Investigation of solid solubility and metastable phase in dilute Th-Co alloys

Jorcrnal of the Less-Common

121

Metals, 157 (1990) 121 - 132

INVESTIGATION OF SOLID SOLUBILITY AND METASTABLE PHASE IN DILUTE Th-Co ALLOYS S. C. AXTELL South Dakota SchooE of Mines and Technology,

Rupid City, SD 57701

(U.S.A.)

A. BEVOLO and 0. N. CARLSON Ames Laboratory-DOE,

Iowa State Uniue~sity~ Ames, IA 50011

(U.S.A.)

(Received May 4,1989)

summary

The solubility of cobalt in (Y thorium is described by the equation c,(at.% Co) = 10970 exp(-13844/Z’). The heat of solution is -115.2 kJ mol-’ Co and the terminal solubility at the eutectic temperature 1100 “C is 0.46 at.% Co. A newly identified metastable phase with a plate morphology is present in thorium-rich alloys on quenching from the high (oTh) region. Upon aging for different times at 500 “C! and above, this phase transforms to rod-like particles of Th,Co3.

1. Introduction The Th-Co phase diagram proposed by Thomson [l] consists of five congruently melting compounds separated by eutectic reactions and negligible terminal solid solubility of cobalt in thorium, as is seen from Fig. 1. Our interest in the fast diffusion behavior of cobalt in f.c.c. thorium, as reported by Weins and Carlson [Z], required a more precise knowledge of the limits of solid solubility and, specifically, the amount of cobalt that can be retained in solution upon quenching. During the course of the investigation a new metastable phase appeared upon rapid cooling. Attempts to identify and characterize this phase are described in this paper.

2. Experimental procedure The thorium metal used in this investigation was obtained from the Materials Preparation Center of Ames Laboratory. Chemical analysis showed it to contain the following major imp~ti~s; 350 ppma oxygen, 350 ppma carbon, 5 ppma tantalum, 7 ppma rhenium, 4 ppma tungsten, 10 ppma @ Elsevier Sequoia/Printed

in The Netherlands

f3ooJIO’ 0

’

20

’

iw

’

40

’

50

’

60

I’I’I’ 80

70

I

90

loo

At.% Co

Fig. 1. Tb-Co phase diagram proposed by Thomson [ 11.

silicon, 5 ppma molybdenum and 0.04 ppma cobalt. Master alloys containing 0.24 and 0.56 at.% Co were prepared by arc-melting small amounts of cobalt and thorium together in an argon atmosphere. Alloys containing 0.004, 0.006, 0.016, 0.05, 0.14 and 0.40 at.% Co were prepared by diluting portions of the master alloys with pure thorium in a subsequent arc-melting step. The cobalt concentration of each of the alloys was determined by atomic absorption. Portions of the 0.05, 0.14, 0.24, 0.40 and 0.56 at.% Co alloys were swaged into rods 0.4 cm in diameter from which thin discs were cut for use in the solvus determination. The discs were equilibrated at various temperatures between 840 and 1300 “C and quenched in oil. The specimens used in the aging studies were wires 0.1 cm in diameter that were held at 1300 “C for 1 h under a pressure of 7 X lo-’ Pa prior to quenching. Heating was by internal electrical resistance and quenching was achieved by aviating the power to the sample while s~uIt~eo~ly directing a stream of liquid-ni~ogen~ooled helium gas at the sample. It is estimated that the quench rate was about 800 “C s-l. Aging experiments were performed on the Th-O.l4at.%Co wire specimens following helium quenching. The wire was cut into short segments which were wrapped individually in tantalum foil and heated in a vacuum furnace. Aging treatments were carried out at temperatures ranging from 500 to 800 “C for periods of 0.5 to 90 h. The specimens were examined by optical microscopy, scanning electron microscopy (SEM) and Auger analysis. A portion of the Th-0.40at.%Co alloy was rolled into a thin strip and this was heated by internal resistance to 1150 “C followed by quenching with helium. A section of the quenched foil was thinned by a combination of jet polishing and ion milling for examination in a JEOL 1OOCX electron microscope.

123

onePhase o Two Phases 0

1400 I3500

600’

0

0

’ 0.1

0

’ 0.2

‘\

‘\

’ 0.3

*

‘.

’ 0.4

’ OS

I

LtbxTh)

’ 0.6

’ 0.7

’ 0.6

’ 0.9

At.%Go

Fig. 2. Thorium-rich

solvus from microstructures

of quenched

alloys.

3. Results The results of metallographic examination of the various quenched alloys are summarized in Fig. 2. The solubility of cobalt in thorium increases from 0.05 at.% at 850 “C to 0.40 at.% at 1075 “C. The solvus was determined by qu~titative metallo~aphic analysis of specimens, with cobalt concentrations from 0.05 to 0.56 at.%, equilibrated and quenched from various temperatures in the two-phase region. The volume per cent of Th7C03 that was present prior to quenching was determined and the solvus concentration c, at the quench temperature was estimated by use of the lever law. The equation used in this estimation is c,(at.% Co) =

Wrh,oo,)(Xrh,c0,) - 100X*) V mco, - 100

(1)

where V is the vol.% and X is the at.% of cobalt in Th,Cos and in the ahoy. The micrographs in Fig. 3 illustrate the difference between the microstructures of a Th-0_24at.~Co alloy that was quenched from the one-phase region and the 0.56 at.% Co alloy from the two-phase region and the same temperature. Note that a plate-like phase (identified here as ThCo,) is observed in the matrix of both micrographs, but large spherical particles are present only in the 0.56 at.% Co alloy (Fig. 3(b)). The spherical particles are Th,Cos that was present prior to quenching whereas the ThCo, phase is a post-quench precipitate. A summary of the solubility data determined by quantitative metallographic analyses is given in Table 1. The solvus points at 1035, 1060 and 1080 “C are the duplicate values for c, calculated from eqn. (1) for the Th-0.24 and Th-0.56at.%Co alloy specimens. At 955 and 840 “C the values for c, were determined solely from the Th-0.14 and ~-0~05at.~~o alloys respectively. As may be seen from Fig. 4, the latter two alloys were quenched

124

(a)

@I

Fig. 3. Optical micrographs of Th-Co alloys after quenching from 1025 “C: (a) Th0.24at.%Co interpreted as one phase at temperature; (b) Th-0.56at.%Co containing spherical particles of Th,C!o3. The plate-like phase is post-quenching metastable ThCo,. Electropolished. 750 x DIC.

TABLE 1 Volume per cent of Th&os and estimated solute concentration quenched from various temperatures in the (rYl’b) + Th&os region

of solvus for alloys

Temperature W)

Alloy composition (at.% Co)

Vol. % Th,Co3

&.% Co)

1080 1080

0.40 0.56

0.08 0.60

0.376 0.381

1060 1060

0.40 0.56

0.14 0.70

0.358 0.352

1035 1035

0.40 0.56

0.42 0.90

0.275 0.293

950

0.14

0.04

0.128

840

0.05

0.02

0.044

from temperatures close to the solvus, as is indicated by the virtual absence of Th,Co3 particles. A linear plot of In c, us. reciprocal temperature for the data given in Table 1 is shown in Fig. 5. The equation that provides the best fit to the data as determined by a linear least-squares analysis is c, (at.% Co) = 10970 exp

-13844

T

(2)

(b)

(a)

Fig. 4. Optical micrographs of (a) Th-O.OSat.%Co at.%Co quenched from 950 “C. Very small amounts matrix. Electropoiished. 187 X DIC.

Fig. 5. Plot of In cs us. reciprocal

temperature

quenched of Th&o3

for cobalt

from 840 “C; (b) Th-0.14can be seen in the thorium

in thorium.

and the heat of solution of cobalt in LYthorium is -115.1 kJ mol”If Extrapolation of this equation to the eutectic temperature gives a terminal solubility of 0.46 at.% at 1100 “CT.

4. Metastable phase

A series of Jfoys containing from 0.004 to 0.47 at.% Co were equilibrated and quenched from 1300 “CLComp~i~n of Figs. 6(a) and 6(b) shows that the 0.004 at.% Co alloy, like pure thorium in the quenched condition, is

127

Fig. 8. Transmission electron micrograph of Th-0.40at.%Co foil quenched from 1150 “C. showing a plate-like precipitate plus unidentified equiaxed particles in the thorium matrix. 7500 X. Fig. 9. Selected area electron diffraction pattern of region of Tt-0.40at.%Co shown in Fig. 8. This shows small satellite spots adjacent to each thorium matrix reflection. Zone axis = [OOl].

In an effort to determine the crystal structure and lattice parameter of this phase, a Debye-Scherrer pattern was taken of a wire specimen of the Th-O.l4at.%Co alloy following a helium quench. As is seen from the optical micrograph of this alloy (Fig. 7(b)), there appears to be a sufficient amount of the precipitate to give a diffraction pattern. No diffraction lines that could be identified with this phase were observed, however. A possible explanation for this will be offered later. The results of a transmission electron microscopy (TEM) study of the quenched Th-0.4Oat.%Co alloy provide some indication of the structure of the metastable phase. A selected area diffraction pattern was taken of the same region of the alloy shown in Fig. 8. It will be noted from the diffraction pattern of Fig. 9 that satellite spots appear adjacent to each reflection. These spots lie closer to the transmitted beam than, and have the same symmetry as, the thorium reflections. Assuming that the extra reflections are associated with the ThCo, phase, it appears to have the same f.c.c. structure as the thorium matrix with a lattice parameter estimated to be 0.56 nm which is about 10% larger than that of CYthorium. 4.2. Stoichiometry Three different techniques were used in an attempt to determine the cobalt content of the precipitate phase. Quantitative metallographic analyses of optical micrographs of the 0.016, 0.05, 0.14 and 0.24 at.% Co alloys using the lever law with c, = 0.004 gave a cobalt concentration of about 0.6 at.% (see Table 2). This concentration corresponds to a stoichiometric composition of approximately ThCo,,oos, which is rather unrealistic. A similar analysis

128 TABLE 2 Volume per cent of plate precipitates and corresponding cobalt concentration from micrographs of quenched ThCo alloys

estimated

Micrograph

Alloy (at.% Co)

Plate precipitates (vol.%)

Co in plate precipitates (at.%)

Optical Optical Optical Optical

0.016 0.05 0.14 0.24

2 8 30 35

0.6 0.6 0.5 0.7

SEM TEM

0.14 0.40

Av. = 0.6

(a)

1.7 5

8.0 8.0

(b)

Fig. 10. Scanning electron micrograph of Th-O.l4at.%Co alloy containing (a) plates of metastable ThCo, after aging at 510 “C for 5 h; (b) rods of Th~Coa after agingat 700 “C for 0.5 h.

of the 0.40 at.% Co alloy from the TEM shown in Fig. 8 and scanning electron micrograph of the 0.14 at.% Co alloy, Fig. IO(a), gives a cobalt concentration of about 8 at.%. This corresponds to a stoichiometry of about ThCoo,os > which is more reasonable and is consistent with the lattice expansion reported above. The low cobalt composition of this phase estimated from the optical micro~aphs is apparently the result of electropol~hing, which greatly magnifies the width of the plates at the surface (see Fig. 7). This would give an unreasonably large volume fraction, whereas the projection of the precipitates observed in the TEM and SEM images should correspond more closely to the actual volume. Additional efforts were made to determine the cobalt concentration of this phase by energy dispersive X-ray (EDX) and Auger microanalyses. A cobalt concentration of 8 at.% should be detectable by either method but no cobalt peak was observed in either case, The reason that the EDX tech-

129

nique did not detect any cobalt (if indeed it is that high) could be because of beam spreading, while the failure of the Auger analysis to detect any cobalt could be due to preferential ion beam sputtering during alloy surface cleaning. 4.3. Time- temperature- transformation studies In an effort to confirm the metastable nature of the plate-like precipitate observed in the quenched specimens, a series of aging experiments was performed on the Th-O.l4at.%Co alloy. The results of these experiments are summarized in Fig. 11. As is seen from the figure the transformation from plates to rods is time and tempemt~e dependent. The plate and rod-like nature of this phase is readily apparent from a comparison of the scanning electron micrographs of the untransformed and transformed Th-O.l$at.%Co alloy in Figs. 10(a) and 10(b).

‘.

Time, h

Fig. 11. Time-temperature rods of Th7C03.

diagram for transformation

of plate precipitates of ThCo, to

A series of optical micrographs showing different stages of the transformation is shown in Fig. 12. In Fig. 12(a) there is no evidence of rod-like particles after 0.5 h at 500 “C but after 0.5 h at 600 “C both rods and plates are observed (Fig. 12(b)) and after aging at 700 “C for 0.5 h the transformation to a rod mo~holo~ is complete, as may be seen from Fig. 12(c). It will be noted that the volume fraction of the rods is considerably less than that of the plates. The time-temperature-transformation (TTT) diagram of Fig. 11 shows that the rod-like phase is stable after aging for 90 h at 500 “C and after 5 h at 600 “C. This suggests that it is the equilibrium phase Th,C!03. In order to confirm this, an EDX microanalysis was performed on one of these particles, a pattern of which is reproduced in Fig. 13. The results of this analysis indicate that the cobalt concentration is approximately 30 at.%, corresponding closely to that of Th,CoJ. In addition to the plate-like precipitates, spherical particles are visible along the gram boundary in Fig. 12(a) with some depletion of the plates in the vicinity of the boundary. These particles were identified as the equilibrium

(a)

Fig. 12. Optical micrographs of Th-O.l4at.%Co quenched from 1300 “C: (a) aged at 500 “C for 0.5 h showing untransformed plates in thorium grains; (b) aged at 600 “C! for 0.5 h showing both plates and rods; (c) aged at 700 “C for 0.5 h showing Th,Co3 rods only in the thorium matrix. Electropolished. 750 x DIC.

Th,Co3 phase which is more readily nucleated in the gram boundaries. The small spherical particles visible within the thorium grams in Fig. 12(c) are probably ThOz, since they are also present in the furnace-cooled thorium base metal. 5. Discussion The results of this investigation suggest that the formation of the metastable phase involves long-range diffusion of cobalt atoms. The precipitate-

131

E(KcV)

Fig. 13. EDX spectrum of a rod particle in Th-O.l4at.%Co aged at 700 “C for 0.5 h.

quenched from 1300 “C and

free regions along the grain boundaries of the quenched specimens and the presence of Th,Cos precipitates at the grain boundaries (see Figs. 3 and 7) indicate that, even during the helium quench, there is sufficient time for the cobalt atoms to diffuse to the grain boundaries. It will be noted from the figures that the width of the precipitate-free zone (PFZ) is about the same as the spacing between the plates. The presence of a PFZ along the grain boundaries also suggests that a critical degree of supersaturation is required for the metastable phase to form. The microstructures of the quenched alloys place that limit between 0.004 and 0.016 at.% Co. This is supported by internal friction measurements that show the maximum amount of cobalt retained in solution upon quenching to be approximately 0.004 at.% Co [3]. From the experimental evidence it is concluded that the me&stable phase forms and transforms by a nucleation and growth process. The fact that this phase rather than the equilibrium phase forms on quenching indicates that the nucleation barrier for formation of the plates is less than for the hexagonal Th,Cos phase. A similarity between the crystal structures and lattice parameters of the ThCo, phase and the thorium matrix would support this conclusion, since these conditions would favor the formation of low energy semicoherent boundaries between the plates and the thorium matrix.

6. Conclusions The solid solubility of cobalt in (Y thorium increases from 0.05 at.% at 850 “C to 0.40 at.% at 1060 “C and is described by the relationship c, (at.% Co) = 10970 exp(-13844/T) and the heat of solution is -115.1 kJ mol-’ Co. A me&&able phase having a plate morphology and an estimated stoichiometry of ThCooaos forms in dilute Th-Co alloys upon rapid quenching from the (oTh) phase field. This phase transforms to the equilibrium phase Th,Cos on aging at temperatures between 700 and 500 “C for times ranging from 0.5 to 90 h.

132

Acknowledgments The authors gratefully acknowledge the technical assistance of Dr. A. R. Pelton, H. Baker, F. Laabs, R. Hofer and L. Lincoln in this investigation. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract No. W-7405-ENG-82.

References 1 J. R. Thomson, J. Less-Common Met., 10 (1966) 432. 2 W. N. Weins and 0. N. Carlson, J. Less-Common Met., 66 (1979) 99. 3 S. C. Axtell, Ph.D. Dissertation, Iowa State University, Ames, IA, 1988.