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Hydrogen solubility in a titanium aluminide alloy
Acta metall, mater. Vol. 40, No. 3, pp. 455-462, 1992 Printed in Great Britain.All rights reserved
0956-7151/92 $5.00+ 0.00 Copyright © 1992Pergamon Press plc
HYDROGEN SOLUBILITY IN A TITANIUM ALUMINIDE ALLOY WU-YANG CHU?, A, W, T H O M P S O N and J, C. WILLIAMS:~ Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. (Received 25 March 1991; in revised form 5 August 1991)
Abstract--The dissolution of hydrogen in a titanium aluminide alloy based on Ti3A1, Ti-24 AI-11 Nb, has been measured as a function of temperature and hydrogen pressure. Both the terminal solubility and the overall solubility, or total uptake of hydrogen, were determined. The terminal solubility of hydrogen in the ~2 matrix phase increases with temperature, while the overall solubility decreases with increasing temperature in the high-temperature region. The partial molar heat of solution in the ~2 phase was determined to be-(21.97 + 1.46) kJ/mol [HI, while the partial molar heat of formation of the hydride phase was -(70.29 + 2.34) kJ/mol [H]. The standard partial molar free energy of hydrogen in the ~2 phase in equilibrium with the hydride was -(74,060 -+ 2000) + (87.8 _+2.1)T J/tool [H]. R6smn6~La dissolution de l'hydrog6ne dans un alliage d'aluminiure de titane fi base de Ti3A1, Ti-24Al-I INb, 6t6 mesur6e en fonction de la temp6rature et de la pression d'hydrog6ne. On a d6termin6 la lois la solubilit6 terminale et la solubilit6 totale (ou prise totale d'hydrog6ne). La solubilit6 terminale de l'hydrog6ne dans la phase matrice ~t: crott avec la temp6rature, alors que la solubilit6 totale d6croit lorsque la temp6rature augmente dans le domaine des hautes temp6ratures. La chaleur molaire partielle de raise en solution dam la phase % est 6gale ~i -(21,97 -+ 1,46) kJ/mol [H], alors que la chaleur molaire partielle de formation de la phase d'hydrure est 6gale ~ -(70,29 +_2,34) kJ/mol [H]. L'6nergie molaire partielle libre classique pour l'hydrog6ne dans la phase ~t: en 6quilibre avec l'hydrure est 6gale - (74,060 _+2000) + (87,8 -+ 2,1)T J/mol [H]. Znsammenfasstmg--Die L6sung von Wasserstoff in einer Titanaluminid-Legierung auf Basis Ti3AI, Ti-24AI-1 INb, wird in Abh.~ngigkeit der Temperatur und des Wasserstoffdruckes gemessen. Sowohl Grenzl6slichkeit wie auch allgemeine L6slichkeit, oder die Gesamtaufnahme an Wasserstoff, werden bcstimmt. Die Grenzl6slichkeit des Wasserstoffes nimmt in der ~2-Matrixphase mit der Temperatur zu, wohingegen die Gesamtl6slichkeit mit zunehmender Temperatur im Bereich hoher Temperaturen absinkt. Die partiale molare L6sungsw~irme in der ~2-Phase wird zu -(21,97_+ 1,46) kJ/Mol [H] ermittelt, die partiale molare Bildungsw~irme der Hydridphase ist -(70,29 5-2,34) kJ/Mol [H]. Die partiale molare freie Energie des Wasserstoffs in der ~t:-Phase im Gleichgewicht mit dem Hydrid ist -(74,060 _+2) + (87,8 5: 2,1)T J/Mol [HI.
INTRODUCTION Alloys based on the compound Ti3A1 have received sufficient attention to show that they may be legitimate candidate materials of construction for aerospace systems [1-3]. Of particular interest are both the engines and the airframe of a hydrogen-burning propulsion system [3, 4]. Since such alloys contain Ti, with its well-known propensity to interact with hydrogen, it is important to have a clear picture of the effects of hydrogen on the properties and rnicrostructure of these aluminide alloys. Yet since the original interest in this material has been for high performance aircraft engines which typically operate under oxidizing conditions, there has been little effort expended in studying the effects of hydrogen. tPresent address: Department of Materials Physics, University of Science and Technology, Beijing 100083, P.R. China. ~Present address: Engineering Materials Technology Laboratory, General Electric Aircraft Engines, Evendale, OH 45215, U.S.A.
On the basis of pioneering studies [5-7], it is now well established [8-10] that the low temperature phase of pure titanium, ~-Ti, has a relatively low solubility for hydrogen and precipitates a hydride when the solubility limit is reached. This hydride, the y phase in the Ti-H system, was originally designated Till 2 but in more recent literature [ l l , 12] has been designated as Till, a better description of its typical stoichiometry, although the limiting composition of the ), phase stoichiometry is indeed Till2. In the case of the Ti3 A1 compound, which is designated ~2 in the titanium-aluminum system, hydrides are also reported to form [13], and the same is true for alloys based on Ti3Al [14-19]. However, the quantitative description of hydride formation conditions is still largely an open question, and no data on the solubility of hydrogen in this compound are available, other than a preliminary report [14]. For some metals, like Ti and Zr, the maximum amount of hydrogen absorbed increases with pressure and decreases with temperature within a high temperature region [8, 20]. Due to this temperature
455
456
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
behavior, they were classed as exothermic occluders. However, some workers have shown that the terminal solubility of ct-Zr in equilibrium with the hydride phase increased as the temperature rose [21, 22]. Among the first alloys based on Ti3A1 was Ti-24 AI-11 Nb (at.%), developed by Blackburn and coworkers [23, 24]. Our preliminary experiments [14, 18] showed that hydride could form in this alloy, designated "Ti-24-11" below, after either cathodic charging in H2SO4 solution with recombination poisons, or after thermal charging in hydrogen gas, and the maximum amount of hydrogen absorbed decreased with increasing temperature within a high temperature region (e.g. T t> 550°C). Therefore, it was the purpose of this paper to investigate the formation of hydride in this alloy, and measure the overall solubility of hydrogen in the alloy as well as the terminal solubility of hydrogen in the ct2 phase in equilibrium with the hydride phase. We define "overall solubility" as total equilibrium hydrogen uptake, combining hydrogen in solution with that contained in hydrides, at a given temperature, while "terminal solubility" refers to the maximum amount of hydrogen in solution at that temperature.
EXPERIMENTS Analytical basis for experiments According to Sieverts' law [20, 25-27], the overall solubility of hydrogen in either the ct2 phase or the hydride 0'-phase in the Ti-H system) is proportional to the square root of the hydrogen gas pressure in contact with the phase, x/~"~, according to (la)
C H = WN~H2,
with W = e-A~/Rr = e-tatmre as/R
(lb)
since AG = A H - TAS. Here C., AG, AH and AS are, respectively, the hydrogen content, partial molar free energy, partial molar enthalpy of solution, and partial molar entropy of solution in the phase under consideration, e.g. ct2, and R is the gas constant. Note that all the relevant thermodynamic quantities can be obtained from measurements varying any two of CH, PH2, and T, while keeping the third constant, as discussed in more detail below, since [20, 27] AG can be obtained from ½R T In Pn2. The dissolution of hydrogen in metals has been treated using statistical mechanics [22, 28]. It can be assumed that the number and position of Nn hydrogen atoms arranged randomly on Ns interstitial sites are related to the partial molar entropy AS as AS = -- R
NH
N,-N~
and
AG=RTln
NH
Ns-N.
(2a)
and the overall solubility should be Nn
g , - g.
= ~
e -AG/Rr.
(2b)
Although it has not been established directly, it can be assumed [ll-13] that the hydrogen atoms occupy tetrahedral sites on the hexagonal symmetry (D019) Ti3A1 parent lattice. There are four sites of tetrahedral co-ordination per unit cell in the Ti3Al structure, therefore NH
CA
Ns - N . = 2 - CA'
(2c)
so that
CA = x/-~n2 e-~/Rr e~s/R 2--CA
(3)
where CA=NH/Nri~ is the atom ratio. Thus equation (3) is the statistical mechanics or "crystallographic" concentration, while equation (1) represents the thermodynamic concentration. Since equation (3) depends on assumptions about site occupancy, it is potentially less reliable than equation (1). To obtain the terminal solubility of hydrogen in the ~2 structure in equilibrium with the hydride, the condition of equilibrium is that the partial molar free energy of hydrogen is equal in the ct2 phase and the phase. Then from equation (1) the H concentration ratio is
C.(~)
Ca(7) = exp[AS('2)- AS(~))/R] x exp - [(AH(,2)-AH(~))/RT ] (4)
where the phase is referenced for each parameter. Equation (4) then describes the terminal solubility. The terminal solubility could be measured in two ways, which are called the x//P vs C method and the P vs I/T method. Their basis is as follows, from equation (1). The square root of the decomposition (or equilibrium) pressure, x/~n2, at a fixed temperature should change linearly with increase of hydrogen concentration Cn, in a single-phase region. However, due to Gibbs' phase rule, in a two-phase region the decomposition pressure would not change with the hydrogen concentration. Thus, the intercept of the straight-line Cn-~P--~.: relation and the constantpressure plateaus give the terminal solubility. It may also be seen from equation (1) that a plot of the decomposition pressure on a logarithmic scale against 1/T is a straight line, for a fixed hydrogen content. Note from equation (I) that AH can be calculated from the slope of this line. But when a single-phase region goes over into a two-phase region or vice versa, the slope of the straight line will change (e.g. Refs [13, 29]). (Observation of a straight line in the two-phase region, incidentally, indicates [20] that
WU-YANG
et al.: H S O L U B I L I T Y I N A Ti A L U M I N I D E
CHU
only the volume fractions of the two phases, and not the composition of either the solid solution or the hydride, are changing appreciably with temperature; or that the compositions of the two phases are changing in exactly the same way with temperature.) The intersection of two such In PH2 vs lIT straight line segments gives the temperature at which the given hydrogen concentration represents the terminal solubility.
101
T (°C) 500
600
700
I
ALLOY
457
400
I 1 H = 490 wppm(2.3 at %
103
Total H, 46 x 10 -4 g 1 O0
1o
• Heating o Cooling
102
10 -1
101
:P
Experimental procedure
All the present experiments were conducted on the titanium aluminide alloy Ti-24-11, which was obtained in bar form from Reactive Metals (RMI). The chemical compositions of the Ti-24-11 were provided by RMI as follows (wt%): A I = 13.0913.34, Nb = 20.57-20.64, Fe = 0.14, O = 0.09-0.1, N = 0.011-0.016. Blanks with dimensions of 20 mm x 20mm x 40mm were machined, cleaned and solution treated in an argon atmosphere at 1149°C (2100°F) for 2 h and air cooled, then aged at 705°C (1300°F) for 2 h and air cooled. The microstructure after this heat treatment was essentially all ct2 phase. The b.c.c, fl phase could not be detected by X-ray, while examination by transmission electron microscopy indicated 0.1-1% of the fl phase, present only as isolated, discontinuous fl regions. Specimens for hydrogen measurement with dimensions of 20 mm x 20 mm x 1 mm, were sectioned from the bulk samples. The specimens were precharged at different temperature for different times. The weight gain, determined by an analytic balance with a sensitivity of 10 -s g was used to calculate the hydrogen concentration in the specimens, which were in the range of 230wppm (1.1 at.%) to 3720 wppm (15.1 at.%). The total weight of the specimens for each time of charging was about 10 g; therefore, the error calculating the hydrogen concentration was less than 10 wppm. A Sieverts apparatus [20, 25, 26, 29, 30] was used to measure the decomposition pressure in this work. During an experimental run, the specimen tube and the operative pressure devices were isolated from the remainder of the system by means of vacuum valves. The whole apparatus was capable of being evacuated initially to a pressure of about 10 #Pa (1 x 10 - 7 torr). The decomposition pressures over the samples were read on two mercury manometers. One of them was a McLeod gage with a range of 125 Pa (1 torr), and
Q_
,~-
Q.
10-2
1
10 -3
10 - I A
10 -4 .0
I 1.1
I 1.2
I 1.3
I 1.4
I 1.5
I 1.6
I O 0 0 1 T (K - I ) Fig. 1. A n e x a m p l e o f d e c o m p o s i t i o n p r e s s u r e in ~ 2 - H
system vs 1/T, 2.3 at.% hydrogen, measured both on heating and on cooling. the other had a range of 2 kPa (15 torr). For each composition, the number of the samples was changed to keep the total amount of hydrogen about constant, e.g. 3.3 to 4.6 mg of H2 for each constant temperature. Temperature was controlled within + I°C. The decomposition pressure was measured many times until a stable pressure was obtained. To assure that the equilibrium pressures were actually determined, measurements were made on both the decomposition (heating cycle) and absorption (cooling cycle) for some samples. Fully reversible behavior was always observed in such runs. Reversibility was also checked by repeating some measurements on the same sample. After measurement, a sample would be heated in the evacuated Sieverts apparatus to remove all hydrogen (it was verified that sample weight would return to the precharging value within 10/~g), recharged with hydrogen, and additional measurements made. Reproducibility of results in such repetitions was excellent. RESULTS
Terminal solubility of hydrogen in ~2, in equilibrium with hydride, was determined by the
Table 1. Decomposition pressure of =:-H system Composition Wppm
at.%
350
400
450
230 490 680 750 930 1290 2620 3720
1.1 2,3 3.2 3.5 4.3 5.8 11.2 15.1
0.13 0.11 0.13 0.13 0.20 0.13 O.11 0,13
0.93 1.2 1.1 0.93 0.93 1.2 1.3 1.1
2.0 5.6 6.0 6.0 6.4 5.3 6,7 7.3
Pressure at temperature °C (Pa) 500 550 600 2.9 9.3 16 20 28 24 27 25
4,1 13 24 32 43 100 93 113
5.5 19 33 43 64 136 293 266
650 8.3 27 47 63 83 180 640 813
675
700
725
253 800 1290
12 33 61 80 109 293 947 1470
1040 1670
W U - Y A N G C H U et al.:
458
H SOLUBILITY IN A Ti A L U M I N I D E ALLOY
T (°C)
30
700 esosoosso soo 4so 400 10 4 ]_ I
I
[
I
I
I
~ -
I
C H (at. %) v 15.1 • 11.2
1o3 - ' ~ , , ~ "
I
~-
[]
5.8
• A
4.3 3.5
650
3so I
j--" °C
- 101 ~
°C
.
oC
q
• 3.2 ~" 102 n
~. W ~ ~
o
•
2.3
1
1.1
~F
'~" g
I1.
10
-r
I0-I a.
,'-:
101
1
1.0
1.1
1.2
1.3
./~P~=
10.2
500 °C
~O
a
f I
I
I
I
2
4
6
8
0
I
I
•
I
I
I
10 12 14 16 18
C H (at. %) 1.4
1.5
Fig. 3. Square root of the decomposition pressure in the =2-H system vs hydrogen concentration (at.%) at different temperatures,
1.6
IO00/T (K-~) Fig. 2. Summary plot of log P m v s I/T. AB line is for e2 and hydride phases, and the other lines represent atomic percent hydrogen concentration in the e2 phase. Different symbols represent different hydrogen concentrations.
methods described above. Figure 1 shows a typical log decomposition pressure (x/~a2) vs I/T graph obtained for the hydrogen concentration of 2.3 at.% (490wppm) hydrogen. Table 1 summarizes the experimental data for eight hydrogen concentrations. In Fig. 1 the slope change at the phase boundary is clearly evident. Fig. 1 indicates the g2 to be the stable phase over the section BC of the graph, while a mixture of the a2 phase and the hydride phase is stable over the section AB. The point B gave the temperature at which the concentration of 2.3 at.% represented the terminal solubility, which was 447°C. The slope of BC gave a partial molar heat of solution as AH~ = - 20.96 kJ/mol [H] for the g2 phase, while the slope of AB gave a value of AH~ = - 74.89 kJ/ mol [H] for the hydride phase. This calculation assumes the composition of the hydride phase was invariant with temperature, as is implied by the reasonably straight AB lines in Figs 1 and 2. The partial molar heats of solution AH=2, AH~ and the partial molar entropies of solution AS=2, AS~ were
calculated using the data listed in Table 1 and are tabulated in Table 2. From these data, 95% confidence limits on the mean values of the thermodynamic parameters were calculated, using t statistics. The data in Table 2 show that neither AH=2 nor AHr were dependent on the hydrogen concentration. The hydrogen content at the phase boundary of the ~-hydride in Ti [8, 31], which was CHv= 60 at.% or NH(¢)/NTi = 1.45, was used to calculate ASr. Figure 2 shows a plot of all decomposition pressure data for the determination of the terminal solubilities. The line AB on the figure represent the average data of all hydrogen concentrations in the two-phase region. The intersection of the decomposition pressure curve for the a2 phase with the line AB gave the temperature at which the given composition represented the terminal solubility. These values are listed in Table 3. Figure 3 shows a plot of the square root of the decomposition pressure as a function of the hydrogen concentrations. In the ~:-phase region, a set of straight lines passing through the zero point were obtained, and the constant-pressure plateaus correspondext to the region where the two phases, hydrogensaturated =~ phase and hydride, co-existed. The
Table 2. AH and AS of hydrogen in the a2 phase and hydride phase Hydrogen concentration
In "2 phase
AH~
In hydride phase
A$~ (J/C/MoI[HI)
AH~
AS~(J/C/tool[H])
At.%
100C^ 2 - CA
kJ/mol[H]
Equation (1)
Equation (3)
kJ/mol[H]
Equation (1)
Equation (3)
1.1 2.3 3.2 3.5 4.3 5.8 11.2 15.1
0.55 1.16 1.65 1.83 2.25 3.20 6.67 9.77
-21.2 -21.0 -21.1 -21.7 -21.4 -24.1 -25.2 -20.1
-10.63 -8.87 -8.70 -9.75 -9.00 -13.22 -13.81 -7.95
-54.98 -53.14 -52.80 -53.72 -52.97 -56.74 -56.69 -50.08
-68.6 -74.9 -72.0 -71.6 -65.3 -69.9 -70.7 -69.5
-46.4 -56.5 -52.3 -51.9 -42.7 -49.4 -50.2 -48.5
-72.8 -8.28 -78.2 -77.8 -68.6 -75.3 -76.2 -74.9
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
459
Table3.Terminalsolubilityof hydrogenin ~2 Method Temperature(°C) At.% Terminal 100C^ solubilities 2 - CA 'Not exactvalue.
403 1.1 0.55
447 2.3 1.16
InP vs liT 489 506 3.5 4.3 1.83 2.25
478 3.2 1.65
intersection of each straight line with its constantpressure plateau gave the terminal solubility, which is the concentration at which the hydride precipitates. The results are included in Table 3. A plot of vs C^/2-C^ [following equation (3)] was similar to that of ~ / ~ , 2 vs Cn (at.%), shown in Fig. 3, although some of the straight lines did not exactly pass through the zero point, suggesting that equation (1) may be a more suitable description for these data (the site occupancy assumption for equation (3) likely needs refinement). The terminal solubilities determined from equation (3) are listed in Table 3. The variation of the terminal solubility (Ca) with temperature, i.e. the hydride solvus, is shown in Fig. 4, using the equation (1) results. It indicates that the terminal solubility of hydrogen in the ~2 in equilibrium with the hydride increases with increasing temperature. The equation of the terminal solubility as function of temperature, at 1 atm pressure, is CH(at.%) = 6.8 x 103 e -575°/r
(5a)
and CA
2-C^
= 57 e -62°°/r
(5b)
where CA is the atomic ratio and T and the exponential constants are in K. Insertion of room temperature into equation (5) predicts a solubility of hydrogen of about 0.2 ppm (atomic); a spline fit to Fig. 4 and extrapolation to room temperature gives a value of about 0.05 ppm. Though these are only estimates, they do indicate that equilibrium room temperature solubility is probably very small, and virtually all charged hydrogen is precipitated as hydride on cooling to ambient.
700
x/P vs C 560 5.8 3.2
638 11.2 6.67
671 15.1 9.77
500 4.3 2.25
550 5.8 2.9
600 8.6 4.74
650 (12.6)' (8.3)"
Table4. CIx/-fi underdifferenttemperature T(°C)
500
550
600
650
C/~PP
9.37
100~ K / ' - P
5.48
6.98 3.85
5.90 3.28
5.11 3.13
Overall solubility The slopes of C , = Wv/P--~.2 (or CA~2- CA = W q/~H2) straight lines of Fig. 3 under constant temperature were calculated and are presented in Table 4. From these data, we obtain the equation of overall solubility or total uptake of hydrogen in the ~2 phase, i.e. Cn(at. %) = 2.67X/~H2e 2650/T
(6)
where T is in K, and the unit of P is Pa. Figure 5 shows a plot of the overall solubility of hydrogen in the ct2 phase as a function of temperature above 400°C. This figure indicates that the overall solubility of hydrogen in the ~t2 phase decreases with increasing temperature when T t> 500°C. In this sense, the ~2 phase is an exothermic occluder [20]. This conclusion has been verified by the thermal charging under high temperature. For the higher pressure, shown in Fig. 5, the hydrogen pressure was adjusted with time to keep hydrogen pressure at 1 atm.
Identification of the hydride Although description of the hydride phase itself was not a primary goal of this study, a few observations may be of interest in view of apparent disagreements in the literature [15, 16, 19] about hydrides in the ~t2 phase. The hydrides appeared as fine markings or lines on the polished surface of the sample without
-
1
700 -
,3 :,0
oO"600 I-500
600 -
o t ~ "
?
v
500 -
7 400
"x,,,.
v
I-
/I 2
° " ~ V s CH I 4
' 6
i 8
' 10
'
I
'
'
12 14 16 18
C H (at. %)
Fig. 4. Terminal solubility of hydrogen in the "2 phase in equilibrium with the hydride phase, i.e. solvus for hydride in *t2, for Ti-24-1 I.
400 0
I 4
I 8
I 12
I 16
I 20
C H (at. %)
Fig. 5. Overall solubility (total uptake) of hydrogen in the ct2 phase vs I/T, for two pressures shown on the curves in units of atm (or 130Pa and 0.1 MPa).
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
460
Uncharged
deg20 I/~ deg 20
Charged (CHffil3.3 at.%) I/~
35.5 15 35.9 75
36.5 15
Table 5. X-ray diffractiondata 38.6 40.6 41.3 15 35 20 39 40.9 20 100
etching after cathodic charging in H2SO4+0.5 g/l NaAsO2 solution, as shown in Fig. 6. In the situation of thermal charging in hydrogen gas, the hydrides precipitated into well-developed platelets, visible as "upsets" on the polished surface without etching, as shown in Fig. 7. It is interesting to note that the distribution of the hydrides is uniform throughout the surface of the sample, but locally the hydride platelets were arranged along certain preferential orientations, as shown in Fig. 7. X-ray diffraction patterns were obtained on a diffractometer using Cu K~ radiation. The diffraction data are listed in Table 5. For "Till2" hydride in the Ti-H system, the three strong diffraction lines were, respectively [8, 31], (111) 20 = 35.9 °, I/Io = 50, (200) 20 = 4 1 °, I/Io= 100, and (311) 20 = 70.8 °, I/Io= 35, which was in good agreement with our observations in Table 5 for the present ~2-H system. Other lines also matched the literature data. Therefore, the hydride we have observed in the ~2 phase of Ti-24-11 has the fluorite structure of ~ Till.
71.4 48
78 100 77.7 20
80.3 20
116.3 20
144.5 30
[H]. The partial molar enthalpy for the hydride is in good agreement with the corresponding value in the T i - H system [8, 29], which was from -125.1 kJ/mol H2 to - 142.3 kJ/mol H2. The mean partial molar heat and entropy of solution in the ~2 phase were AH,2 = - 2 1 . 9 7 + 1.4 kJ/mol [H], and AS~2= - 10.25 + 1.80J/K/mol [H], with 95% confidence intervals. These data are believed to represent adequately the ~2 phase, although a minor amount of/~ phase was present. The straight-line appearance of Figs 1 to 3, and the general similarities to behavior both of ~ titanium [8, 28] and of binary Ti3A1 [13], indicate that the behavior we observed was essentially that of the ~2 phase. The very small and discontinuous content of fl does not seem to have had a major effect on the results. That conclusion is also consistent with the relatively small differences in thermodynamic parameters for the ~2 and fl phases [8, 131.
DISCUSSION
Thermodynamic properties It was shown in Fig. 2 that all data of the twophase region formed a straight line, thus, the partial molar heat and entropy of formation for the hydride phase could be calculated from equation (1) and (3). As the calculations were more consistent with equation (1), those results are emphasized here; restilts from equation (3) have been collected in Table 2. The mean values showed AH~ = - 7 0 . 2 9 kJ/mol [H] or - 140.6 kJ/mol H2, with AS~ = - 49.8 J/K/tool
Fig. 6. Hydride marking on the polished surface of the sample after cathodic charging in 1N H2SO4+0.5g/1 NaAsO2 solution with an applied current density of 0.2 A/cm2.
Fig. 7. Hydride platelets precipitated during thermal charging in hydrogen gas; unetched surface viewed with polarized light. (a) CH = 5.8 at.%. (b) CH= 11.2 at.%.
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY The standard partial molar free energies of solution for the :t2 phase and the hydride phase, therefore, are, from equation (1) with 95% confidence intervals: AG°~: = AH°~2 - TAS°~2 = --(21,970 __ 1400) +(10.25 _+ 1.8)T J/mol [H];
(7a)
AG°r = AH°~.- TAS°~ = -(70,290 + 2340) + (49.8 + 3.3)T J/tool [H].
(7b)
Substituting the equation of the terminal solubility, equation (5), and the partial molar free energy of solution for the eq phase, equation (7), into equation (I), the standard partial molar free energy of hydrogen in the a 2 phase in equilibrium with the hydride phase can be obtained, i.e. AG °~2 = -(74,060 +_ 2000) + (87.8 __ 2.1) TJ/mol [H] (8a) and for hydrogen in the hydride phase, using equation (3) and the values of NH(~)/NTi = 1.45 and Ns/NTi = 2, we obtain AG °~ = - 70.29 + 0.084T kJ/mol [H].
(8b)
As would be expected, the AG ° value for hydrogen in the ~t2 phase in equilibrium with the hydride phase, equation (8a), is in very good agreement with the value for the hydride phase, equation (8b). The result in equation (88) is derived from equation (1) and has better statistical support, and thus is preferred.
Overall solubility and terminal solubility Comparing the values of equation (7) for ct2 and phases shows that the difference in partial molar heats of formation of the ct2 and ~ is 48.3 kJ/mol [H], i.e. greater than zero, therefore, based on equation (5), the terminal solubility of hydrogen in the 0~2 phase in equilibrium with hydride should increase with increasing temperature. However, equation (7a) indicates AH~2 = - 2 2 kJ/mol [H], or less than zero, so according to equation (1) or (3), the overall solubility of hydrogen in the ct2 phase should decrease with increasing temperature. Therefore, it can be explained why the terminal solubility can show an endothermic behavior while the overall solubility is exothermic in nature. Since equation (7) contains the thermodynamic parameters for hydrogen in the ~q phase in equilibrium with the hydride phase, we can get the terminal solubility equation for 1 atm pressure from equation (l), for which the result is CH (at.%) = 60 e 39s± 2/R e-48300+ 1200/RT = (22.6 - 37.2) × 103 e -24z°°± 6oo/r
(9)
where R is in units of J/K. Comparing this equation to the experimental results in equation (5), it is obvious that the terminal solubility equation calculated from Sieverts' Law is in very good agreement with the experimental results. Almost equally good agreement
461
with equation (5) is obtained with the statistical treatment, equation (3). Substituting the parameters of equation (8a) into (1), we obtain the equation of overall solubility of hydrogen in the 0t2 phase, with P in Pa, i.e. CH(at.%)=(3.35 _ 0.81) x/~n2 e2650-+150/T (10) The results are in agreement with equation (6). A similar calculation for equation (3) should be used in the situation of large hydrogen concentration, since that equation is more appropriate for the condition of high hydrogen pressure. However, if we extend equation (10) [from either equations (1) or (3)] to 1 atm pressure, the calculated hydrogen concentration is higher than that measured under thermal charging in hydrogen gas of 1 atm. Although we are not sure of the precise reason for this, it might have arisen from the size of the reaction chamber used in the present work (57 mm diameter x 460 mm length), so that the cooling rate of the samples after charging was not very high. Hydrogen loss during cooling might explain why observed hydrogen concentrations after charging at 1 atm pressure were lower than those inferred from thermodynamic parameters measured at sub-atmospheric pressure.
Hydrides We have determined that the hydride phase observed had the fluorite structure of ? Till, although we have not determined to what extent the present hydrides incorporate Nb or A1 [13-16, 32-34]. For binary Ti3AI, i.e. no Nb, there is evidence [13] that at the temperatures of Fig. 4, the hydride is the binary Ti-H compound. We also have not determined the value of the atom ratio of Ti to H, though presumably [6-8, 13, 22, 29] it is a function of hydrogen content; variations in stoichiometry are accommodated in the y structure by hydrogen vacancies [35, 36]. The degree of tetragonality of ~: also varies with hydrogen content [8, 13, 35,37]. The literature on Ti and Ti alloy hydrides consistently refers to the tetragonal structure as f.c.t., to avoid confusion as the f.c.c, symmetry of the 7 becomes tetragonal, but of course the f.c.t. structure is equivalent to b.c.t, and has occasionally been reported as b.c.t. [19, 31]. The posssible roles of AI and Nb in these effects, by analogy [9, 38] with et-Ti, are not known. More work is clearly needed on the hydride phase. CONCLUSIONS 1. For the ~t2-H system, the terminal solubility of hydrogen in the ct2 phase in equilibrium with the hydride phase increases with increasing temperature, while the overall solubility decreases with increasing temperature in the high temperature region. 2. The experimental equations of the terminal and overall solubility in the ~t2-H system are, respectively CH(at.%) = 6.8 x l03 e -575°/r
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
462 and
CH(at.%) = 2.67 X/~a2 e265°/r where T is in K, and the unit of P is Pa. 3. The standard partial molar heats and free energies of solution for the ~2, and the hydride phase are respectively as follows, with 95% confidence limits:
11. 12. 13. 14.
AH,°2 = - (21.97:1: 1.4) LI/mol[H], AH~ = - ( 7 0 . 2 9 _+ 2.3) kJ/mol[H];
and
AG,°2 = - ( 2 1 , 9 7 0 :t: 1400)+(10.25 + 1.8)T J/mol[H]; AG~ =
-
(70,290 + 2300) + (49.8 _+ 3.3)T J/mol[H].
15. 16. 17.
The AG ° values for hydrogen in the ~t2 phase in equilibrium with the hydride phase are AG°~2 = AG°v = -(74,060 + 2000)
18.
+(87.8 + 2.1)T J/tool[H]. 4. The hydride phase at room temperature represents nearly all the hydrogen content of the material, and precipitates with the ~, fluorite structure of titanium hydride. Acknowledgements--We appreciate helpful discussions with I. M. Bernstein and R. J. Fruehan, and are grateful for the sponsorship of the Defense Advanced Research Projects Agency through DARPA Order 6155, monitored by the Air Force Office of Scientific Research under Contract F4962088-C-0013. This is a joint contract of the University of Pittsburgh and Carnegie MeUon University.
19. 20. 21. 22. 23. 24.
REFERENCES
1. H. A. Lipsitt, in High- Temperature Ordered Intermetallic Alloys (MRS Syrup. 39) (Edited by C. C. Koch et al.), pp. 351-364. Mater. Res. Soc., Pittsburgh, Pa (1985). 2. R. L. Fleischer, D. M. Dimiduk and H. A. Lipsitt, Ann. Rev. Mater. Sci. 19, 231 (1989). 3. H. G. Nelson, SAMPE Q 20(1), 20 (1988). 4. T. M. Ronald, in Summary Proceedings of the 2rid Workshop on Hydrogen-Materials Interaction, NASP Joint Program Office Workshop Pub. 1004 (edited by H . G . Nelson), pp. 3-17. NASA-Ames, Moffett Field, Calif. (1988). 5. L. Kirschfeld and A. Sieverts, Z. Phys. Chem. 145A, 227 (1929). 6. A. D. McQuillan, Proc. R. Soc. A 204, 309 (1950). 7. G. A. Lenning, C. M. Craighead and R. I. Jaffee, Trans. Am. Inst. Min. Engrs 200, 367 (1954). 8. W. M. Mueller, in Metal Hydrides (edited by W. M. Mueller, J. P. Blackledge and G. G. Libowitz) pp. 337-383. Academic Press, New York (1968). 9. N. E. Paton and J. C. Williams, in Hydrogen in Metals (edited by I. M. Bernstein and A. W. Thompson), pp. 409-431. Am. Soc. Metals, Metals Park, Ohio (1974). 10. J. C. Williams, in Effect of Hydrogen on Behavior of Materials (edited by A. W. Thompson and I. M.
25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
Bcrnstein), pp. 367-375. Am. Inst. Min. Engrs, New York (1976). H. Numakura and M. Koiwa, Acta metall. 32, 1799 (1984). O. T. Woo, G. C. Weatherly, C. E. Coleman and R. W. Gilbert, Acta metall. 33, 1897 (1985). P. S. Rudman, J. J. Reilly and R. H. WiswaU, J. less-common Metals 58, 231 (1978). A. W. Thompson, W.-Y. Chu and J. C. Williams, in Summary Proc. 2nd Workshop on Hydrogen-Materials Interaction, NASP Joint Program Office Workshop Pub. 1004 (edited by H. G. Nelson), pp. 133-135. NASA-Ames, Moffet Field, Calif. (1988). D. S. Shih, G. K. Scarr and G. E. Wasielewski, Scripta metall. 23, 973 (1989). E. Manor and D. Eliezer, Scripta metall. 23, 1313 (1989). D. Eliezer, E. Manor and F. H. Froes, in Hydrogen Effects on Material Behavior (edited by N. R. Moody and A. W. Thompson), pp. 523-532. T.M.S.-A.I.M.E., Warrendale, Pa (1990). W.-Y. Chu, A. W. Thompson and J. C. Williams, in Hydrogen Effects on Material Behavior (edited by N. R. Moody and A. W. Thompson), pp. 543-553. T.M.S.-A.I.M.E., Warrendale, Pa (1990). M. Gao, J. B. Boodey and R. P. Wei, Scripta metall. mater. 24, 2135 (1990). R. Speiser, in Metal Hydrides (edited by W. M. Mueller, J. P. Blackledge and G. G. Libowitz), pp. 6949. Academic Press, New York (1968). C. M. Schwartz and M. W. Mallett, Trans. Am. Soc, Metals 46, 640 (1954). E. A. Gulbransen and K. F. Andrew, Trans. Am. Inst. Min. Engrs 203, 136 (1955). M. J. Blackburn, D. L. Ruckle and C. E. Bevan, Tech. Rep. No. AFML-TR-78-18, Air Force Materials Laboratory, Wright-Petterson AFB, Ohio (1978). M. J. Blackburn and M. P. Smith, Tech. Rep. No. AFWAL-TR-82-4086, Air Force Wright Aeronautical Laboratory, Wright-Patterson AFB, Ohio (1982). A. Sieverts, Z. Metall. 21, 37 (1929). C. J. Smithells, Gases and Metals, pp. 140-159. Chapman & Hall, London (1937). C. H. P. Lupis, Chemical Thermodynamics of Materials. North-Holland, Amsterdam (1983). A. L. G. Rees, Trans. Faraday Soc. 50, 335 (1954). R. M. Haag and F. J. Shipko, J. Am. Chem. Soc. 78, 5155 (1956). J. P. Blackledge, in Metal Hydrides (edited by W. M. Mueller, J. P. Blackledge and G. G. Libowitz), pp. 151-156. Academic Press, New York (1968). L. D. Jaffe, Trans. Am. Inst. Min. Engrs 206, 861 (1956). A. W. Thompson, in Environmental Effects in Advanced Materials (edited by R. E. Ricker and R. H. Jones), pp. 21-33. T.M.S.-A.I.M.E., Warrendale, Pa (1991). W.-Y. Chu and A. W. Thompson, Metall. Trans. A. In press. W.-Y. Chu and A. W. Thompson, Metall. Trans. 22A, 71 (1991). G. G. Libowitz, Adv. Chem. Serv. 39, 74 (1963). P. E. Irving and C. J. Beevers, Metall. Trans. 2, 613 (1971). G. G. Libowitz, J. nucl. Mater. 2, 1 (1960). N. E. Paton, B. S. Hickman and D. H. Leslie, Metall. Trans. 2, 2791 (1971).
0956-7151/92 $5.00+ 0.00 Copyright © 1992Pergamon Press plc
HYDROGEN SOLUBILITY IN A TITANIUM ALUMINIDE ALLOY WU-YANG CHU?, A, W, T H O M P S O N and J, C. WILLIAMS:~ Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. (Received 25 March 1991; in revised form 5 August 1991)
Abstract--The dissolution of hydrogen in a titanium aluminide alloy based on Ti3A1, Ti-24 AI-11 Nb, has been measured as a function of temperature and hydrogen pressure. Both the terminal solubility and the overall solubility, or total uptake of hydrogen, were determined. The terminal solubility of hydrogen in the ~2 matrix phase increases with temperature, while the overall solubility decreases with increasing temperature in the high-temperature region. The partial molar heat of solution in the ~2 phase was determined to be-(21.97 + 1.46) kJ/mol [HI, while the partial molar heat of formation of the hydride phase was -(70.29 + 2.34) kJ/mol [H]. The standard partial molar free energy of hydrogen in the ~2 phase in equilibrium with the hydride was -(74,060 -+ 2000) + (87.8 _+2.1)T J/tool [H]. R6smn6~La dissolution de l'hydrog6ne dans un alliage d'aluminiure de titane fi base de Ti3A1, Ti-24Al-I INb, 6t6 mesur6e en fonction de la temp6rature et de la pression d'hydrog6ne. On a d6termin6 la lois la solubilit6 terminale et la solubilit6 totale (ou prise totale d'hydrog6ne). La solubilit6 terminale de l'hydrog6ne dans la phase matrice ~t: crott avec la temp6rature, alors que la solubilit6 totale d6croit lorsque la temp6rature augmente dans le domaine des hautes temp6ratures. La chaleur molaire partielle de raise en solution dam la phase % est 6gale ~i -(21,97 -+ 1,46) kJ/mol [H], alors que la chaleur molaire partielle de formation de la phase d'hydrure est 6gale ~ -(70,29 +_2,34) kJ/mol [H]. L'6nergie molaire partielle libre classique pour l'hydrog6ne dans la phase ~t: en 6quilibre avec l'hydrure est 6gale - (74,060 _+2000) + (87,8 -+ 2,1)T J/mol [H]. Znsammenfasstmg--Die L6sung von Wasserstoff in einer Titanaluminid-Legierung auf Basis Ti3AI, Ti-24AI-1 INb, wird in Abh.~ngigkeit der Temperatur und des Wasserstoffdruckes gemessen. Sowohl Grenzl6slichkeit wie auch allgemeine L6slichkeit, oder die Gesamtaufnahme an Wasserstoff, werden bcstimmt. Die Grenzl6slichkeit des Wasserstoffes nimmt in der ~2-Matrixphase mit der Temperatur zu, wohingegen die Gesamtl6slichkeit mit zunehmender Temperatur im Bereich hoher Temperaturen absinkt. Die partiale molare L6sungsw~irme in der ~2-Phase wird zu -(21,97_+ 1,46) kJ/Mol [H] ermittelt, die partiale molare Bildungsw~irme der Hydridphase ist -(70,29 5-2,34) kJ/Mol [H]. Die partiale molare freie Energie des Wasserstoffs in der ~t:-Phase im Gleichgewicht mit dem Hydrid ist -(74,060 _+2) + (87,8 5: 2,1)T J/Mol [HI.
INTRODUCTION Alloys based on the compound Ti3A1 have received sufficient attention to show that they may be legitimate candidate materials of construction for aerospace systems [1-3]. Of particular interest are both the engines and the airframe of a hydrogen-burning propulsion system [3, 4]. Since such alloys contain Ti, with its well-known propensity to interact with hydrogen, it is important to have a clear picture of the effects of hydrogen on the properties and rnicrostructure of these aluminide alloys. Yet since the original interest in this material has been for high performance aircraft engines which typically operate under oxidizing conditions, there has been little effort expended in studying the effects of hydrogen. tPresent address: Department of Materials Physics, University of Science and Technology, Beijing 100083, P.R. China. ~Present address: Engineering Materials Technology Laboratory, General Electric Aircraft Engines, Evendale, OH 45215, U.S.A.
On the basis of pioneering studies [5-7], it is now well established [8-10] that the low temperature phase of pure titanium, ~-Ti, has a relatively low solubility for hydrogen and precipitates a hydride when the solubility limit is reached. This hydride, the y phase in the Ti-H system, was originally designated Till 2 but in more recent literature [ l l , 12] has been designated as Till, a better description of its typical stoichiometry, although the limiting composition of the ), phase stoichiometry is indeed Till2. In the case of the Ti3 A1 compound, which is designated ~2 in the titanium-aluminum system, hydrides are also reported to form [13], and the same is true for alloys based on Ti3Al [14-19]. However, the quantitative description of hydride formation conditions is still largely an open question, and no data on the solubility of hydrogen in this compound are available, other than a preliminary report [14]. For some metals, like Ti and Zr, the maximum amount of hydrogen absorbed increases with pressure and decreases with temperature within a high temperature region [8, 20]. Due to this temperature
455
456
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
behavior, they were classed as exothermic occluders. However, some workers have shown that the terminal solubility of ct-Zr in equilibrium with the hydride phase increased as the temperature rose [21, 22]. Among the first alloys based on Ti3A1 was Ti-24 AI-11 Nb (at.%), developed by Blackburn and coworkers [23, 24]. Our preliminary experiments [14, 18] showed that hydride could form in this alloy, designated "Ti-24-11" below, after either cathodic charging in H2SO4 solution with recombination poisons, or after thermal charging in hydrogen gas, and the maximum amount of hydrogen absorbed decreased with increasing temperature within a high temperature region (e.g. T t> 550°C). Therefore, it was the purpose of this paper to investigate the formation of hydride in this alloy, and measure the overall solubility of hydrogen in the alloy as well as the terminal solubility of hydrogen in the ct2 phase in equilibrium with the hydride phase. We define "overall solubility" as total equilibrium hydrogen uptake, combining hydrogen in solution with that contained in hydrides, at a given temperature, while "terminal solubility" refers to the maximum amount of hydrogen in solution at that temperature.
EXPERIMENTS Analytical basis for experiments According to Sieverts' law [20, 25-27], the overall solubility of hydrogen in either the ct2 phase or the hydride 0'-phase in the Ti-H system) is proportional to the square root of the hydrogen gas pressure in contact with the phase, x/~"~, according to (la)
C H = WN~H2,
with W = e-A~/Rr = e-tatmre as/R
(lb)
since AG = A H - TAS. Here C., AG, AH and AS are, respectively, the hydrogen content, partial molar free energy, partial molar enthalpy of solution, and partial molar entropy of solution in the phase under consideration, e.g. ct2, and R is the gas constant. Note that all the relevant thermodynamic quantities can be obtained from measurements varying any two of CH, PH2, and T, while keeping the third constant, as discussed in more detail below, since [20, 27] AG can be obtained from ½R T In Pn2. The dissolution of hydrogen in metals has been treated using statistical mechanics [22, 28]. It can be assumed that the number and position of Nn hydrogen atoms arranged randomly on Ns interstitial sites are related to the partial molar entropy AS as AS = -- R
NH
N,-N~
and
AG=RTln
NH
Ns-N.
(2a)
and the overall solubility should be Nn
g , - g.
= ~
e -AG/Rr.
(2b)
Although it has not been established directly, it can be assumed [ll-13] that the hydrogen atoms occupy tetrahedral sites on the hexagonal symmetry (D019) Ti3A1 parent lattice. There are four sites of tetrahedral co-ordination per unit cell in the Ti3Al structure, therefore NH
CA
Ns - N . = 2 - CA'
(2c)
so that
CA = x/-~n2 e-~/Rr e~s/R 2--CA
(3)
where CA=NH/Nri~ is the atom ratio. Thus equation (3) is the statistical mechanics or "crystallographic" concentration, while equation (1) represents the thermodynamic concentration. Since equation (3) depends on assumptions about site occupancy, it is potentially less reliable than equation (1). To obtain the terminal solubility of hydrogen in the ~2 structure in equilibrium with the hydride, the condition of equilibrium is that the partial molar free energy of hydrogen is equal in the ct2 phase and the phase. Then from equation (1) the H concentration ratio is
C.(~)
Ca(7) = exp[AS('2)- AS(~))/R] x exp - [(AH(,2)-AH(~))/RT ] (4)
where the phase is referenced for each parameter. Equation (4) then describes the terminal solubility. The terminal solubility could be measured in two ways, which are called the x//P vs C method and the P vs I/T method. Their basis is as follows, from equation (1). The square root of the decomposition (or equilibrium) pressure, x/~n2, at a fixed temperature should change linearly with increase of hydrogen concentration Cn, in a single-phase region. However, due to Gibbs' phase rule, in a two-phase region the decomposition pressure would not change with the hydrogen concentration. Thus, the intercept of the straight-line Cn-~P--~.: relation and the constantpressure plateaus give the terminal solubility. It may also be seen from equation (1) that a plot of the decomposition pressure on a logarithmic scale against 1/T is a straight line, for a fixed hydrogen content. Note from equation (I) that AH can be calculated from the slope of this line. But when a single-phase region goes over into a two-phase region or vice versa, the slope of the straight line will change (e.g. Refs [13, 29]). (Observation of a straight line in the two-phase region, incidentally, indicates [20] that
WU-YANG
et al.: H S O L U B I L I T Y I N A Ti A L U M I N I D E
CHU
only the volume fractions of the two phases, and not the composition of either the solid solution or the hydride, are changing appreciably with temperature; or that the compositions of the two phases are changing in exactly the same way with temperature.) The intersection of two such In PH2 vs lIT straight line segments gives the temperature at which the given hydrogen concentration represents the terminal solubility.
101
T (°C) 500
600
700
I
ALLOY
457
400
I 1 H = 490 wppm(2.3 at %
103
Total H, 46 x 10 -4 g 1 O0
1o
• Heating o Cooling
102
10 -1
101
:P
Experimental procedure
All the present experiments were conducted on the titanium aluminide alloy Ti-24-11, which was obtained in bar form from Reactive Metals (RMI). The chemical compositions of the Ti-24-11 were provided by RMI as follows (wt%): A I = 13.0913.34, Nb = 20.57-20.64, Fe = 0.14, O = 0.09-0.1, N = 0.011-0.016. Blanks with dimensions of 20 mm x 20mm x 40mm were machined, cleaned and solution treated in an argon atmosphere at 1149°C (2100°F) for 2 h and air cooled, then aged at 705°C (1300°F) for 2 h and air cooled. The microstructure after this heat treatment was essentially all ct2 phase. The b.c.c, fl phase could not be detected by X-ray, while examination by transmission electron microscopy indicated 0.1-1% of the fl phase, present only as isolated, discontinuous fl regions. Specimens for hydrogen measurement with dimensions of 20 mm x 20 mm x 1 mm, were sectioned from the bulk samples. The specimens were precharged at different temperature for different times. The weight gain, determined by an analytic balance with a sensitivity of 10 -s g was used to calculate the hydrogen concentration in the specimens, which were in the range of 230wppm (1.1 at.%) to 3720 wppm (15.1 at.%). The total weight of the specimens for each time of charging was about 10 g; therefore, the error calculating the hydrogen concentration was less than 10 wppm. A Sieverts apparatus [20, 25, 26, 29, 30] was used to measure the decomposition pressure in this work. During an experimental run, the specimen tube and the operative pressure devices were isolated from the remainder of the system by means of vacuum valves. The whole apparatus was capable of being evacuated initially to a pressure of about 10 #Pa (1 x 10 - 7 torr). The decomposition pressures over the samples were read on two mercury manometers. One of them was a McLeod gage with a range of 125 Pa (1 torr), and
Q_
,~-
Q.
10-2
1
10 -3
10 - I A
10 -4 .0
I 1.1
I 1.2
I 1.3
I 1.4
I 1.5
I 1.6
I O 0 0 1 T (K - I ) Fig. 1. A n e x a m p l e o f d e c o m p o s i t i o n p r e s s u r e in ~ 2 - H
system vs 1/T, 2.3 at.% hydrogen, measured both on heating and on cooling. the other had a range of 2 kPa (15 torr). For each composition, the number of the samples was changed to keep the total amount of hydrogen about constant, e.g. 3.3 to 4.6 mg of H2 for each constant temperature. Temperature was controlled within + I°C. The decomposition pressure was measured many times until a stable pressure was obtained. To assure that the equilibrium pressures were actually determined, measurements were made on both the decomposition (heating cycle) and absorption (cooling cycle) for some samples. Fully reversible behavior was always observed in such runs. Reversibility was also checked by repeating some measurements on the same sample. After measurement, a sample would be heated in the evacuated Sieverts apparatus to remove all hydrogen (it was verified that sample weight would return to the precharging value within 10/~g), recharged with hydrogen, and additional measurements made. Reproducibility of results in such repetitions was excellent. RESULTS
Terminal solubility of hydrogen in ~2, in equilibrium with hydride, was determined by the
Table 1. Decomposition pressure of =:-H system Composition Wppm
at.%
350
400
450
230 490 680 750 930 1290 2620 3720
1.1 2,3 3.2 3.5 4.3 5.8 11.2 15.1
0.13 0.11 0.13 0.13 0.20 0.13 O.11 0,13
0.93 1.2 1.1 0.93 0.93 1.2 1.3 1.1
2.0 5.6 6.0 6.0 6.4 5.3 6,7 7.3
Pressure at temperature °C (Pa) 500 550 600 2.9 9.3 16 20 28 24 27 25
4,1 13 24 32 43 100 93 113
5.5 19 33 43 64 136 293 266
650 8.3 27 47 63 83 180 640 813
675
700
725
253 800 1290
12 33 61 80 109 293 947 1470
1040 1670
W U - Y A N G C H U et al.:
458
H SOLUBILITY IN A Ti A L U M I N I D E ALLOY
T (°C)
30
700 esosoosso soo 4so 400 10 4 ]_ I
I
[
I
I
I
~ -
I
C H (at. %) v 15.1 • 11.2
1o3 - ' ~ , , ~ "
I
~-
[]
5.8
• A
4.3 3.5
650
3so I
j--" °C
- 101 ~
°C
.
oC
q
• 3.2 ~" 102 n
~. W ~ ~
o
•
2.3
1
1.1
~F
'~" g
I1.
10
-r
I0-I a.
,'-:
101
1
1.0
1.1
1.2
1.3
./~P~=
10.2
500 °C
~O
a
f I
I
I
I
2
4
6
8
0
I
I
•
I
I
I
10 12 14 16 18
C H (at. %) 1.4
1.5
Fig. 3. Square root of the decomposition pressure in the =2-H system vs hydrogen concentration (at.%) at different temperatures,
1.6
IO00/T (K-~) Fig. 2. Summary plot of log P m v s I/T. AB line is for e2 and hydride phases, and the other lines represent atomic percent hydrogen concentration in the e2 phase. Different symbols represent different hydrogen concentrations.
methods described above. Figure 1 shows a typical log decomposition pressure (x/~a2) vs I/T graph obtained for the hydrogen concentration of 2.3 at.% (490wppm) hydrogen. Table 1 summarizes the experimental data for eight hydrogen concentrations. In Fig. 1 the slope change at the phase boundary is clearly evident. Fig. 1 indicates the g2 to be the stable phase over the section BC of the graph, while a mixture of the a2 phase and the hydride phase is stable over the section AB. The point B gave the temperature at which the concentration of 2.3 at.% represented the terminal solubility, which was 447°C. The slope of BC gave a partial molar heat of solution as AH~ = - 20.96 kJ/mol [H] for the g2 phase, while the slope of AB gave a value of AH~ = - 74.89 kJ/ mol [H] for the hydride phase. This calculation assumes the composition of the hydride phase was invariant with temperature, as is implied by the reasonably straight AB lines in Figs 1 and 2. The partial molar heats of solution AH=2, AH~ and the partial molar entropies of solution AS=2, AS~ were
calculated using the data listed in Table 1 and are tabulated in Table 2. From these data, 95% confidence limits on the mean values of the thermodynamic parameters were calculated, using t statistics. The data in Table 2 show that neither AH=2 nor AHr were dependent on the hydrogen concentration. The hydrogen content at the phase boundary of the ~-hydride in Ti [8, 31], which was CHv= 60 at.% or NH(¢)/NTi = 1.45, was used to calculate ASr. Figure 2 shows a plot of all decomposition pressure data for the determination of the terminal solubilities. The line AB on the figure represent the average data of all hydrogen concentrations in the two-phase region. The intersection of the decomposition pressure curve for the a2 phase with the line AB gave the temperature at which the given composition represented the terminal solubility. These values are listed in Table 3. Figure 3 shows a plot of the square root of the decomposition pressure as a function of the hydrogen concentrations. In the ~:-phase region, a set of straight lines passing through the zero point were obtained, and the constant-pressure plateaus correspondext to the region where the two phases, hydrogensaturated =~ phase and hydride, co-existed. The
Table 2. AH and AS of hydrogen in the a2 phase and hydride phase Hydrogen concentration
In "2 phase
AH~
In hydride phase
A$~ (J/C/MoI[HI)
AH~
AS~(J/C/tool[H])
At.%
100C^ 2 - CA
kJ/mol[H]
Equation (1)
Equation (3)
kJ/mol[H]
Equation (1)
Equation (3)
1.1 2.3 3.2 3.5 4.3 5.8 11.2 15.1
0.55 1.16 1.65 1.83 2.25 3.20 6.67 9.77
-21.2 -21.0 -21.1 -21.7 -21.4 -24.1 -25.2 -20.1
-10.63 -8.87 -8.70 -9.75 -9.00 -13.22 -13.81 -7.95
-54.98 -53.14 -52.80 -53.72 -52.97 -56.74 -56.69 -50.08
-68.6 -74.9 -72.0 -71.6 -65.3 -69.9 -70.7 -69.5
-46.4 -56.5 -52.3 -51.9 -42.7 -49.4 -50.2 -48.5
-72.8 -8.28 -78.2 -77.8 -68.6 -75.3 -76.2 -74.9
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
459
Table3.Terminalsolubilityof hydrogenin ~2 Method Temperature(°C) At.% Terminal 100C^ solubilities 2 - CA 'Not exactvalue.
403 1.1 0.55
447 2.3 1.16
InP vs liT 489 506 3.5 4.3 1.83 2.25
478 3.2 1.65
intersection of each straight line with its constantpressure plateau gave the terminal solubility, which is the concentration at which the hydride precipitates. The results are included in Table 3. A plot of vs C^/2-C^ [following equation (3)] was similar to that of ~ / ~ , 2 vs Cn (at.%), shown in Fig. 3, although some of the straight lines did not exactly pass through the zero point, suggesting that equation (1) may be a more suitable description for these data (the site occupancy assumption for equation (3) likely needs refinement). The terminal solubilities determined from equation (3) are listed in Table 3. The variation of the terminal solubility (Ca) with temperature, i.e. the hydride solvus, is shown in Fig. 4, using the equation (1) results. It indicates that the terminal solubility of hydrogen in the ~2 in equilibrium with the hydride increases with increasing temperature. The equation of the terminal solubility as function of temperature, at 1 atm pressure, is CH(at.%) = 6.8 x 103 e -575°/r
(5a)
and CA
2-C^
= 57 e -62°°/r
(5b)
where CA is the atomic ratio and T and the exponential constants are in K. Insertion of room temperature into equation (5) predicts a solubility of hydrogen of about 0.2 ppm (atomic); a spline fit to Fig. 4 and extrapolation to room temperature gives a value of about 0.05 ppm. Though these are only estimates, they do indicate that equilibrium room temperature solubility is probably very small, and virtually all charged hydrogen is precipitated as hydride on cooling to ambient.
700
x/P vs C 560 5.8 3.2
638 11.2 6.67
671 15.1 9.77
500 4.3 2.25
550 5.8 2.9
600 8.6 4.74
650 (12.6)' (8.3)"
Table4. CIx/-fi underdifferenttemperature T(°C)
500
550
600
650
C/~PP
9.37
100~ K / ' - P
5.48
6.98 3.85
5.90 3.28
5.11 3.13
Overall solubility The slopes of C , = Wv/P--~.2 (or CA~2- CA = W q/~H2) straight lines of Fig. 3 under constant temperature were calculated and are presented in Table 4. From these data, we obtain the equation of overall solubility or total uptake of hydrogen in the ~2 phase, i.e. Cn(at. %) = 2.67X/~H2e 2650/T
(6)
where T is in K, and the unit of P is Pa. Figure 5 shows a plot of the overall solubility of hydrogen in the ct2 phase as a function of temperature above 400°C. This figure indicates that the overall solubility of hydrogen in the ~t2 phase decreases with increasing temperature when T t> 500°C. In this sense, the ~2 phase is an exothermic occluder [20]. This conclusion has been verified by the thermal charging under high temperature. For the higher pressure, shown in Fig. 5, the hydrogen pressure was adjusted with time to keep hydrogen pressure at 1 atm.
Identification of the hydride Although description of the hydride phase itself was not a primary goal of this study, a few observations may be of interest in view of apparent disagreements in the literature [15, 16, 19] about hydrides in the ~t2 phase. The hydrides appeared as fine markings or lines on the polished surface of the sample without
-
1
700 -
,3 :,0
oO"600 I-500
600 -
o t ~ "
?
v
500 -
7 400
"x,,,.
v
I-
/I 2
° " ~ V s CH I 4
' 6
i 8
' 10
'
I
'
'
12 14 16 18
C H (at. %)
Fig. 4. Terminal solubility of hydrogen in the "2 phase in equilibrium with the hydride phase, i.e. solvus for hydride in *t2, for Ti-24-1 I.
400 0
I 4
I 8
I 12
I 16
I 20
C H (at. %)
Fig. 5. Overall solubility (total uptake) of hydrogen in the ct2 phase vs I/T, for two pressures shown on the curves in units of atm (or 130Pa and 0.1 MPa).
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
460
Uncharged
deg20 I/~ deg 20
Charged (CHffil3.3 at.%) I/~
35.5 15 35.9 75
36.5 15
Table 5. X-ray diffractiondata 38.6 40.6 41.3 15 35 20 39 40.9 20 100
etching after cathodic charging in H2SO4+0.5 g/l NaAsO2 solution, as shown in Fig. 6. In the situation of thermal charging in hydrogen gas, the hydrides precipitated into well-developed platelets, visible as "upsets" on the polished surface without etching, as shown in Fig. 7. It is interesting to note that the distribution of the hydrides is uniform throughout the surface of the sample, but locally the hydride platelets were arranged along certain preferential orientations, as shown in Fig. 7. X-ray diffraction patterns were obtained on a diffractometer using Cu K~ radiation. The diffraction data are listed in Table 5. For "Till2" hydride in the Ti-H system, the three strong diffraction lines were, respectively [8, 31], (111) 20 = 35.9 °, I/Io = 50, (200) 20 = 4 1 °, I/Io= 100, and (311) 20 = 70.8 °, I/Io= 35, which was in good agreement with our observations in Table 5 for the present ~2-H system. Other lines also matched the literature data. Therefore, the hydride we have observed in the ~2 phase of Ti-24-11 has the fluorite structure of ~ Till.
71.4 48
78 100 77.7 20
80.3 20
116.3 20
144.5 30
[H]. The partial molar enthalpy for the hydride is in good agreement with the corresponding value in the T i - H system [8, 29], which was from -125.1 kJ/mol H2 to - 142.3 kJ/mol H2. The mean partial molar heat and entropy of solution in the ~2 phase were AH,2 = - 2 1 . 9 7 + 1.4 kJ/mol [H], and AS~2= - 10.25 + 1.80J/K/mol [H], with 95% confidence intervals. These data are believed to represent adequately the ~2 phase, although a minor amount of/~ phase was present. The straight-line appearance of Figs 1 to 3, and the general similarities to behavior both of ~ titanium [8, 28] and of binary Ti3A1 [13], indicate that the behavior we observed was essentially that of the ~2 phase. The very small and discontinuous content of fl does not seem to have had a major effect on the results. That conclusion is also consistent with the relatively small differences in thermodynamic parameters for the ~2 and fl phases [8, 131.
DISCUSSION
Thermodynamic properties It was shown in Fig. 2 that all data of the twophase region formed a straight line, thus, the partial molar heat and entropy of formation for the hydride phase could be calculated from equation (1) and (3). As the calculations were more consistent with equation (1), those results are emphasized here; restilts from equation (3) have been collected in Table 2. The mean values showed AH~ = - 7 0 . 2 9 kJ/mol [H] or - 140.6 kJ/mol H2, with AS~ = - 49.8 J/K/tool
Fig. 6. Hydride marking on the polished surface of the sample after cathodic charging in 1N H2SO4+0.5g/1 NaAsO2 solution with an applied current density of 0.2 A/cm2.
Fig. 7. Hydride platelets precipitated during thermal charging in hydrogen gas; unetched surface viewed with polarized light. (a) CH = 5.8 at.%. (b) CH= 11.2 at.%.
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY The standard partial molar free energies of solution for the :t2 phase and the hydride phase, therefore, are, from equation (1) with 95% confidence intervals: AG°~: = AH°~2 - TAS°~2 = --(21,970 __ 1400) +(10.25 _+ 1.8)T J/mol [H];
(7a)
AG°r = AH°~.- TAS°~ = -(70,290 + 2340) + (49.8 + 3.3)T J/tool [H].
(7b)
Substituting the equation of the terminal solubility, equation (5), and the partial molar free energy of solution for the eq phase, equation (7), into equation (I), the standard partial molar free energy of hydrogen in the a 2 phase in equilibrium with the hydride phase can be obtained, i.e. AG °~2 = -(74,060 +_ 2000) + (87.8 __ 2.1) TJ/mol [H] (8a) and for hydrogen in the hydride phase, using equation (3) and the values of NH(~)/NTi = 1.45 and Ns/NTi = 2, we obtain AG °~ = - 70.29 + 0.084T kJ/mol [H].
(8b)
As would be expected, the AG ° value for hydrogen in the ~t2 phase in equilibrium with the hydride phase, equation (8a), is in very good agreement with the value for the hydride phase, equation (8b). The result in equation (88) is derived from equation (1) and has better statistical support, and thus is preferred.
Overall solubility and terminal solubility Comparing the values of equation (7) for ct2 and phases shows that the difference in partial molar heats of formation of the ct2 and ~ is 48.3 kJ/mol [H], i.e. greater than zero, therefore, based on equation (5), the terminal solubility of hydrogen in the 0~2 phase in equilibrium with hydride should increase with increasing temperature. However, equation (7a) indicates AH~2 = - 2 2 kJ/mol [H], or less than zero, so according to equation (1) or (3), the overall solubility of hydrogen in the ct2 phase should decrease with increasing temperature. Therefore, it can be explained why the terminal solubility can show an endothermic behavior while the overall solubility is exothermic in nature. Since equation (7) contains the thermodynamic parameters for hydrogen in the ~q phase in equilibrium with the hydride phase, we can get the terminal solubility equation for 1 atm pressure from equation (l), for which the result is CH (at.%) = 60 e 39s± 2/R e-48300+ 1200/RT = (22.6 - 37.2) × 103 e -24z°°± 6oo/r
(9)
where R is in units of J/K. Comparing this equation to the experimental results in equation (5), it is obvious that the terminal solubility equation calculated from Sieverts' Law is in very good agreement with the experimental results. Almost equally good agreement
461
with equation (5) is obtained with the statistical treatment, equation (3). Substituting the parameters of equation (8a) into (1), we obtain the equation of overall solubility of hydrogen in the 0t2 phase, with P in Pa, i.e. CH(at.%)=(3.35 _ 0.81) x/~n2 e2650-+150/T (10) The results are in agreement with equation (6). A similar calculation for equation (3) should be used in the situation of large hydrogen concentration, since that equation is more appropriate for the condition of high hydrogen pressure. However, if we extend equation (10) [from either equations (1) or (3)] to 1 atm pressure, the calculated hydrogen concentration is higher than that measured under thermal charging in hydrogen gas of 1 atm. Although we are not sure of the precise reason for this, it might have arisen from the size of the reaction chamber used in the present work (57 mm diameter x 460 mm length), so that the cooling rate of the samples after charging was not very high. Hydrogen loss during cooling might explain why observed hydrogen concentrations after charging at 1 atm pressure were lower than those inferred from thermodynamic parameters measured at sub-atmospheric pressure.
Hydrides We have determined that the hydride phase observed had the fluorite structure of ? Till, although we have not determined to what extent the present hydrides incorporate Nb or A1 [13-16, 32-34]. For binary Ti3AI, i.e. no Nb, there is evidence [13] that at the temperatures of Fig. 4, the hydride is the binary Ti-H compound. We also have not determined the value of the atom ratio of Ti to H, though presumably [6-8, 13, 22, 29] it is a function of hydrogen content; variations in stoichiometry are accommodated in the y structure by hydrogen vacancies [35, 36]. The degree of tetragonality of ~: also varies with hydrogen content [8, 13, 35,37]. The literature on Ti and Ti alloy hydrides consistently refers to the tetragonal structure as f.c.t., to avoid confusion as the f.c.c, symmetry of the 7 becomes tetragonal, but of course the f.c.t. structure is equivalent to b.c.t, and has occasionally been reported as b.c.t. [19, 31]. The posssible roles of AI and Nb in these effects, by analogy [9, 38] with et-Ti, are not known. More work is clearly needed on the hydride phase. CONCLUSIONS 1. For the ~t2-H system, the terminal solubility of hydrogen in the ct2 phase in equilibrium with the hydride phase increases with increasing temperature, while the overall solubility decreases with increasing temperature in the high temperature region. 2. The experimental equations of the terminal and overall solubility in the ~t2-H system are, respectively CH(at.%) = 6.8 x l03 e -575°/r
WU-YANG CHU et al.: H SOLUBILITY IN A Ti ALUMINIDE ALLOY
462 and
CH(at.%) = 2.67 X/~a2 e265°/r where T is in K, and the unit of P is Pa. 3. The standard partial molar heats and free energies of solution for the ~2, and the hydride phase are respectively as follows, with 95% confidence limits:
11. 12. 13. 14.
AH,°2 = - (21.97:1: 1.4) LI/mol[H], AH~ = - ( 7 0 . 2 9 _+ 2.3) kJ/mol[H];
and
AG,°2 = - ( 2 1 , 9 7 0 :t: 1400)+(10.25 + 1.8)T J/mol[H]; AG~ =
-
(70,290 + 2300) + (49.8 _+ 3.3)T J/mol[H].
15. 16. 17.
The AG ° values for hydrogen in the ~t2 phase in equilibrium with the hydride phase are AG°~2 = AG°v = -(74,060 + 2000)
18.
+(87.8 + 2.1)T J/tool[H]. 4. The hydride phase at room temperature represents nearly all the hydrogen content of the material, and precipitates with the ~, fluorite structure of titanium hydride. Acknowledgements--We appreciate helpful discussions with I. M. Bernstein and R. J. Fruehan, and are grateful for the sponsorship of the Defense Advanced Research Projects Agency through DARPA Order 6155, monitored by the Air Force Office of Scientific Research under Contract F4962088-C-0013. This is a joint contract of the University of Pittsburgh and Carnegie MeUon University.
19. 20. 21. 22. 23. 24.
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