Journal of the Less-Common Metals, 30 (1973) 205-209 K Elsevier Sequoia S.A.. Lausanne - Printed in The Netherlands
J. D. BORNAND*.
IN THE MOLYBDENUM-GALLIUM
R. E. SIEMENS,
L. L. ODEN
Albany Metallurgy Research Center, Bureau of Mines.
U.S. Departmetlt q/’ the Iutrrior. Albun.v,
June 23, 1972)
Diffusion couples and alloys in the molybdenum-gallium system wereexamined by thermal analysis, X-ray diffraction, electron microprobe and metallography. Two intermetallic compounds were identified and characterized as follows : 1820f 20” C (1) Mo,Ga peritectic 835 f s”C. (2) MoGa, peritectic The solubility of Ga in MO increased from 1.79 at.% Ga at 1275°C to 10.34 at.“,, Ga at 1700°C.
Very little information has been published on the phase relations in the Mo-Ga system. Wood et al.’ determined the structure of the first compound, Mo,Ga. and Matthias et al.’ reported the superconducting transition temperature of a nominal composition corresponding to MoGa,. The compound they studied was probably the one we identify as MoGa,. We found no evidence to support the existence of another compound, MoGaa, for which only the formula and superconducting transition temperature have appeared in the literature3*4. EXPERIMENTAL
Diffusion couple preparation and interpretation
The diffusion couples, prepared from high-purity, pressed, and sintered MO billets (99.9%: MO) and Ga of 99.99% purity, were formed in a manner previously described’ by sealing gallium within molybdenum cylinders by arc welding. The cylinders were then heated at 835” to 950°C for periods up to 4 weeks, after which they were sectioned transversely, mounted in plastic epoxide resin, and examined by metallography, electron microprobe, and X-ray diffraction. The composition of phases and the solubility of Ga in MO, as presented in Table 1, were determined with the electron microprobe by recording gallium Kc * land.
fellow at AMRC,
1180 Rolle. Switzer-
J. D. BORNAND,
R. E. SIEMENS,
L. L. ODEN
Phases present (at.% Ga)
Diffusion temp. (“C)
1OOh 1 week 4 weeks 4 weeks 2 weeks 2 weeks 2 weeks 258 h 1OOh 120h 30 h 46 h 314 h
1400 600 835 945 1060 1100 1275 1300 1400 1530 1600 1700 1950 X phase identified
24.7-25.4 25.C-25.6 24.7 25.7 X 24.3-26.8 24.6 24.8-26.9 24.9 24.7725.8 X
84.0 X X 83.0 X X 83.4
Ga solubility in MO (at.:,,)
1.79 1.86 2.33 5.96 7.16 10.34
but not analyzed.
and molybdenum La intensities and comparing them with pure standard materials. The solubility of Ga in MO was found to decrease from 10.34 at.% at 17OO’C to 1.79 at.% at 1275°C. The logarithm ofthe solubilitywhenplotted us. reciprocal temperature yields the Sieverts Law plot shown in Fig. 1. The curve can be extrapolated with confidence to obtain solubilities both above and below the experimental temperature interval.
I 0 5
Fig. 1. Solubility
I 06 1,000/T,
Fig. 2. Molybdenum-gallium
I 07 OK
in molybdenum. phase diagram.
To aid in identifying the phases by metallography, the specimens were anodized in a solution similar to that described by Keil and Salomon’. The solution used in this study contained 19 g of Na,B,O,. iOH, per liter of glacial acetic acid. Prior to anodization, the solution was heated to 60°C and just enough water was added to dissolve all the salt (31+ 1.5 ml of water per liter of acetic acid). A potential of 50 V was applied during anodization for the time necessary to produce a red-pink color on the molybdenum (1545 s). Under these conditions Mo,Ga appeared yellow or light-brown, and MoGa, was grey-blue.
Fig. 3. Diffusion couple held $ h at 1950°C. The phases are, from left to right: MO, Mo-Ga solid solution (A). Mo,Ga (B), and MoGa, (C). (x250)
Two diffusion couples were chosen to illustrate the phase relations in the MoGa system, as shown by the phase diagram on Fig. 2. Figure 3 shows a diffusion couple annealed above [email protected]
(the formation isotherm of the first compound). There is no indication in Fig. 3 of stable diffusion phases other than molybdenum solid solution (phase A) at the annealing temperature. Additional dendrites of solid solution formed on cooling, thereby proving that the annealing temperature exceeded the formation isotherm of Mo,Ga. Immediately surrounding the solid solution is a narrow band of Mo,Ga (phase B) which formed on cooling, apparently by peritectic reaction of solid solution and liquid. Additional small islands of Mo,Ga formed directly from the liquid on further cooling as the liquid sought to maintain equilibrium. The remainder of the liquid solidified as MoGa, (phase C) at temperatures equal to or below the formation isotherm of MoGa,. This phase was the last to solidify, as evidenced by the fact that it contains all of the observed porosity. Diffusion couples annealed at temperat~ires between 1820°C and 835°C had
J. D. BORNAND, R. E. SIEMENS, L. L. ODE?:
Fig. 4. Diffusion couple held 46 hat 1700°C. The phases are, from left to right : MO, Mo,Ga (B), and MoGa, (C).(x 150)
the appearance of Fig. 4. Mo,Ga (phase B) existed as a discrete diffusion phase adjacent to MO at temperature, and it also formed from the liquid, on cooling, as large globular particles. Surrounding the Mo,Ga is a continuous region of MoGa, (phase C) that contains all of the observed porosity. Peritectic formation of MoGa, is indicated by the smooth interface between Mo,Ga and MoGa,. Alloy preparation
Alloys could not be prepared ‘by arc-melting but were successfully formed by diffusing MO powder and Ga at high temperature. The powder and liquid Ga were premixed in the desired proportions and then sealed within MO cylinders by arcwelding under helium. The sealed capsules were heated for periods up to a week at temperatures to 13OO”C,and the resulting alloys were ground to fines and used for thermal analysis and X-ray diffraction. Thermal analysis
Thermal analyses, both time-temperature plots and differential thermal analysis, were performed on small (0.2 g) samples of compacted powders that were sealed within MO cylinders 1.6 cm long by 0.33 cm o.d. with 0.06 cm walls. Thermocouples of W5ReW26Re were spot welded onto the outside of the cylinders. Heating and cooling rates of 4-25°C per min were used to locate both isotherms in the system. The formation isotherms for Mo,Ga and MoGa, were detected at [email protected]
and 835 f 5”C, respectively, where the possible error allows for thermal gradients within the furnace, hysteresis on heating and cooling, and possible error in thermocouple
calibration. A reproducible thermal indication that was attributed to the liquids occurred at 1895°C for an alloy corresponding in composition to Mo,Ga. X-ray results and identification of MoGa,
X-ray diffraction analysis of the powder samples using CuKa radiation and a Ni filter confirmed the presence of two compounds. The first, Mo,Ga, with a lattice parameter of 4.944+0.0015 I%,had the beta tungsten structure and was identical to that reported by Wood et al.’ who listed the parameter as 4.943 +0.002 A. The second compound has not been reported in the literature. The formula, as determined by microprobe, is MoGa,, and the observed X-ray pattern is given in Table II. TABLE
4.01 2.87 2.83 2.65 2.62 2.57 2.55 2.52 2.51 2.322 2.310
2.282 2.220 2.159 2.130 2.008 1.990 1.960 1.830 1.689 1.638 1.512
49 71 50 39 100
1.560 1.538 1.510 1.436 1.425 1.415 1.353 1.313 1.280 1.269 1.258
60 32 23 9 17 13 25 27 77 71
11 8 4 16 3
4 7 5 5 5 5 5 17 11 8 7
The authors are very thankful for assistance from the following personnel of the Albany Metallurgy Research Center : P. A. Romans of the Physics Laboratory, who performed the microprobe evaluations, R. A. McCune of the Diffraction Laboratory, and C. W. Antrim for assistance in metallography. REFERENCES 1 E. A. Wood, V. B. Compton, B. T. Matthias and E. Corenzwit, Acta Cryst.. 11 (1958) 604. 2 B. T. Matthias. V. B. Compton and E. Corenzwit. J. Phys. Chem. Solids, 19 (1961) 130. 3 B. T. Matthias, in C. J. Gorter (ed.), Proyress in Low Temperature Physics, Vol. II, North-Holland Publ. Co.. Amsterdam, 1957, p. 146. 4 B. W. Roberts. in K. Mendelssohn (ed.), Progress in Cryogenics, Vol. 4. Academic Press. New York. 1964, p. 185. 5 L. L. Oden and R. E. Siemens, J. Less-Common Metals. 14 (1968) 33. 6 R. G. Keil and R. E. Salomon. J. Electrochem. Sot.. 115 (1968) 628.