquid sodium in a nickel crucible became coated with a
molybdenum
nickel alloy.
More recently a survey in liquid bismuth, lead and tin, has established some of the conditions for isothermal mass transfer and has provided further examples. Aluminium, which is slightly soluble in liquid metals at 5OO”C,was found to react at the surface of molybdenum, which is almost insoluble (< 1 ppm), to form intermetallic compounds. The rates of growth of these compounds have now been measured in liquid bismuth, cadmium, indium, lead, thallium and tin. Mass transfer of aluminium may be divided into three elementary steps: (1) solution of aluminium, (2) transfer of aluminium through the liquid metal to the molybdenum surface by diffusion and convection and (3) reaction with molybdenum to form inter-metallic compounds by solid-state diffusion. Any one, or more of these steps could be rate determining. WARD and (COVINGTON and WOOLF, 1959) of pairs of metals
2. EXPERIMENTAL
The tests were carried out at 500°C in borosilicate glass ampoules which were unattacked by any of the liquid metals. The molybdenum specimens (18 x 5 mm diameter) were abraded to a bright finish with successively finer grades of emery paper down to grade 600. The aluminium specimens (grain size 0.5 mm) of the same length but 3.5 mm square section were abraded with a fine file. Both metals were the spectroscopically standardized materials supplied by Johnson, Matthey and Co. Ltd. A few experiments were carried out with molybdenum prepared from sintered powder (A.E.I., Rugby) purified by electron bombardment zone melting. Specimens were degreased for 15-30 min in benzene vapour. Bismuth (Mining and Chemical Products Ltd.) was purified by melting and degassing under vacuum, followed by hydrogen treatment at 500°C and
TAYLOR (1956) have shown that liquid lead or bismuth
becomes saturated with copper within an hour, and the * Present address: Chemistry Department, King’s College, University of Durham, Newcastle-upon-Tyne 1. t Present address: Colville’s Research Laboratory, Motherwell. 355
356
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
subsequent filtration through a 4 cm dia. sintered glass disk of pore size 40-50~, (HORSLEY,1957). The other metals were not given preliminary purification. Their sources and stated purities were: indium 99.99 per cent (Mining and Chemical Products Ltd.) ; cadmium 99.99 per cent (Consolidated Zinc Corporation); lead 99.998 per cent (Johnson, Matthey and Co. Ltd.), 99.992 per cent (Britannia Lead Co.); thallium 99.95 per cent (New Metals and Chemicals Ltd.); tin 99.997 per cent (Capper Pass and Co. Ltd.). The metal specimens were held loosely in horizontal positions 2.5 cm apart in the ampoules described previously (COVINGTONand WOOLF, 1959). The ampoules were evacuated and flushed with argon several times. Small pieces of the required metal were melted in uucuo and forced by a small argon pressure through a sintered glass disk into the ampoule to give about 5 cm3 of liquid metal around the specimens. This process successfully removed all oxide scum and the resultant liquid metal surfaces were always bright. Ampoules were sealed at pressures less than 10e4 mm of Hg, and heated for times up to 1500 hr in an aircirculation furnace similar to that described by WARD and TAYLOR(1956). The temperature was controlled by a resistance thermometer element (Sunvic RT2) and was measured with a calibrated chromel-alumel thermocouple situated in the centre of the turntable which held the ampoules. The high thermal capacity of the stainless-steel turn-table and baskets reduced temperature fluctuations and gradients to less than 12°C including fluctuations caused by opening the port to remove or replace an ampoule. A smaller furnace, used for a few runs at other temperatures, was controlled by a Temcon bimetal unit to f4”C (WARD and TAYLOR,1956). After treatment the ampoules were removed from the furnace and inverted to drain the liquid metal. The molybdenum specimens were mounted, sectioned transversely and examined metallographically. The growth of the layers was not regular and the thickness of each layer was therefore measured at intervals around each specimen using a Vickers projection microscope at a 500-fold magnification. The 70-120 readings for each layer were plotted on a frequency distribution diagram and the most probable value (the height of the distribution curve) was selected as the best measure of the layer thickness. A few experiments which gave distribution curves without a single clear maximum tendency were rejected. The reproducibility of layer thicknesses between tests was approximately & 10~. Experiments with zone-refined molybdenum rod gave similar results to those with the other material but the layers were more uniform.
3. THE ALUMINIUM-MOLYBDENUM PHASE DIAGRAM When two metals interdiffuse, the intermediate phase shown in the cross section of the binary equilibrium diagram at the diffusing temperature, should be present as single-phase layers at the interface (RHINES,1940; BAIRD,1960). In practice, however, not all phases appear in diffusion couples for reasons (BAIRD, 1960) such as insufficiency of one of the diffusing metals caused by poor contact, oxide films, vacancy condensation etc. and the slowness of solid state diffusion in compounds of high melting point relative to the diffusion temperature. Nevertheless a knowledge of the phase diagram is helpful in identifying layers produced in diffusion experiments such as isothermal mass transfer, The aluminium-molybdenum system has not been thoroughly studied by any one investigator and many of the observations are contradictory (HANSEN,1958). The aluminium-rich end of the diagram was first studied by REIMANN(1922) who found thermal arrests at 735” and 1130°C. YAMAGUCHIand SIMIZU(1940) proposed a tentative diagram showing the components A&MO and A&MO determined by chemical analysis of compounds extracted by solution of the aluminium matrix with 10 per cent hydrochloric acid. A&MO was stated to result from a peritectic reaction at 703°C between Al,Mo and a liquid containing 0.24 per cent MO, but the (high) congruent melting point of Al,Mo was not determined. WACHTELL(1952) identified two phases formed by diffusion between an aluminium coating and the molybdenum base as A&MO and AI,Mo following a diagram proposed by WALTHER(1950). The molybdenum end of the diagram has been studied by X-ray techniques and metallography (see NORTHCOTT,1956). A phase identified as AIMo, having the B-tungsten structure is formed peritectically at 2150°C. Alloys in the range lo-40 per cent MO showed a eutectic structure consisting of AlMo, and a compound described as A&MO whose structure was not determined but which gave rise to a complex X-ray pattern. The eutectic temperature was found to be 1760°C at 23 per cent Al. Summers-Smith in this Laboratory found two intermetallic phases which he formulated as A&MO, Al,Mo and a third cubic phase which subsequently was shown to possess an X-ray diffraction pattern of the p-tungsten type consistent with the American work. A&MO was found to form peritectically at 715 k 5°C in reasonable agreement with the German and Japanese work. Knapton and Hall, continuing this investigation, concluded that the arrest at 7 15°C was due to the peritectic formation, not of A&MO, but of AI,,Mo, a phase whose existence had
Isothermal mass transfer of aluminium onto molybdenum
in liquid metals
351
TABLEl.-INTERMEDIATE PHASES IN THEAl-MOSYSTEM Wt. % Al
M.P.
Structure
Density (g cm-s)
V.P.N.
Al,,Mo
11.1
715” peritectic
b.c.c. a = 7.573 - 7528 A
3.22 3*28*
280400
A&MO
58.5
1130” peritectic
Hexagonal a = 4.98 A c/a = 1.80
3.90*
460660
&Mot
45.7
4.60*
640175
Phase
c. 1520”
a = 6.297 A c/a = 1.588 tetragonal
A&MO(?)
AlMo,
36.0
8.6
> 1760”
2150 peritectic
Not known (complex)
-
@-tungsten a = 4.936 - 4.948 8,
900
* Assuming linear density relation. t According to FORSYTH(1960) this is the compound A&MO, with a monoclinic cell. (a = 9.208 A, b = 3.637 A, c = 10.065 A, /Y = 100”-47’).
been reported and structure determined by ADAM and RICH (1954) by analogy with the phase A&,W. The peritectic melting point of A&MO was not determined but was thought to be above 755°C; it could perhaps be at 1130°C the arrest detected by REIMANN (1922). Recently SPERNER (1959) has reported a phase A&MO, anisotropic like Al,Mo and melting at a temperature above the eutectic at 1760°C reported by the American SPERNER’Sconclusion differs from the workers. foregoing in that A&MO is assigned a peritectic melting point of 1130°C and A&MO is the congruently melting phase of the system. In order to resolve this discrepancy a series of arc melted alloys in the range 40-80 wt. per cent MO were examined. Duplicate preparations of composition A&MO were shown by chemical analysis to be close to the theoretical composition and by metallography to be essentially single phase. They remained single phase after annealing for one week at 1000°C and three hours at 1300°C. An approximate melting point curve carried out in the arc furnace showed a small but broad inflexion at the composition AI,Mo around 1520°C. Alloys corresponding to the composition Al,Mo melted considerably below the congruent melting point of above 1760°C ascribed to it by SPERNER.X-ray patterns of annealed alloys on the molybdenum-rich side of A&MO showed them to be mixtures of A&MO and AIMo,, and on the aluminium-rich side to be mixtures of Al,Mo and A&MO. It was concluded from the above evidence that A&MO* is the congruently * A recent complete X-ray structural analysis at Cambridge (FORSYTH, 1960)has shown this compound to be A&MO,. X-ray powder photographs of FORSYTH’Salloy and our own are identical. The compound has a composition range.
melting compound and not A&MO, and the diagram suggested by SPERNER (1959) cannot be entirely correct. The properties of compounds reported in the system are collected in Table 1. 4. RESULTS
(a) IdentiJicution oy layers In mass transfer experiments never more than three layers were observed. Thus of the five compounds Al,,Mo, A&MO, A&MO, A&MO and AlMo,, reported by various investigators, two are apparently missing. The two extreme compounds Al,,Mo and AlMo, are easily differentiated from the others because their structures are cubic and they do not interact with polarized light. The outer layer in mass transfer specimens, when three layers are present, is isotropic. This layer was confirmed as Al,,Mo by comparison of a glancing angle X-ray photograph of a molybdenum specimen (530 hr at 500°C in liquid tin, after removal of the adhering tin with hot mercury) with those from Hall and Knapton’s alloys. The intermediate layer when three layers form (or the outer layer when only two form) was identified as A&MO by a similar comparison of glancing angle X-ray photographs of a molybdenum specimen (724 hr at 500°C in liquid cadmium, adhering cadmium removed with nitric acid) with powder photographs of prepared alloys. The layer adjacent to the molybdenum was anisotropic with a typical columnar structure and could not be AlMo,, the most molybdenum-rich compound, as might have been expected. Pieces removed from a specimen on which only one layer had formed gave an X-ray powder pattern identical with that from alloys of composition AlaM0 prepared by Summers-Smith
358
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
and probably the same as that indexed as tetragonal by SPERNER. The change of colour under polarized light from orange to blue is also in agreement with SPERNER’Sobservation on A&MO but not for A&MO which he states changes from red to grey-green. Further evidence of the identity of the inner layer with arcmelted alloys of composition around A&MO, was provided by microhardness measurements and by X-ray microanalysis (MULVEY, 1959). A micron diameter beam was allowed to impinge on a singlephase alloy and on the inner diffusion layer and the counts were found to be the same. The layer was also scanned by the beam and found to be homogeneous in molybdenum content. It was not possible to obtain an absolute molybdenum content by this method because of uncertain absorption corrections. However, even allowing for the uncertainty in these corrections the indicated composition of the layer was nearer A&MO than A&MO. Possible reasons for the absence of the molybdenumrich component AlMo, adjacent to the molybdenum are: (a) that it is unstable at the testing temperature, (b) that its growth rate is too low for a finite thickness to be visible within the testing time and (c) nucleation difficulties. To test which of these possibilities was correct, specimens coated with A&MO from the mass transfer experiments were sealed in evacuated silica capsules and heated at temperatures of 700”, 800”, 900” and 950°C. No additional layer between the molybdenum and the A&MO was visible at 700°C. At the other temperatures even thicknesses of 2, 3, 9 and 16,~respectively of an optically isotropic layer of AlMo, were measured after 98 hr. From these results the activitation energy for the growth of AlMo, was 38 kcal and by extrapolation no layer would be observed under the optical microscope for a reasonable testing time at temperatures below 700°C. The AIMo, layer after 98 hr at 900°C is shown in Fig. 1. This specimen was re-heated for 1000 hr at 500°C. No change in thickness of the AlMo, layer was apparent, and hence the compound is stable at this temperature. Under polarized light the anisotropic compound A&MO would have been distinguished from A&MO if it had been formed at the higher temperatures. (b) Growth of layers Results obtained in the six liquid metals are shown in Fig. 2(a-f). These experiments were carried out under identical geometry, except in liquid tin where additional aluminium was needed to saturate the liquid metal. Behaviour was different in all liquid metals, although there were some similarities. In bismuth,
following an initial growth of A&MO, a small amount of A&MO appeared and then after an induction period of 175 hr Al,,Mo grew according to a parabolic growth law (Figs. 2a and 3). Thereafter the thicknesses of the A&MO and A&MO layers remained constant within experimental error or possibly fell off slightly. Growth in indium (Fig. 2b) was very similar. The final layer of Al,Mo was slightly thicker, and the induction period before AI,,Mo appeared was longer (225 hr) although it grew at the same rate as in bismuth. In tin A&MO and Al,,Mo were found (Figs. 2c and 4) but A&MO could only be observed in small patches (1~) on some specimens. Growth of AI,,Mo was again the same as in bismuth, but the induction period was reduced to 35 hr. Thallium (Fig. 2d) resembled lead (Fig. 2e) in
.2 ,
50
D c
IO
20
ti
(hr
x
x 30
‘)
FIG. 2(a).-Layer growth at 500°C in bismuth. Al,
MO
x Al,
MO
0 +
Al,, MO
ti
FIG.2(b).-Layer
(hr
i)
growth at 500°C in indium.
1 40
Isothermal mass transfer of alumlnium onto molybdenum
100
2
s
0
Al,Mo x AlsMo + AllZMo
in liquid metals
359
o Al,Mo
xA’SMo
-2 r
.t
)-
10 , -
FIG. 2(e).-Layer
20 ti (hri)
30
40
growth at 500°C in lead.
o Al,uo
200 t
x AI, k.40
,-
150 , -
I lo
_I ti (hr i)
FIG. 2(c).-Layer
1 2o
I
35
growth at 500°C in tin.
e Al, MO x AIS.MO
tf FIG. 2(f).-Layer
FIG. 2(d).-Layer
growth at 500°C in thallium.
that no Al,,Mo was found and A&MO grew parabolically.
Cadmium
(Fig.
2f) tias
intermediate
with
of A&MO after 90 hr. AI,,Mo appeared only after 800 hr when it grew at the same rate as in bismuth, indium and tin, without apparent diminution of the A&MO layer.
rapid
parabolic
growth
(hr t)
growth at 500°C
in cadmium.
These facts are summarized in Table 2 together with the saturation solubilities of aluminium in these liquid metals at 500°C determined as described later. There is a correlation between solubility and the presence of Al,,Mo in the intermetallic compound layer. In tin, indium and bismuth, the three metals in which aluminium is most soluble, AI,,Mo appears after an induction period which increases as the solubility (atomic per cent) in the liquid metal decreases. With lead and thallium, in which aluminium has a low
TABLE 2.-ISOTHERMAL MASSTRANSFERAT 500°C Solubility of Al (wt. %) (atomic %) Initial rate of A&MO (U hr-h) Final extent of Al,Mo (JJ> Rate Al,Mo (,u k-t) Final extent A&MO (u) Induction period (hr) Rate Al,,Mo (,u hr-*) Induction period (hr) * SULLY, HARDY and HEAL (1949).
Sn 7.6* 26.5 4.0 50 G 10 35
Bi 0.544 4.01 4.2 70 1 15 10 175
In 0.854 3.63 4.2 15 2 30 10 225
Cd 0.296
1.22 3.5 50 7 90 G’ 7
$6) (0.45) 3.2 80 1 30 Nil
Pb 0.0386 0.296 4.0 90 0.9 65 Nil
A. K. COVINGTON,J. D. BAIRD and A. A. WOOLF
360
solubility, no Al,,Mo is observed during the test period. Cadmium is intermediate and Al,,Mo appears only after about 800 hr. The logarithm of the solubility of aluminium in bismuth, indium, cadmium and tin decreases linearly with the logarithm of the observed induction period. Extrapolating this relationship the expected induction periods for thallium and lead would be greater than the testing period. If there were sufficient aluminium at the molybdenum surface,
d
t!f FIG. 5(a).-Layer
FIG. 5(c).-Layer
growth in lead at 460°C.
(0 A&MO.)
FIG. 5(d).-Layer
growth in lead at 540°C. x A&MO.)
(0 A&MO;
(hr i)
growth in bismuth at 460°C. (0 A&MO; x A&MO.)
however, Al&Jo should have grown immediately in all liquid metals and a correlation with solubility does not explain the behaviour in the various solvents. A few runs were performed in bismuth and lead at temperatures of 460°C and 540°C and the results are shown in Fig. 5(a-d). In bismuth at 460°C after an induction period of 100 hr Al,Mo grew more rapidly than at 500°C. Some Al,Mo appeared but no Al,,Mo in the time of testing. At 540°C a very small constant amount of A&MO (20~) was accompanied by a very rapid growth of Al,Mo similar to that found with cadmium (Fig. 2f), with only a small rate of growth of Al,,Mo. In lead at 460°C there was a faster growth of A&MO than at 500°C and the rate was the same as that in bismuth at 460°C. No Al,Mo or AI,,Mo was found. At 540°C less A&MO grew than at 500°C and there was a small parabolic growth of A&MO. The effect of temperature is complex and it is not possible to derive activation energies for diffusion in each layer. 5. FURTHER
EXPERIMENTS
Additional experiments were carried out to elucidate the behaviour in different solvents and in particular the late growth of Al,,Mo and its non-appearance in lead and thallium. I
I
IO
ti FIG. S(b).--Layer
20 (hr i)
I
30
growth in bismuth at 540°C. (0 A&MO; x AI,Mo).
(a) Variation of aluminium concentration To investigate the effect on layer growth of aluminium concentrations below saturation, amounts
Isothermal mass transfer of ahuninium onto molybdenum
.
‘;: -
I
b/o.
o % - 0.29
b= 0.155
0
b/or
b= 0.12
.
b/a = 0.12
0.22
361
saturation slowly. This was confirmed by results in lead in which A&MO grew faster and the final amount of Al,Mo was reduced compared with the original results.
b. 0.292
0.53
in liquid metals
b- 0.067
(c) Rate of solution and solubility of aluminium in liquid metals
tf FIG. 6.-Layer
(hr
i)
growth in bismuth not saturated with Al.
of aluminium, insufficient to saturate the bismuth when completely dissolved, were introduced into the ampoules together with a constant amount of bismuth. The results are shown in Fig. 6. The amount ofAl,Mo, the only compound to appear, was less than that in the previous bismuth runs, and the amount decreased the further the liquid was below saturation, until at one tenth saturation no layer was observed even after 400 hr. The 100 hr specimens in the intermediate concentration runs also showed no layer and were quite clean when removed. The amount of aluminium consumed in layer growth was insufficient to affect appreciably the concentration of aluminium during an experiment. The results show that compounds of higher aluminium content do not form without sufficient aluminium in solution. However, in the lead and thallium runs, described previously, there was sufficient solid present to maintain saturation and Al,,Mo should have formed. Its absence in lead and thallium could be explained if the liquid metal were not saturated with aluminium in the vicinity of the molybdenum. Hence the original assumption, of solution and transport processes which were rapid compared with the layer growth, became questionable. (b) Reversal of the positions molybdenum
specimens
of the aluminium in the ampoules
The construction of ampoules was exactly the same, and the thermal conditions were as nearly as possible the same, as those in the layer growth experiments. The air-circulation furnace was adapted by fitting an auxiliary furnace above the entrance port to minimize temperature fluctuations caused by opening. The liquid metals were degassed under vacuum, treated with hydrogen and given a preliminary filtration as described earlier for bismuth. For solution times of the order of minutes the liquid metal was filtered into the ampoule through a side-arm which was then sealed off. Inversion of the ampoule then brought the whole volume of liquid metal into contact with the aluminium bar instantaneously. Ampoules were removed from the furnace after a prescribed time and inverted to drain the liquid metal, which was retained for chemical analysis (see Appendix). For rates of solution of the order of hours a more complicated ampoule was employed which allowed the solution to be filtered and sampled for analysis at the temperature of the experiment. The extent of solution was also followed by the loss in weight of the aluminium after adhering metal was removed with concentrated nitric acid. This checked with chemical analysis of the solidified solution. It could not be applied to lead solutions, because the presence of lead destroyed the passivity of aluminium to concentrated nitric acid, and also because the weight loss was small. Values of the saturation solubility were determined over a range of temperature using the appropriate type
and
With the aluminium at the bottom of the ampoule instead of at the top as in all previously described experiments, circulation caused by density differences on dissolution should be increased. Results for liquid bismuth are shown in Fig. 7 (compare Fig. 2a). There was a significant reduction from 175 to 50 hr in the induction period for the growth of Al,,Mo, and the Al,Mo layer after 100 hr was thicker. Both these facts indicate that the liquid near the molybdenum reached
20
lo
tf
30
(hrf)
FIG. 7.-Layer growth in bismuth MO above Al. (0 A&MO; x A&MO; + AI,,Mo.)
A. K. COVINGTON, J. D.
362
of ampoule. The results are shown in Fig. 8 and the constants in the equation log,, s = A - 5) where S is the solubility in atomic per cent and T is absolute temperature, are given in Table 3. The heats of solution of aluminium AH,, obtained from the 550 I
OC
500 I
400 I
450 I
BAIRD
and A. A. WOOLF
lead at 1lOO”C,cooled to the required temperature and then quenched. The lower part of the ingot was analysed chemically by an unspecified method. Extensive segregation in the ingots was always found in our work. DARDEL’S higher temperature results are coincident with an extrapolated log S against l/T line through the present results but his lower temperature results diverge greatly. The previous values are compared with the present results in Fig. 8 which also includes ‘the literature values for tin. The solution rates are shown in Fig. 9(a). Only in bismuth was the solution process completed in 90 min. It was difficult to obtain consistent results with abraded specimens in cadmium and impossible in lead and indium. There were indications, however, that the process may be complete after 10 hr. Some experiments were performed with aluminium specimens chemically polished in phosphoric-nitric acid mixture (94/6 v/v) at 85°C. (This polish removes 5 ,Uper min.) There was little difference in the rate of solution in bismuth between abraded and polished specimens but the reproducibility of the results was improved in cadmium and indium. The results were analysed in terms of the equation (BIRCUMSHAW and RIDDIFORD, 1952) k’t
,
-\
I.2
I
I
I.3
I.4 h
FIG.
x I03
=
7 = In
I
d_-
(1)
a-x
I.5 (“KY
T
8.-Variation of solubility with temperature of aluminium in liquid metals.
relation AH, = 2.303RB are also given. HILDEBRAND’S solubility parameters (HILDEBRAND and SCOTT,1950)
I
-
Saturnlion value
3
83
0.9
do not predict the correct order of solubilities. The only solubilities previously reported are for aluminium in indium and lead. CAMPBELL,BUCHANAN, KUZMAK and TUXWORTH (1952) analysed
7
\
In
(500’)
/
their samples
TABLE3 0.6 log,,S (atomic%) = A - 5
Solvent
A
Bi In Cd Pb
3.094 2.990 3.248 2.667
&9
1,920 1,884 2,443 2,472
AH, = 2.303 RB (kcal mole-l) 8.7 (8) 8.6 (2) 11.1 (8) 11.3 (1) --
for the major constituent, indium, polarographically, supplemented by a rather dubious density check. For samples of low aluminium content this could lead to errors as great as 20 per cent in the weight per cent aluminium. DARDEL (1946) dissolved aluminium in
hr
FIG. 9(a).-Rate
of solution of aluminium in liquid metals.
Isothermal mass transfer of aluminium onto molybdenum in liquid metals Polished Abraded
0
x
/
---_
0.6 / H 0 ; I
0.4
/
w
/
/
/
’
/
constant for copper in bismuth (WARD and TAYLOR,
Al 502’ Al 495’
Corrected
for change
1956). of orea
x
x
* x
x I
I
I
I
5
IO
I5
20
363
min
FIG. 9(b).-Accordance of solution rate of aluminium in bismuth with transport control.
where a is the saturation concentration, x the concentration at time t, k the.solution rate constant, S the surface area of the specimen and V the volume of the liquid. Equation (1) has been found to fit the dissolution of metals in liquid metals (WARD and TAYLOR, 1956).
A plot of log a/a - x against t for the bismuth runs at 500°C is shown in Fig. 9(b). After correction for the change in surface area assuming a uniform attack, the results for abraded and polished specimens conform to equation (1) and yield k = 5 x 1O-3cm set-l at 500°C which is of the same order as the solution rate
In cadmium and indium there is a rapid initial solution followed by a slow approach to equilibrium. With chemically polished specimens the apparent induction period is removed. In lead, polished specimens also dissolve quicker initially. Even when the approximate corrections are made for change in surface area, the results for cadmium, indium and lead do not appear to conform to equation (1) although approximate constants of k’ = O-145hr-l and 0.19 hr-l may be calculated for abraded aluminium in lead and cadmium respectively. A few experiments were carried out at other temperatures in cadmium, lead and bismuth. At 454°C in cadmium the rate of solution with polished specimens was greater than that for abraded specimens at 501°C. The results for three temperatures in bismuth are shown in Fig. 9(c). At the highest temperature the area of specimens was substantially reduced, which accounts for some of the diminution in rate but the time to reach saturation was At 454°C there was an considerably extended. induction period of 5-7 min compared with approximately + min at 500°C. The induction period may be associated with the time for the liquid metal to dissolve or break through a coherent oxide film. Finite wetting times which decrease with increased temperatures have been observed in surface tension studies (TAMMANN and R~~HENBECK,1935; BONDI, 1953). The presence of tenacious surface
-,
5470 1
2
4
6
a
IO
12
14
I6
18
20
min
FIG. 9(c).-Rate
of solution of aluminium in bismuth at different temperatures.
oxide films
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
364
accounts for the irreproducibility of some of the other runs. The time of ‘break through’ will not necessarily be the same for all specimens, nor need the wetted area be constant from specimen to specimen. The thin uniform film produced by chemical polishing will be more quickly dissolved or disrupted by the liquid metal, and the dissolution is correspondingly quicker. The observed slowing down of the solution rate with chemically polished specimens in indium and cadmium may be attributed to the reformation of an oxide film competing with the solution process, or alternatively to the formation of an oxide film on parts of the surface reducing the effective area of the specimen. Elements with stable oxides (e.g. Zr, Ti, Mg) are used to remove oxides from liquid metals and aluminium would be expected to behave similarly.
_. 100 -
experiments were carried out under 100 atm pressure of argon in an attempt to maintain continuous contact. Specimens tested up to 2000 hr were mounted and measured in the same way as the mass transfer specimens. The growth of Al,Mo was the same in both the clamp and pressure-couple experiments (Fig. 10). A&MO and AI,,Mo were found in smaller amounts than in the experiments in liquid bismuth. Metallographic examination showed that good contact between the metals had still not been maintained, so that the supply of aluminium necessary for the rapid growth of the compounds A&MO and Al,,Mo was not achieved. A&MO could be seen (Fig. 11) in the grain boundaries of the columnar grains of A&MO. This may indicate that grain boundary diffusion is important in the transfer of aluminium through A&MO. BAIRD (1960) has shown theoretically that grain boundary diffusion may give rise to a fourth power growth rate equation instead of the usual parabolic rate equation, and this may be one reason why growth of AlaM0 does not maintain its initial parabolic rate. In the pressurecouple specimens, the fine-grained structure of the A&MO produced on dipping persists on annealing, showing that there was insufficient diffusion of molybdenum at 500°C to allow recrystallization. There was a small but irregular growth of Al,,Mo in these specimens but little A&MO. 6. DISCUSSION
0
IO
20
ti
(hri)
30
40
FIG. lO.-Layer growthin diffusioncouples.
(d) D@ision couples The growth of alloy layers, formed by the direct contact of aluminium with molybdenum, was studied to find if maximum growth rates were being reached in the liquid metals. However, a good contact is difficult to achieve because of the rapidly reformed oxide film on aluminium. When the two metals were polished, and then clamped or rolled together, they did not react evenly at 500°C. The molybdenum was therefore dipped through a flux into molten aluminium to form a continuous alloy layer, the thickness of which (10,~) was subtracted from the final measurement. Several zone-refined molybdenum strips thus coated were placed in hand-tightened steel clamps. These were sealed in evacuated glass capsules and heated at 500°C for times up to 2000 hr. Even in this method of preparation there were indications that the contact between the aluminium and the alloy layer had deteriorated during the runs probably due to vacancy condensation (BAIRD, 1960), and a further series of
The solution process at the aluminium interface may be regarded as taking place in two steps: (a) reaction at the interface (chemical control), (b) diffusion from a saturated laminar layer into the bulk of the solution (transport control). Either step or both may be rate-controlling. A difference in the rate of dissolution produced by a change in the method of surface preparation is usually taken as an indication of some chemical control (BIRCUMSHAWand RIDDIFORD,1952), whilst a change produced by a variation of stirring rate is taken as an indication of some transport control. WARD and TAYLOR (1956) concluded from their studies of the solution of copper in bismuth and lead, that the results were consistent with transport across a laminar layer. EPSTEIN(1957) came to a similar conclusion about iron in mercury but suggested that in sodium, the dissolution of iron was chemically controlled. This is feasible in view of the difficulty in completely removing oxides (HORSLEY,1956). TAYLORand WARD (1957) believed that the solution of iron in bismuth was also transport controlled, but in an extension of this work GRAHAM and WILSON (1959) attributed differences between the solution rate constants for certain chrome steels to
Isothermal mass transfer of aluminium onto molybdenum in liquid metals
oxide films of different composition and tenacity. All these results (apart from those of EPSTEINwho did not present experimental data) may be fitted to the firstorder equation (1). LOMMELLand CHALMERS(1959) found that the rate of solution of a lead-tin alloy in liquid lead was diffusion controlled with no stirring, but with high stirring rates the process was controlled by the surface reaction. The results for aluminium show that its dissolution is also controlled by surface reactions probably involving the removal of oxide films. The rates of solution were slowest for those liquid metals whose freezing points are closest to the testing temperature. However, even though the dissolution time for some liquid metals is of the order of hours, this time is still short compared with that to grow an appreciable alloy layer. The rate of growth of alloy layers on the surface of the molybdenum would be expected to depend on the concentration of aluminium in the liquid metal, and this is demonstrated by the results shown in Fig. 6. It might also be expected to be controlled by the rate of diffusion of one or both metal atoms through the already formed alloy layer and hence upon its thickness. The parabolic growth of the outer alloy layers confirms this. The overall process of isothermal mass transfer is complicated and any analytical treatment would be complex. The following treatment demonstrates that under the experimental conditions chosen for testing in those liquid metals with low solubility of aluminium, the amount of aluminium passing into solution may be comparable to that removed by alloying. The liquid
metal is only slowly saturated under these conditions so that the aluminium-rich compound Al,,Mo may not form. Let x = concentration of aluminium in liquid metal at time t a = saturated concentration of aluminium w1 = weight of aluminium removed from solid rod at time t w’~= weight of aluminium removed from solution by alloying at time t y = thickness of alloy layer at time t c = concentration of aluminium in alloy layer
365
A = mean cylindrical area of alloy layers on molybdenum (2.9 cm2). Experimentally the growth of alloy layers is found to obey the equation y = k,‘t* (2) then w, = ycA = k2’cAtt = k2tf where k, = k,‘cA. The dissolution process has been shown to conform approximately to equation (I) which may be expressed in a corresponding differential form as
dw, - = dt
kS(a - x).
Since x = (wl - w2)/V and from above ‘2
= - k2
2vt”
dx
kS -_-(~_+k~. dt
Multiplying by the factor ekStlVand integrating gives x = a(1 _ e-kStIV) _ .!$ @St/v!>-*
ekS/Vdt
where 5 is an integration variable. The second term may be transformed into an integral 5 of the form F(x) = e-“* ev2dy, for which tabulated s values are available, b; substituting y2 for kS/V (MILLERand GORDON,1931). Then
where k’ = kS/V.
Values of 5 for various times have been calculated using k, = 4 x 10d5 g sect derived from the parabolic rate constant k,’ = 4 ,u hr-* which is approximately the same in all liquid metals tested (Table 4). Thus although bismuth is effectively saturated with aluminium from the start of the experiments, the lead is much slower to reach saturation. After 1000 hr it is still only 99 per cent saturated. If the growth or nucleation of the compound Al,,Mo requires an aluminium concentration very close to saturation, the compound might not be expected to appear under the
(2.1 g cmP3 for Al,Mo) Mat. %I Density at 500°C (g cm+)
Bi 9.8 Pb 10.1
k
s
V
(cm set-I)
(cm?
(cm3)
5 x 10-S 1.1 x 10-d
2.52
5.6
0.99978
0.99984
2.52
6.7
0.388
0.653
-1 0.968
300 (W
1000
-1 0.982
-1 0.990
(hr)
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
366
conditions of the present experiments. In the calculation, the approximate value of the rate constant for first-order kinetics of dissolution has been used. The actual rate of solution in its later stages was actually slower than predicted by this equation, and saturation would be reached even later than predicted above. These predicticns were later confirmed by two additional mass transfer experiments using a modified ampoule, which allowed filtering and sampling for chemical analysis of the liquid lead at the testing temperature. Values of x/a found were: 0.63 after 100 hr and 0.89 after 300 hr. For those experiments with bismuth in which insufficient aluminium was added to saturate the liquid metal, assuming that the rate of removal of aluminium by alloying is proportional to its concentration in solution, then
3
7. CONCLUSIONS Isothermal mass transfer of aluminium onto molybdenum is controlled by diffusion in the solid state provided that a sufficient concentration of aluminium is available at the surface of the molybdenum. The dissolution process is not always rapid compared with the rate of layer growth and may be impeded by a slow chemical process at the solid surface or diffusion through a coherent oxide film. Isothermal mass transfer may be troublesome in nuclear reactors employing liquid metals as coolants or solvents for fissile material. Studies with liquid bismuth contained in steel have shown that thermal gradient mass transfer arising from the large temperature coefficient of solubility of iron in liquid bismuth, occurs (HORSLEY,1959), even with the use of inhibiting films provided by the addition of zirconium
= ks(w- WJ
dt
W2
where w2 = weight of aluminium removed in time t w = weight of aluminium originally added to bismuth (assumed to dissolve completely). However, in these experiments w2 < w so the above equation simplifies to dw, -=dt
k,w w2
but z = b, the concentration of aluminium in solution and wg = ycA therefore
dr
ksb
dt=rv
On integration : where k,” is the parabolic growth rate constant. These growth constants are proportional to b”, as can be seen from Fig. 12. They have been evaluated by drawing the initial slopes in Fig. 6. When b = a, k,’ = k,” and the slope of the graph (Fig. 11) is given correctly by k,‘a-* within the accuracy of estimation of the slopes in Fig. 6. Since the initial rate of growth of A&MO is little affected by unsaturation of liquid metal up to 50 per cent (see Fig. 6), the rate of growth of Al,Mo appears to be approximately the same in all liquid metals. Hence the small effect of aluminium concentration on growth of A&MO can be ignored in calculating the time variation of aluminium concentration (equation 3).
FIG. 12.-Growth
rates in non-saturated
bismuth solutions.
or titanium to the liquid metal. Despite the cost and fabrication difficulties, refractory metals such as molybdenum would seem to be the only reasonable alternatives for containing liquid bismuth. Because of its minute solubility, thermal gradient mass transfer of molybdenum will not take place but dissimilar mass transfer may. Our previous study (COVINGTONand WOOLF, 1959) showed that few metals will react with molybdenum or the other refractory metals at present reactor temperatures, although they may do so a few hundred degrees higher. In an L.M.F.R. contained in a refractory metal, the accumulation of fission-product metals may lead to isothermal mass transfer, but if the concentration of these is kept low by continuous removal and processing of the bismuth, the growth of alloy layers should be small, and the deposition of low melting point compounds with rapid growth rates should be prevented.
Isothermal mass transfer of aluminium onto molybdenum 8. APPENDIX
in liquid metals
361
A. (1953) Chem. Rev. 52.417. BRASUNAS‘A. DE S. (1953) Corr&ion 9, 78. CAMPBELLA, N., BUCHANANL. B., KUZMAK J. M. and TuxThe determination of small amounts of aluminium is diflicult WORTHR. H. (1952) J. Amer. them. Sot. 74,1962. because the available reagents are far from specific and reagents COVINGTONA. K., GEACH G. A. and WOOLF A. A. (1958) and solvent media themselves contain appreciable quantities of Atomics 9, 10. aluminium. A reliable analysis requires quantitative separation COVINGTON A. K. and WOOLFA. A. (1959) Reactor Sci. Technol. from almost all other elements, a mimimun use of solvents and (J. Nucl. Energy Part B) 1, 35. reagents and careful blank determinations. DARDELY. (1946) Light Metals 9, 220. Aluminium can be extracted along with many other elements EPSTEINL. F. (1957) Chem. Engng. Progr. Symposium Series 53, by I-hydtoxy-quinoline (oxine) from solutions of pH 2-10 by No. 20. chloroform or carbon tetrachloride. The related I-hydroxyFORSYTHJ. B. (1960) Private communication. quinaldine does not extract aluminium but does extract other GRAHAML. W. and WILSON G. W. (1959) J. Iron. Steel Inst. metals. A combination of both reagents has been used to deter193, 103. mine aluminium in its alloys (HYNEKand WRANGELL,1956) and HANSEN M. and ANDERKO K. (1958) Constitution of Binary the present method is a modification of this procedure. Alloys, 2nd Ed., McGraw-Hill, New York. After solution of the sample in a suitable acid mixture, HILDEBRANDJ. H. and SCOTTR. L. (1950) Solubility of Nonelectrolysis with a mercury cathode (SCHERRERand MOGERMAN, electrolytes, Reinhold, New York. 1938; JOHNSONet al., 1947) was performed at a (cathodic) HORSLEYG. W. (1956) J. Iron Steel Inst. 382, 43. current density of 0.25 A cm-2 for about 5 hr to remove bismuth HORSLEYG. W. (1957) J. Nucl. Energy 6,41. and indium. Lead was slowly precipitated as sulphate over a HORSLEYG. W. (1959) Reactor Technol. (J. Nucl. Energy Part B) period of 1 hr. Aliquots were extracted at pH = 9.5 into 1, 84. chloroform with 8-hydroxyquinaldine to remove other metals, HYNEKJ. R. and WRANGELLL. J. (1956) Analyt. Chem. 28,152l. followed by extraction of aluminium with oxine, and their JOHNSONH. O., WEAVERJ. R. and LYKKENL. (1947) Zndustr. absorption was measured on a spectrophotometer at 400 rnp. Engng. Chem. (Analyt.) 19,481 Aluminium in cadmium was estimated by the standard graviLOMMELL J. M. and CHALMERS B. (1959) Trans. Amer. Inst. Mech. metric method (precipitation of aluminium hydroxide) and Engrs. 215, 499. checked against the weight loss method. MILLERW. L. and GORDONA. R. (1931) J.phys. Chem. 35,2878. MULVEYT. (1959) J. sci. Znstrum. 36, 350. Analysis of molybdenum in aluminium-molybdenum alloys MULVEYT., BERNARDH. and BRYSON-HAYNES D. (1959) J. sci. Instrum. 36, 438. Molybdenum was determined volumetrically. The alloy was dissolved in the minimum amount of HNO,: HF mixture (20 : 1). NORTHCOTTL. (1956) Molybdenum, Butterworths, London. PARKS T. D., JOHNSONH. D. and LYKKEN L. (1948) Industr. Sulphuric acid was then added and the solution fumed to remove Engng. Chem. (Analyt.) 20, 148. all nitric acid which would interfere with the subsequent reducREIMANNH. (1922) Z. Metal. 14, 195. tion. The solution was diluted and passed through an activated RHINES F. N. (1940) Surface Treatment of Metals, A. S. M. Jones reductor into standard potassium dichromate containing Symposium, 122-16. some phosphoric acid. The excess dichromate was back-titrated SCHERRERA. and MOGERMANW. D. (1938) J. Res. Nat. bur. with ferrous ammonium sulphate using barium diphenylamine Stand. 21, 105. sulphonate as internal indicator. SPERNERF. (1959) Z. Metal. 50, 588. Acknowledgments-We wish to thank DR. B. W. HOWLETTand SULLYA. H., HARDYH. K. and HEAL T. J. (1949) J. Inst. Metal DR. A. G. KNAPTONfor assistance with the metallography and 76, 269. for many useful discussions, DR. E. SMITH for mathematical TAMMANNG. and R~~HENBECK A. (1935) Z. anorg. Chem. 223, advice, MR. J. N. BRAZIER, MR. T. M. COTTONand MR. J. 193. RAMIREZfor experimental assistance, particularly with chemical TAYLORJ. W. and WARD A. G. (1958) Nucl. Power 3,101. analysis, DR. G. A. GEACH and DR. B. R. T. FROST for their TAYLORJ. W. and WARD A. G. (1957) Kinetics and Equilibrium continued interest and DR. T. E. ALLIBONE,C.B.E., F.R.S., Solubility Studies in the Iron-Bismuth System. AERE Report Director of this Laboratory, for permission to publish this paper. M/R 2295. WACHTELLR. L. (1952) Powder MetaN. Bull. 6, 99. REFERENCES WALTHERW. D. (1950) Molybdenum-Aluminium System. Thesis, Massachussetts Institute of Technology. ADAM J. and RICH J. B. (1954) Acta Cryst. 7, 813. WARD A. G. and TAYLORJ. W. (1956) J. Inst. Metal 85,145. BAIRDD. (1960) Reactor Sci. (J. Nucl. Energy Part A) 11, 81. YAMAGUCHIK. and SIMIZU K. (1940) Trans. Japan. Inst. Metal BIRCUM~HAWL. L. and RIDDIFORDA. C. (1952) Quart. Rev. them. Sot. 6, 157. 4, 390 Chemical analysis of aluminium dissolved in liquid metals
BONDI
ISOTHERMAL MASS TRANSFER OF ALUMINIUM MOLYBDENUM IN LIQUID METALS
ONTO
A. K. COVINGTON,* J. D. BAIRD? and A. A. WOOLF Associated Electrical Industries, Aldermaston Court, Aldermaston, Berks. (Received 25 October 1961)
Abstract-The rate of growth has been studied of the alloy layer formed on molybdenum when it is immersed in solutions of aluminium in the liquid metals, bismuth, cadmium, indium, lead, thallium or tin at 500°C. The published work on the aluminium-molybdenum phase diagram is surveyed and from this and some additional studies the compounds in the alloy layer were identified as Al,Mo, A&MO and Al,,Mo. The only other well-established compound in the system AlMo,, was found to grow too slowly at this temperature to be observed in the layer. Considerable differences were found in the rates of growth and composition of the alloy layer in the different solvent metals. The solubilities and rates of solution of aluminium in the solvent metals were measured in order to elucidate the rate controlling step in the mass transfer and its dependence on the liquid metal. Provided that the liquid metal was saturated with aluminium in the vicinity of the molybdenum, diffusion in the solid state was the rate controlling step. However, this condition was not satisfied for those liquid metals
in which aluminium was sparingly soluble (e.g. lead) and under our test conditions the rate of solution was found to be of the same order as the rate of removal of aluminium by alloying. The implications of these results on the use of liquid metals in reactor technology is discussed. 1. INTRODUCTION
first two steps were expected to be rapid compared with the formation time of a measurable layer (IO-100 hr). Hence in the initial experiments it was assumed that the liquid would be saturated throughout the test period, provided sufficient aluminium were present. Later, differences in behaviour in the different liquid metals showed that this assumption was unjustified and that the first two steps could not be ignored. The latter were studied by investigating the effect on layer growth of varying the concentration of aluminium, and the position of aluminium relative to molybdenum, and also by measuring the solubility and rates ofsolution of aluminium at different temperatures.
phenomenon of isothermal mass transfer in liquid metals (TAYLOR and WARD, 1958; COVINGTON, GEACH and WOOLF, 1958) was discovered incidentally while investigating the corrosion of solid metals by liquid metals insupposedlyinert containers. BRASUNAS(1953) THE
found that a molybdenum specimen immersed in liquid sodium in a nickel crucible became coated with a
molybdenum
nickel alloy.
More recently a survey in liquid bismuth, lead and tin, has established some of the conditions for isothermal mass transfer and has provided further examples. Aluminium, which is slightly soluble in liquid metals at 5OO”C,was found to react at the surface of molybdenum, which is almost insoluble (< 1 ppm), to form intermetallic compounds. The rates of growth of these compounds have now been measured in liquid bismuth, cadmium, indium, lead, thallium and tin. Mass transfer of aluminium may be divided into three elementary steps: (1) solution of aluminium, (2) transfer of aluminium through the liquid metal to the molybdenum surface by diffusion and convection and (3) reaction with molybdenum to form inter-metallic compounds by solid-state diffusion. Any one, or more of these steps could be rate determining. WARD and (COVINGTON and WOOLF, 1959) of pairs of metals
2. EXPERIMENTAL
The tests were carried out at 500°C in borosilicate glass ampoules which were unattacked by any of the liquid metals. The molybdenum specimens (18 x 5 mm diameter) were abraded to a bright finish with successively finer grades of emery paper down to grade 600. The aluminium specimens (grain size 0.5 mm) of the same length but 3.5 mm square section were abraded with a fine file. Both metals were the spectroscopically standardized materials supplied by Johnson, Matthey and Co. Ltd. A few experiments were carried out with molybdenum prepared from sintered powder (A.E.I., Rugby) purified by electron bombardment zone melting. Specimens were degreased for 15-30 min in benzene vapour. Bismuth (Mining and Chemical Products Ltd.) was purified by melting and degassing under vacuum, followed by hydrogen treatment at 500°C and
TAYLOR (1956) have shown that liquid lead or bismuth
becomes saturated with copper within an hour, and the * Present address: Chemistry Department, King’s College, University of Durham, Newcastle-upon-Tyne 1. t Present address: Colville’s Research Laboratory, Motherwell. 355
356
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
subsequent filtration through a 4 cm dia. sintered glass disk of pore size 40-50~, (HORSLEY,1957). The other metals were not given preliminary purification. Their sources and stated purities were: indium 99.99 per cent (Mining and Chemical Products Ltd.) ; cadmium 99.99 per cent (Consolidated Zinc Corporation); lead 99.998 per cent (Johnson, Matthey and Co. Ltd.), 99.992 per cent (Britannia Lead Co.); thallium 99.95 per cent (New Metals and Chemicals Ltd.); tin 99.997 per cent (Capper Pass and Co. Ltd.). The metal specimens were held loosely in horizontal positions 2.5 cm apart in the ampoules described previously (COVINGTONand WOOLF, 1959). The ampoules were evacuated and flushed with argon several times. Small pieces of the required metal were melted in uucuo and forced by a small argon pressure through a sintered glass disk into the ampoule to give about 5 cm3 of liquid metal around the specimens. This process successfully removed all oxide scum and the resultant liquid metal surfaces were always bright. Ampoules were sealed at pressures less than 10e4 mm of Hg, and heated for times up to 1500 hr in an aircirculation furnace similar to that described by WARD and TAYLOR(1956). The temperature was controlled by a resistance thermometer element (Sunvic RT2) and was measured with a calibrated chromel-alumel thermocouple situated in the centre of the turntable which held the ampoules. The high thermal capacity of the stainless-steel turn-table and baskets reduced temperature fluctuations and gradients to less than 12°C including fluctuations caused by opening the port to remove or replace an ampoule. A smaller furnace, used for a few runs at other temperatures, was controlled by a Temcon bimetal unit to f4”C (WARD and TAYLOR,1956). After treatment the ampoules were removed from the furnace and inverted to drain the liquid metal. The molybdenum specimens were mounted, sectioned transversely and examined metallographically. The growth of the layers was not regular and the thickness of each layer was therefore measured at intervals around each specimen using a Vickers projection microscope at a 500-fold magnification. The 70-120 readings for each layer were plotted on a frequency distribution diagram and the most probable value (the height of the distribution curve) was selected as the best measure of the layer thickness. A few experiments which gave distribution curves without a single clear maximum tendency were rejected. The reproducibility of layer thicknesses between tests was approximately & 10~. Experiments with zone-refined molybdenum rod gave similar results to those with the other material but the layers were more uniform.
3. THE ALUMINIUM-MOLYBDENUM PHASE DIAGRAM When two metals interdiffuse, the intermediate phase shown in the cross section of the binary equilibrium diagram at the diffusing temperature, should be present as single-phase layers at the interface (RHINES,1940; BAIRD,1960). In practice, however, not all phases appear in diffusion couples for reasons (BAIRD, 1960) such as insufficiency of one of the diffusing metals caused by poor contact, oxide films, vacancy condensation etc. and the slowness of solid state diffusion in compounds of high melting point relative to the diffusion temperature. Nevertheless a knowledge of the phase diagram is helpful in identifying layers produced in diffusion experiments such as isothermal mass transfer, The aluminium-molybdenum system has not been thoroughly studied by any one investigator and many of the observations are contradictory (HANSEN,1958). The aluminium-rich end of the diagram was first studied by REIMANN(1922) who found thermal arrests at 735” and 1130°C. YAMAGUCHIand SIMIZU(1940) proposed a tentative diagram showing the components A&MO and A&MO determined by chemical analysis of compounds extracted by solution of the aluminium matrix with 10 per cent hydrochloric acid. A&MO was stated to result from a peritectic reaction at 703°C between Al,Mo and a liquid containing 0.24 per cent MO, but the (high) congruent melting point of Al,Mo was not determined. WACHTELL(1952) identified two phases formed by diffusion between an aluminium coating and the molybdenum base as A&MO and AI,Mo following a diagram proposed by WALTHER(1950). The molybdenum end of the diagram has been studied by X-ray techniques and metallography (see NORTHCOTT,1956). A phase identified as AIMo, having the B-tungsten structure is formed peritectically at 2150°C. Alloys in the range lo-40 per cent MO showed a eutectic structure consisting of AlMo, and a compound described as A&MO whose structure was not determined but which gave rise to a complex X-ray pattern. The eutectic temperature was found to be 1760°C at 23 per cent Al. Summers-Smith in this Laboratory found two intermetallic phases which he formulated as A&MO, Al,Mo and a third cubic phase which subsequently was shown to possess an X-ray diffraction pattern of the p-tungsten type consistent with the American work. A&MO was found to form peritectically at 715 k 5°C in reasonable agreement with the German and Japanese work. Knapton and Hall, continuing this investigation, concluded that the arrest at 7 15°C was due to the peritectic formation, not of A&MO, but of AI,,Mo, a phase whose existence had
Isothermal mass transfer of aluminium onto molybdenum
in liquid metals
351
TABLEl.-INTERMEDIATE PHASES IN THEAl-MOSYSTEM Wt. % Al
M.P.
Structure
Density (g cm-s)
V.P.N.
Al,,Mo
11.1
715” peritectic
b.c.c. a = 7.573 - 7528 A
3.22 3*28*
280400
A&MO
58.5
1130” peritectic
Hexagonal a = 4.98 A c/a = 1.80
3.90*
460660
&Mot
45.7
4.60*
640175
Phase
c. 1520”
a = 6.297 A c/a = 1.588 tetragonal
A&MO(?)
AlMo,
36.0
8.6
> 1760”
2150 peritectic
Not known (complex)
-
@-tungsten a = 4.936 - 4.948 8,
900
* Assuming linear density relation. t According to FORSYTH(1960) this is the compound A&MO, with a monoclinic cell. (a = 9.208 A, b = 3.637 A, c = 10.065 A, /Y = 100”-47’).
been reported and structure determined by ADAM and RICH (1954) by analogy with the phase A&,W. The peritectic melting point of A&MO was not determined but was thought to be above 755°C; it could perhaps be at 1130°C the arrest detected by REIMANN (1922). Recently SPERNER (1959) has reported a phase A&MO, anisotropic like Al,Mo and melting at a temperature above the eutectic at 1760°C reported by the American SPERNER’Sconclusion differs from the workers. foregoing in that A&MO is assigned a peritectic melting point of 1130°C and A&MO is the congruently melting phase of the system. In order to resolve this discrepancy a series of arc melted alloys in the range 40-80 wt. per cent MO were examined. Duplicate preparations of composition A&MO were shown by chemical analysis to be close to the theoretical composition and by metallography to be essentially single phase. They remained single phase after annealing for one week at 1000°C and three hours at 1300°C. An approximate melting point curve carried out in the arc furnace showed a small but broad inflexion at the composition AI,Mo around 1520°C. Alloys corresponding to the composition Al,Mo melted considerably below the congruent melting point of above 1760°C ascribed to it by SPERNER.X-ray patterns of annealed alloys on the molybdenum-rich side of A&MO showed them to be mixtures of A&MO and AIMo,, and on the aluminium-rich side to be mixtures of Al,Mo and A&MO. It was concluded from the above evidence that A&MO* is the congruently * A recent complete X-ray structural analysis at Cambridge (FORSYTH, 1960)has shown this compound to be A&MO,. X-ray powder photographs of FORSYTH’Salloy and our own are identical. The compound has a composition range.
melting compound and not A&MO, and the diagram suggested by SPERNER (1959) cannot be entirely correct. The properties of compounds reported in the system are collected in Table 1. 4. RESULTS
(a) IdentiJicution oy layers In mass transfer experiments never more than three layers were observed. Thus of the five compounds Al,,Mo, A&MO, A&MO, A&MO and AlMo,, reported by various investigators, two are apparently missing. The two extreme compounds Al,,Mo and AlMo, are easily differentiated from the others because their structures are cubic and they do not interact with polarized light. The outer layer in mass transfer specimens, when three layers are present, is isotropic. This layer was confirmed as Al,,Mo by comparison of a glancing angle X-ray photograph of a molybdenum specimen (530 hr at 500°C in liquid tin, after removal of the adhering tin with hot mercury) with those from Hall and Knapton’s alloys. The intermediate layer when three layers form (or the outer layer when only two form) was identified as A&MO by a similar comparison of glancing angle X-ray photographs of a molybdenum specimen (724 hr at 500°C in liquid cadmium, adhering cadmium removed with nitric acid) with powder photographs of prepared alloys. The layer adjacent to the molybdenum was anisotropic with a typical columnar structure and could not be AlMo,, the most molybdenum-rich compound, as might have been expected. Pieces removed from a specimen on which only one layer had formed gave an X-ray powder pattern identical with that from alloys of composition AlaM0 prepared by Summers-Smith
358
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
and probably the same as that indexed as tetragonal by SPERNER. The change of colour under polarized light from orange to blue is also in agreement with SPERNER’Sobservation on A&MO but not for A&MO which he states changes from red to grey-green. Further evidence of the identity of the inner layer with arcmelted alloys of composition around A&MO, was provided by microhardness measurements and by X-ray microanalysis (MULVEY, 1959). A micron diameter beam was allowed to impinge on a singlephase alloy and on the inner diffusion layer and the counts were found to be the same. The layer was also scanned by the beam and found to be homogeneous in molybdenum content. It was not possible to obtain an absolute molybdenum content by this method because of uncertain absorption corrections. However, even allowing for the uncertainty in these corrections the indicated composition of the layer was nearer A&MO than A&MO. Possible reasons for the absence of the molybdenumrich component AlMo, adjacent to the molybdenum are: (a) that it is unstable at the testing temperature, (b) that its growth rate is too low for a finite thickness to be visible within the testing time and (c) nucleation difficulties. To test which of these possibilities was correct, specimens coated with A&MO from the mass transfer experiments were sealed in evacuated silica capsules and heated at temperatures of 700”, 800”, 900” and 950°C. No additional layer between the molybdenum and the A&MO was visible at 700°C. At the other temperatures even thicknesses of 2, 3, 9 and 16,~respectively of an optically isotropic layer of AlMo, were measured after 98 hr. From these results the activitation energy for the growth of AlMo, was 38 kcal and by extrapolation no layer would be observed under the optical microscope for a reasonable testing time at temperatures below 700°C. The AIMo, layer after 98 hr at 900°C is shown in Fig. 1. This specimen was re-heated for 1000 hr at 500°C. No change in thickness of the AlMo, layer was apparent, and hence the compound is stable at this temperature. Under polarized light the anisotropic compound A&MO would have been distinguished from A&MO if it had been formed at the higher temperatures. (b) Growth of layers Results obtained in the six liquid metals are shown in Fig. 2(a-f). These experiments were carried out under identical geometry, except in liquid tin where additional aluminium was needed to saturate the liquid metal. Behaviour was different in all liquid metals, although there were some similarities. In bismuth,
following an initial growth of A&MO, a small amount of A&MO appeared and then after an induction period of 175 hr Al,,Mo grew according to a parabolic growth law (Figs. 2a and 3). Thereafter the thicknesses of the A&MO and A&MO layers remained constant within experimental error or possibly fell off slightly. Growth in indium (Fig. 2b) was very similar. The final layer of Al,Mo was slightly thicker, and the induction period before AI,,Mo appeared was longer (225 hr) although it grew at the same rate as in bismuth. In tin A&MO and Al,,Mo were found (Figs. 2c and 4) but A&MO could only be observed in small patches (1~) on some specimens. Growth of AI,,Mo was again the same as in bismuth, but the induction period was reduced to 35 hr. Thallium (Fig. 2d) resembled lead (Fig. 2e) in
.2 ,
50
D c
IO
20
ti
(hr
x
x 30
‘)
FIG. 2(a).-Layer growth at 500°C in bismuth. Al,
MO
x Al,
MO
0 +
Al,, MO
ti
FIG.2(b).-Layer
(hr
i)
growth at 500°C in indium.
1 40
Isothermal mass transfer of alumlnium onto molybdenum
100
2
s
0
Al,Mo x AlsMo + AllZMo
in liquid metals
359
o Al,Mo
xA’SMo
-2 r
.t
)-
10 , -
FIG. 2(e).-Layer
20 ti (hri)
30
40
growth at 500°C in lead.
o Al,uo
200 t
x AI, k.40
,-
150 , -
I lo
_I ti (hr i)
FIG. 2(c).-Layer
1 2o
I
35
growth at 500°C in tin.
e Al, MO x AIS.MO
tf FIG. 2(f).-Layer
FIG. 2(d).-Layer
growth at 500°C in thallium.
that no Al,,Mo was found and A&MO grew parabolically.
Cadmium
(Fig.
2f) tias
intermediate
with
of A&MO after 90 hr. AI,,Mo appeared only after 800 hr when it grew at the same rate as in bismuth, indium and tin, without apparent diminution of the A&MO layer.
rapid
parabolic
growth
(hr t)
growth at 500°C
in cadmium.
These facts are summarized in Table 2 together with the saturation solubilities of aluminium in these liquid metals at 500°C determined as described later. There is a correlation between solubility and the presence of Al,,Mo in the intermetallic compound layer. In tin, indium and bismuth, the three metals in which aluminium is most soluble, AI,,Mo appears after an induction period which increases as the solubility (atomic per cent) in the liquid metal decreases. With lead and thallium, in which aluminium has a low
TABLE 2.-ISOTHERMAL MASSTRANSFERAT 500°C Solubility of Al (wt. %) (atomic %) Initial rate of A&MO (U hr-h) Final extent of Al,Mo (JJ> Rate Al,Mo (,u k-t) Final extent A&MO (u) Induction period (hr) Rate Al,,Mo (,u hr-*) Induction period (hr) * SULLY, HARDY and HEAL (1949).
Sn 7.6* 26.5 4.0 50 G 10 35
Bi 0.544 4.01 4.2 70 1 15 10 175
In 0.854 3.63 4.2 15 2 30 10 225
Cd 0.296
1.22 3.5 50 7 90 G’ 7
$6) (0.45) 3.2 80 1 30 Nil
Pb 0.0386 0.296 4.0 90 0.9 65 Nil
A. K. COVINGTON,J. D. BAIRD and A. A. WOOLF
360
solubility, no Al,,Mo is observed during the test period. Cadmium is intermediate and Al,,Mo appears only after about 800 hr. The logarithm of the solubility of aluminium in bismuth, indium, cadmium and tin decreases linearly with the logarithm of the observed induction period. Extrapolating this relationship the expected induction periods for thallium and lead would be greater than the testing period. If there were sufficient aluminium at the molybdenum surface,
d
t!f FIG. 5(a).-Layer
FIG. 5(c).-Layer
growth in lead at 460°C.
(0 A&MO.)
FIG. 5(d).-Layer
growth in lead at 540°C. x A&MO.)
(0 A&MO;
(hr i)
growth in bismuth at 460°C. (0 A&MO; x A&MO.)
however, Al&Jo should have grown immediately in all liquid metals and a correlation with solubility does not explain the behaviour in the various solvents. A few runs were performed in bismuth and lead at temperatures of 460°C and 540°C and the results are shown in Fig. 5(a-d). In bismuth at 460°C after an induction period of 100 hr Al,Mo grew more rapidly than at 500°C. Some Al,Mo appeared but no Al,,Mo in the time of testing. At 540°C a very small constant amount of A&MO (20~) was accompanied by a very rapid growth of Al,Mo similar to that found with cadmium (Fig. 2f), with only a small rate of growth of Al,,Mo. In lead at 460°C there was a faster growth of A&MO than at 500°C and the rate was the same as that in bismuth at 460°C. No Al,Mo or AI,,Mo was found. At 540°C less A&MO grew than at 500°C and there was a small parabolic growth of A&MO. The effect of temperature is complex and it is not possible to derive activation energies for diffusion in each layer. 5. FURTHER
EXPERIMENTS
Additional experiments were carried out to elucidate the behaviour in different solvents and in particular the late growth of Al,,Mo and its non-appearance in lead and thallium. I
I
IO
ti FIG. S(b).--Layer
20 (hr i)
I
30
growth in bismuth at 540°C. (0 A&MO; x AI,Mo).
(a) Variation of aluminium concentration To investigate the effect on layer growth of aluminium concentrations below saturation, amounts
Isothermal mass transfer of ahuninium onto molybdenum
.
‘;: -
I
b/o.
o % - 0.29
b= 0.155
0
b/or
b= 0.12
.
b/a = 0.12
0.22
361
saturation slowly. This was confirmed by results in lead in which A&MO grew faster and the final amount of Al,Mo was reduced compared with the original results.
b. 0.292
0.53
in liquid metals
b- 0.067
(c) Rate of solution and solubility of aluminium in liquid metals
tf FIG. 6.-Layer
(hr
i)
growth in bismuth not saturated with Al.
of aluminium, insufficient to saturate the bismuth when completely dissolved, were introduced into the ampoules together with a constant amount of bismuth. The results are shown in Fig. 6. The amount ofAl,Mo, the only compound to appear, was less than that in the previous bismuth runs, and the amount decreased the further the liquid was below saturation, until at one tenth saturation no layer was observed even after 400 hr. The 100 hr specimens in the intermediate concentration runs also showed no layer and were quite clean when removed. The amount of aluminium consumed in layer growth was insufficient to affect appreciably the concentration of aluminium during an experiment. The results show that compounds of higher aluminium content do not form without sufficient aluminium in solution. However, in the lead and thallium runs, described previously, there was sufficient solid present to maintain saturation and Al,,Mo should have formed. Its absence in lead and thallium could be explained if the liquid metal were not saturated with aluminium in the vicinity of the molybdenum. Hence the original assumption, of solution and transport processes which were rapid compared with the layer growth, became questionable. (b) Reversal of the positions molybdenum
specimens
of the aluminium in the ampoules
The construction of ampoules was exactly the same, and the thermal conditions were as nearly as possible the same, as those in the layer growth experiments. The air-circulation furnace was adapted by fitting an auxiliary furnace above the entrance port to minimize temperature fluctuations caused by opening. The liquid metals were degassed under vacuum, treated with hydrogen and given a preliminary filtration as described earlier for bismuth. For solution times of the order of minutes the liquid metal was filtered into the ampoule through a side-arm which was then sealed off. Inversion of the ampoule then brought the whole volume of liquid metal into contact with the aluminium bar instantaneously. Ampoules were removed from the furnace after a prescribed time and inverted to drain the liquid metal, which was retained for chemical analysis (see Appendix). For rates of solution of the order of hours a more complicated ampoule was employed which allowed the solution to be filtered and sampled for analysis at the temperature of the experiment. The extent of solution was also followed by the loss in weight of the aluminium after adhering metal was removed with concentrated nitric acid. This checked with chemical analysis of the solidified solution. It could not be applied to lead solutions, because the presence of lead destroyed the passivity of aluminium to concentrated nitric acid, and also because the weight loss was small. Values of the saturation solubility were determined over a range of temperature using the appropriate type
and
With the aluminium at the bottom of the ampoule instead of at the top as in all previously described experiments, circulation caused by density differences on dissolution should be increased. Results for liquid bismuth are shown in Fig. 7 (compare Fig. 2a). There was a significant reduction from 175 to 50 hr in the induction period for the growth of Al,,Mo, and the Al,Mo layer after 100 hr was thicker. Both these facts indicate that the liquid near the molybdenum reached
20
lo
tf
30
(hrf)
FIG. 7.-Layer growth in bismuth MO above Al. (0 A&MO; x A&MO; + AI,,Mo.)
A. K. COVINGTON, J. D.
362
of ampoule. The results are shown in Fig. 8 and the constants in the equation log,, s = A - 5) where S is the solubility in atomic per cent and T is absolute temperature, are given in Table 3. The heats of solution of aluminium AH,, obtained from the 550 I
OC
500 I
400 I
450 I
BAIRD
and A. A. WOOLF
lead at 1lOO”C,cooled to the required temperature and then quenched. The lower part of the ingot was analysed chemically by an unspecified method. Extensive segregation in the ingots was always found in our work. DARDEL’S higher temperature results are coincident with an extrapolated log S against l/T line through the present results but his lower temperature results diverge greatly. The previous values are compared with the present results in Fig. 8 which also includes ‘the literature values for tin. The solution rates are shown in Fig. 9(a). Only in bismuth was the solution process completed in 90 min. It was difficult to obtain consistent results with abraded specimens in cadmium and impossible in lead and indium. There were indications, however, that the process may be complete after 10 hr. Some experiments were performed with aluminium specimens chemically polished in phosphoric-nitric acid mixture (94/6 v/v) at 85°C. (This polish removes 5 ,Uper min.) There was little difference in the rate of solution in bismuth between abraded and polished specimens but the reproducibility of the results was improved in cadmium and indium. The results were analysed in terms of the equation (BIRCUMSHAW and RIDDIFORD, 1952) k’t
,
-\
I.2
I
I
I.3
I.4 h
FIG.
x I03
=
7 = In
I
d_-
(1)
a-x
I.5 (“KY
T
8.-Variation of solubility with temperature of aluminium in liquid metals.
relation AH, = 2.303RB are also given. HILDEBRAND’S solubility parameters (HILDEBRAND and SCOTT,1950)
I
-
Saturnlion value
3
83
0.9
do not predict the correct order of solubilities. The only solubilities previously reported are for aluminium in indium and lead. CAMPBELL,BUCHANAN, KUZMAK and TUXWORTH (1952) analysed
7
\
In
(500’)
/
their samples
TABLE3 0.6 log,,S (atomic%) = A - 5
Solvent
A
Bi In Cd Pb
3.094 2.990 3.248 2.667
&9
1,920 1,884 2,443 2,472
AH, = 2.303 RB (kcal mole-l) 8.7 (8) 8.6 (2) 11.1 (8) 11.3 (1) --
for the major constituent, indium, polarographically, supplemented by a rather dubious density check. For samples of low aluminium content this could lead to errors as great as 20 per cent in the weight per cent aluminium. DARDEL (1946) dissolved aluminium in
hr
FIG. 9(a).-Rate
of solution of aluminium in liquid metals.
Isothermal mass transfer of aluminium onto molybdenum in liquid metals Polished Abraded
0
x
/
---_
0.6 / H 0 ; I
0.4
/
w
/
/
/
’
/
constant for copper in bismuth (WARD and TAYLOR,
Al 502’ Al 495’
Corrected
for change
1956). of orea
x
x
* x
x I
I
I
I
5
IO
I5
20
363
min
FIG. 9(b).-Accordance of solution rate of aluminium in bismuth with transport control.
where a is the saturation concentration, x the concentration at time t, k the.solution rate constant, S the surface area of the specimen and V the volume of the liquid. Equation (1) has been found to fit the dissolution of metals in liquid metals (WARD and TAYLOR, 1956).
A plot of log a/a - x against t for the bismuth runs at 500°C is shown in Fig. 9(b). After correction for the change in surface area assuming a uniform attack, the results for abraded and polished specimens conform to equation (1) and yield k = 5 x 1O-3cm set-l at 500°C which is of the same order as the solution rate
In cadmium and indium there is a rapid initial solution followed by a slow approach to equilibrium. With chemically polished specimens the apparent induction period is removed. In lead, polished specimens also dissolve quicker initially. Even when the approximate corrections are made for change in surface area, the results for cadmium, indium and lead do not appear to conform to equation (1) although approximate constants of k’ = O-145hr-l and 0.19 hr-l may be calculated for abraded aluminium in lead and cadmium respectively. A few experiments were carried out at other temperatures in cadmium, lead and bismuth. At 454°C in cadmium the rate of solution with polished specimens was greater than that for abraded specimens at 501°C. The results for three temperatures in bismuth are shown in Fig. 9(c). At the highest temperature the area of specimens was substantially reduced, which accounts for some of the diminution in rate but the time to reach saturation was At 454°C there was an considerably extended. induction period of 5-7 min compared with approximately + min at 500°C. The induction period may be associated with the time for the liquid metal to dissolve or break through a coherent oxide film. Finite wetting times which decrease with increased temperatures have been observed in surface tension studies (TAMMANN and R~~HENBECK,1935; BONDI, 1953). The presence of tenacious surface
-,
5470 1
2
4
6
a
IO
12
14
I6
18
20
min
FIG. 9(c).-Rate
of solution of aluminium in bismuth at different temperatures.
oxide films
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
364
accounts for the irreproducibility of some of the other runs. The time of ‘break through’ will not necessarily be the same for all specimens, nor need the wetted area be constant from specimen to specimen. The thin uniform film produced by chemical polishing will be more quickly dissolved or disrupted by the liquid metal, and the dissolution is correspondingly quicker. The observed slowing down of the solution rate with chemically polished specimens in indium and cadmium may be attributed to the reformation of an oxide film competing with the solution process, or alternatively to the formation of an oxide film on parts of the surface reducing the effective area of the specimen. Elements with stable oxides (e.g. Zr, Ti, Mg) are used to remove oxides from liquid metals and aluminium would be expected to behave similarly.
_. 100 -
experiments were carried out under 100 atm pressure of argon in an attempt to maintain continuous contact. Specimens tested up to 2000 hr were mounted and measured in the same way as the mass transfer specimens. The growth of Al,Mo was the same in both the clamp and pressure-couple experiments (Fig. 10). A&MO and AI,,Mo were found in smaller amounts than in the experiments in liquid bismuth. Metallographic examination showed that good contact between the metals had still not been maintained, so that the supply of aluminium necessary for the rapid growth of the compounds A&MO and Al,,Mo was not achieved. A&MO could be seen (Fig. 11) in the grain boundaries of the columnar grains of A&MO. This may indicate that grain boundary diffusion is important in the transfer of aluminium through A&MO. BAIRD (1960) has shown theoretically that grain boundary diffusion may give rise to a fourth power growth rate equation instead of the usual parabolic rate equation, and this may be one reason why growth of AlaM0 does not maintain its initial parabolic rate. In the pressurecouple specimens, the fine-grained structure of the A&MO produced on dipping persists on annealing, showing that there was insufficient diffusion of molybdenum at 500°C to allow recrystallization. There was a small but irregular growth of Al,,Mo in these specimens but little A&MO. 6. DISCUSSION
0
IO
20
ti
(hri)
30
40
FIG. lO.-Layer growthin diffusioncouples.
(d) D@ision couples The growth of alloy layers, formed by the direct contact of aluminium with molybdenum, was studied to find if maximum growth rates were being reached in the liquid metals. However, a good contact is difficult to achieve because of the rapidly reformed oxide film on aluminium. When the two metals were polished, and then clamped or rolled together, they did not react evenly at 500°C. The molybdenum was therefore dipped through a flux into molten aluminium to form a continuous alloy layer, the thickness of which (10,~) was subtracted from the final measurement. Several zone-refined molybdenum strips thus coated were placed in hand-tightened steel clamps. These were sealed in evacuated glass capsules and heated at 500°C for times up to 2000 hr. Even in this method of preparation there were indications that the contact between the aluminium and the alloy layer had deteriorated during the runs probably due to vacancy condensation (BAIRD, 1960), and a further series of
The solution process at the aluminium interface may be regarded as taking place in two steps: (a) reaction at the interface (chemical control), (b) diffusion from a saturated laminar layer into the bulk of the solution (transport control). Either step or both may be rate-controlling. A difference in the rate of dissolution produced by a change in the method of surface preparation is usually taken as an indication of some chemical control (BIRCUMSHAWand RIDDIFORD,1952), whilst a change produced by a variation of stirring rate is taken as an indication of some transport control. WARD and TAYLOR (1956) concluded from their studies of the solution of copper in bismuth and lead, that the results were consistent with transport across a laminar layer. EPSTEIN(1957) came to a similar conclusion about iron in mercury but suggested that in sodium, the dissolution of iron was chemically controlled. This is feasible in view of the difficulty in completely removing oxides (HORSLEY,1956). TAYLORand WARD (1957) believed that the solution of iron in bismuth was also transport controlled, but in an extension of this work GRAHAM and WILSON (1959) attributed differences between the solution rate constants for certain chrome steels to
Isothermal mass transfer of aluminium onto molybdenum in liquid metals
oxide films of different composition and tenacity. All these results (apart from those of EPSTEINwho did not present experimental data) may be fitted to the firstorder equation (1). LOMMELLand CHALMERS(1959) found that the rate of solution of a lead-tin alloy in liquid lead was diffusion controlled with no stirring, but with high stirring rates the process was controlled by the surface reaction. The results for aluminium show that its dissolution is also controlled by surface reactions probably involving the removal of oxide films. The rates of solution were slowest for those liquid metals whose freezing points are closest to the testing temperature. However, even though the dissolution time for some liquid metals is of the order of hours, this time is still short compared with that to grow an appreciable alloy layer. The rate of growth of alloy layers on the surface of the molybdenum would be expected to depend on the concentration of aluminium in the liquid metal, and this is demonstrated by the results shown in Fig. 6. It might also be expected to be controlled by the rate of diffusion of one or both metal atoms through the already formed alloy layer and hence upon its thickness. The parabolic growth of the outer alloy layers confirms this. The overall process of isothermal mass transfer is complicated and any analytical treatment would be complex. The following treatment demonstrates that under the experimental conditions chosen for testing in those liquid metals with low solubility of aluminium, the amount of aluminium passing into solution may be comparable to that removed by alloying. The liquid
metal is only slowly saturated under these conditions so that the aluminium-rich compound Al,,Mo may not form. Let x = concentration of aluminium in liquid metal at time t a = saturated concentration of aluminium w1 = weight of aluminium removed from solid rod at time t w’~= weight of aluminium removed from solution by alloying at time t y = thickness of alloy layer at time t c = concentration of aluminium in alloy layer
365
A = mean cylindrical area of alloy layers on molybdenum (2.9 cm2). Experimentally the growth of alloy layers is found to obey the equation y = k,‘t* (2) then w, = ycA = k2’cAtt = k2tf where k, = k,‘cA. The dissolution process has been shown to conform approximately to equation (I) which may be expressed in a corresponding differential form as
dw, - = dt
kS(a - x).
Since x = (wl - w2)/V and from above ‘2
= - k2
2vt”
dx
kS -_-(~_+k~. dt
Multiplying by the factor ekStlVand integrating gives x = a(1 _ e-kStIV) _ .!$ @St/v!>-*
ekS/Vdt
where 5 is an integration variable. The second term may be transformed into an integral 5 of the form F(x) = e-“* ev2dy, for which tabulated s values are available, b; substituting y2 for kS/V (MILLERand GORDON,1931). Then
where k’ = kS/V.
Values of 5 for various times have been calculated using k, = 4 x 10d5 g sect derived from the parabolic rate constant k,’ = 4 ,u hr-* which is approximately the same in all liquid metals tested (Table 4). Thus although bismuth is effectively saturated with aluminium from the start of the experiments, the lead is much slower to reach saturation. After 1000 hr it is still only 99 per cent saturated. If the growth or nucleation of the compound Al,,Mo requires an aluminium concentration very close to saturation, the compound might not be expected to appear under the
(2.1 g cmP3 for Al,Mo) Mat. %I Density at 500°C (g cm+)
Bi 9.8 Pb 10.1
k
s
V
(cm set-I)
(cm?
(cm3)
5 x 10-S 1.1 x 10-d
2.52
5.6
0.99978
0.99984
2.52
6.7
0.388
0.653
-1 0.968
300 (W
1000
-1 0.982
-1 0.990
(hr)
A. K. COVINGTON, J. D. BAIRDand A. A. WOOLF
366
conditions of the present experiments. In the calculation, the approximate value of the rate constant for first-order kinetics of dissolution has been used. The actual rate of solution in its later stages was actually slower than predicted by this equation, and saturation would be reached even later than predicted above. These predicticns were later confirmed by two additional mass transfer experiments using a modified ampoule, which allowed filtering and sampling for chemical analysis of the liquid lead at the testing temperature. Values of x/a found were: 0.63 after 100 hr and 0.89 after 300 hr. For those experiments with bismuth in which insufficient aluminium was added to saturate the liquid metal, assuming that the rate of removal of aluminium by alloying is proportional to its concentration in solution, then
3
7. CONCLUSIONS Isothermal mass transfer of aluminium onto molybdenum is controlled by diffusion in the solid state provided that a sufficient concentration of aluminium is available at the surface of the molybdenum. The dissolution process is not always rapid compared with the rate of layer growth and may be impeded by a slow chemical process at the solid surface or diffusion through a coherent oxide film. Isothermal mass transfer may be troublesome in nuclear reactors employing liquid metals as coolants or solvents for fissile material. Studies with liquid bismuth contained in steel have shown that thermal gradient mass transfer arising from the large temperature coefficient of solubility of iron in liquid bismuth, occurs (HORSLEY,1959), even with the use of inhibiting films provided by the addition of zirconium
= ks(w- WJ
dt
W2
where w2 = weight of aluminium removed in time t w = weight of aluminium originally added to bismuth (assumed to dissolve completely). However, in these experiments w2 < w so the above equation simplifies to dw, -=dt
k,w w2
but z = b, the concentration of aluminium in solution and wg = ycA therefore
dr
ksb
dt=rv
On integration : where k,” is the parabolic growth rate constant. These growth constants are proportional to b”, as can be seen from Fig. 12. They have been evaluated by drawing the initial slopes in Fig. 6. When b = a, k,’ = k,” and the slope of the graph (Fig. 11) is given correctly by k,‘a-* within the accuracy of estimation of the slopes in Fig. 6. Since the initial rate of growth of A&MO is little affected by unsaturation of liquid metal up to 50 per cent (see Fig. 6), the rate of growth of Al,Mo appears to be approximately the same in all liquid metals. Hence the small effect of aluminium concentration on growth of A&MO can be ignored in calculating the time variation of aluminium concentration (equation 3).
FIG. 12.-Growth
rates in non-saturated
bismuth solutions.
or titanium to the liquid metal. Despite the cost and fabrication difficulties, refractory metals such as molybdenum would seem to be the only reasonable alternatives for containing liquid bismuth. Because of its minute solubility, thermal gradient mass transfer of molybdenum will not take place but dissimilar mass transfer may. Our previous study (COVINGTONand WOOLF, 1959) showed that few metals will react with molybdenum or the other refractory metals at present reactor temperatures, although they may do so a few hundred degrees higher. In an L.M.F.R. contained in a refractory metal, the accumulation of fission-product metals may lead to isothermal mass transfer, but if the concentration of these is kept low by continuous removal and processing of the bismuth, the growth of alloy layers should be small, and the deposition of low melting point compounds with rapid growth rates should be prevented.
Isothermal mass transfer of aluminium onto molybdenum 8. APPENDIX
in liquid metals
361
A. (1953) Chem. Rev. 52.417. BRASUNAS‘A. DE S. (1953) Corr&ion 9, 78. CAMPBELLA, N., BUCHANANL. B., KUZMAK J. M. and TuxThe determination of small amounts of aluminium is diflicult WORTHR. H. (1952) J. Amer. them. Sot. 74,1962. because the available reagents are far from specific and reagents COVINGTONA. K., GEACH G. A. and WOOLF A. A. (1958) and solvent media themselves contain appreciable quantities of Atomics 9, 10. aluminium. A reliable analysis requires quantitative separation COVINGTON A. K. and WOOLFA. A. (1959) Reactor Sci. Technol. from almost all other elements, a mimimun use of solvents and (J. Nucl. Energy Part B) 1, 35. reagents and careful blank determinations. DARDELY. (1946) Light Metals 9, 220. Aluminium can be extracted along with many other elements EPSTEINL. F. (1957) Chem. Engng. Progr. Symposium Series 53, by I-hydtoxy-quinoline (oxine) from solutions of pH 2-10 by No. 20. chloroform or carbon tetrachloride. The related I-hydroxyFORSYTHJ. B. (1960) Private communication. quinaldine does not extract aluminium but does extract other GRAHAML. W. and WILSON G. W. (1959) J. Iron. Steel Inst. metals. A combination of both reagents has been used to deter193, 103. mine aluminium in its alloys (HYNEKand WRANGELL,1956) and HANSEN M. and ANDERKO K. (1958) Constitution of Binary the present method is a modification of this procedure. Alloys, 2nd Ed., McGraw-Hill, New York. After solution of the sample in a suitable acid mixture, HILDEBRANDJ. H. and SCOTTR. L. (1950) Solubility of Nonelectrolysis with a mercury cathode (SCHERRERand MOGERMAN, electrolytes, Reinhold, New York. 1938; JOHNSONet al., 1947) was performed at a (cathodic) HORSLEYG. W. (1956) J. Iron Steel Inst. 382, 43. current density of 0.25 A cm-2 for about 5 hr to remove bismuth HORSLEYG. W. (1957) J. Nucl. Energy 6,41. and indium. Lead was slowly precipitated as sulphate over a HORSLEYG. W. (1959) Reactor Technol. (J. Nucl. Energy Part B) period of 1 hr. Aliquots were extracted at pH = 9.5 into 1, 84. chloroform with 8-hydroxyquinaldine to remove other metals, HYNEKJ. R. and WRANGELLL. J. (1956) Analyt. Chem. 28,152l. followed by extraction of aluminium with oxine, and their JOHNSONH. O., WEAVERJ. R. and LYKKENL. (1947) Zndustr. absorption was measured on a spectrophotometer at 400 rnp. Engng. Chem. (Analyt.) 19,481 Aluminium in cadmium was estimated by the standard graviLOMMELL J. M. and CHALMERS B. (1959) Trans. Amer. Inst. Mech. metric method (precipitation of aluminium hydroxide) and Engrs. 215, 499. checked against the weight loss method. MILLERW. L. and GORDONA. R. (1931) J.phys. Chem. 35,2878. MULVEYT. (1959) J. sci. Znstrum. 36, 350. Analysis of molybdenum in aluminium-molybdenum alloys MULVEYT., BERNARDH. and BRYSON-HAYNES D. (1959) J. sci. Instrum. 36, 438. Molybdenum was determined volumetrically. The alloy was dissolved in the minimum amount of HNO,: HF mixture (20 : 1). NORTHCOTTL. (1956) Molybdenum, Butterworths, London. PARKS T. D., JOHNSONH. D. and LYKKEN L. (1948) Industr. Sulphuric acid was then added and the solution fumed to remove Engng. Chem. (Analyt.) 20, 148. all nitric acid which would interfere with the subsequent reducREIMANNH. (1922) Z. Metal. 14, 195. tion. The solution was diluted and passed through an activated RHINES F. N. (1940) Surface Treatment of Metals, A. S. M. Jones reductor into standard potassium dichromate containing Symposium, 122-16. some phosphoric acid. The excess dichromate was back-titrated SCHERRERA. and MOGERMANW. D. (1938) J. Res. Nat. bur. with ferrous ammonium sulphate using barium diphenylamine Stand. 21, 105. sulphonate as internal indicator. SPERNERF. (1959) Z. Metal. 50, 588. Acknowledgments-We wish to thank DR. B. W. HOWLETTand SULLYA. H., HARDYH. K. and HEAL T. J. (1949) J. Inst. Metal DR. A. G. KNAPTONfor assistance with the metallography and 76, 269. for many useful discussions, DR. E. SMITH for mathematical TAMMANNG. and R~~HENBECK A. (1935) Z. anorg. Chem. 223, advice, MR. J. N. BRAZIER, MR. T. M. COTTONand MR. J. 193. RAMIREZfor experimental assistance, particularly with chemical TAYLORJ. W. and WARD A. G. (1958) Nucl. Power 3,101. analysis, DR. G. A. GEACH and DR. B. R. T. FROST for their TAYLORJ. W. and WARD A. G. (1957) Kinetics and Equilibrium continued interest and DR. T. E. ALLIBONE,C.B.E., F.R.S., Solubility Studies in the Iron-Bismuth System. AERE Report Director of this Laboratory, for permission to publish this paper. M/R 2295. WACHTELLR. L. (1952) Powder MetaN. Bull. 6, 99. REFERENCES WALTHERW. D. (1950) Molybdenum-Aluminium System. Thesis, Massachussetts Institute of Technology. ADAM J. and RICH J. B. (1954) Acta Cryst. 7, 813. WARD A. G. and TAYLORJ. W. (1956) J. Inst. Metal 85,145. BAIRDD. (1960) Reactor Sci. (J. Nucl. Energy Part A) 11, 81. YAMAGUCHIK. and SIMIZU K. (1940) Trans. Japan. Inst. Metal BIRCUM~HAWL. L. and RIDDIFORDA. C. (1952) Quart. Rev. them. Sot. 6, 157. 4, 390 Chemical analysis of aluminium dissolved in liquid metals
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