X-Ray Analysis of Chromium-Molybdenum and' Chrow ium-Tungsten Alloys W. TRZEBIATOWSKI, H. PLOSZEK, ASD J. LOBZOWSKI Institute of Inorganic Chemistry, University and Polytechnic Institute, Wroclaw, Poland (Silesia)
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Chromium-molj bdenum and chromium-tungsten alloys have been examined by means of x-ray analysis. Chromium and molybdenum formed an unlimited solid sylution at all temperatures which w-ere applied. The melting points of a series of alloys were determined and a minimum at 15 atomic q ' molybdenum and 1700" C. was found. Alloys of chromium with tungsten behave differently. The existence of an unlimited solid solution has been proved at a temperature of 1700"; however, they show two limited solid solutions at 1430' and the solubility of the latter diminishes with lowering of temperature. RIutual solubilities ha5 e been determined also at 1200", 1000°, and 600' C. In comparison with Vegard's law, solid solutions in both cases show an expansion of the lattice constants.
tungsten alloys rvere prepared from the above-mentioned materials by weighing mixtures containing 10 to 90 atomic % of each. These mixtures were pressed into a cylindrical die under pressure of 2000 to 4000 atmospheres t o form small pieces 10 mm. in diameter and in height (IO). The chromium-molybdenum mixtures were also pressed into small bars for determining their melting points. Preliminary sintering was applied during 2 hours at 1000' or 1100' C. in a hydrogen atmosphere. Annealing. The samples designed for x-ray analysis were subjected to further annealing during 5 to 8 hours a t 1430" in a Tammann furnace; they were placed in a corundum tube through which a current of pure hydrogen was conducted. The samples cooled in the furnace after cutting off the electric heating current. The samples, after having been alloyed in this way, were cut into several sections and submitted to annealing a t different temperatures and times as listed in Table I in order to obtain defined states of equilibrium. The samples were placed in small silica tubes, evacuated, and sealed off. These tubes were placed in a platinum resistance furnace, the temperatures of which were maintained constant within *loo. At 1700' C. it was necessary to heat the samples in the Tammann furnace in a corundum tube through which a purified hydrogen current was passed.
LLOYS of chromium and molybdenum and of chromium and tungsten have been relatively rarely investigated on account of the numerous experimental difficulties involved. In prior work exceedingly pure components were not used in alloy preparation, so that as a matter of course impurities such as carbon or aluminum affected the results. Sargent (5) first stated that chromium, molybdenum, and tungsten, obtained from their oxides by carbon reduction, easily formed alloys. Subsequently the binary system chromium-molybdenum was systematically investigated by Siedschlag ( 7 ) , who with the aid of thermal and microscopic analysis found a eutectic mixture a t 1460' C. containing 22.7% molybdenum. His experimental technique as well as the use of impure chromium made these results very doubtful. Subsequently this system was subjected to x-ray analysis by the a-rikrs as well as by Schneider and Kubaschewski (6). Results of both investigations contradicted Siedschlag's point of view. The binary system chromium-tungsten was investigated by Isida, Asada, and Higasimura (4) by means of thermal, microscopic, and x-ray analysis. The alloys were prepared aluminothermically, which caused the content of aluminum to reach 6% at times. As a result of their investigations these authors concluded that the eutectic occurs a t 35y0 tungsten. There exist two limited solid solutions, one containing about 32% tungsten i n chromium, and the other 26% chromium in tungsten. Schneider and Kubascheivski (6) obtained different results by means of x-ray and microscopic analysis. They reported the existence of unlimited solid solutions in the alloys containing more than 5070 chromium prepared by melting or by sintering of the components in the hydrogen atmosphere of the Tammann furnace.
Determination of Melting Points in the Chromium-Molybdenum System. CHEXICAL ANALYSIS.For this purpose samples in form of bars additionally sintered in a corundum tube and hydrogen atmosphere a t 1200"were used. The samples were melted in an apparatus resembling that of Agte ( I ) , but improved by the writers. The gradually increased electric current passed directly through the small bars 50 mm. in length and 3 X 2 mm. in cross section. The sample was placed in two massive molybdenum braces, one of which was connected with a copper spring in order t o accommodate the expansion or shrinkage of the sample during the heating. A hardglass bulb 22 cm. in diameter was pressed by means of a flange and a ring of rubber to the brass support cooled by water. The bulb contained the sample in an atmo3phere of pure hydrogen under a pressure of 180 to 200 mm. of mercury. A small hole in the sample --as observed by a calibrated micropyrometer which indicated the true temperature. Because of the high vapor pressure of chromium, it v,-as neces-
EXPERIMENTAL
Materials. All the metals were of a high degree of purity. A t the beginning electrolytic chromium was used, but on account of the difficulties of pulverizing this very hard material, causing contamination with iron, a very pure and fine powder prepared by I. G. Farbenindustrie (Bitterfeld) was used for further investigations. The only contamination was less than 1 % of chromium oxide. Molybdenum and tungsten of the high purity required for the incandescent lamp industry were used. For removing all traces of oxides the metals were reduced with hydrogen a t temperatures of 1000' and 1100' C., respectively. In all cases hydrogen was carefully purified from oxygen, mater, and nitrogen. A palladium catalyst and a system of wash flasks with sulfuric acid, chromic acid, phosphorus pentoxide, and magnesium activated Ivith sodium were employed in the purification. Preparation of Alloys. Chromium-molybdenum and chromium-
Table I. Annealing Procedures for Cr-1\10 and Cr-W Alloys Temperature of Annealing
c.
600 1000 1200 1430 1700
93
Time Hours 1500 350 240
;
Cooling
*
Quenched in water Quenched in water Quenched in water Quenchedin water Rapid cooling in Hz stream
ANALYTICAL CHEMISTRY
94
X-RAYANALYSIS.All patterns for this system showed only one phase-Le., the primary solid solution of the components. The lattice constants of their body-centered cubic structure varied regularly with the atomic composition of the alloys. Numerical results are given in Table 111. The l@tice constants of identical samples varied in a few cases by 0.01 A., but these differences did not have a systematic character and hence did not indicate any separation of the solid solution. THE CHROMIUM-TUNGSTEN SYsrmf. X-ray analysis gives different results in comparison with the chromium-molybdenum system. A t 1700' C. the two components form an unlimited solid solution, but a t lower temperatures they separate into two limited solid solutions. These results are shown in Table IV. The parameter values of the limited solid solutions permit determination of the mutual solubilities'of the components a t different temperatures. When compared graphically with the lattice constants of the primary solid solution obtained by annealing at 1700" C. the approximate values of solubility given in Table V are derived. The solubility increases markedly at temperatures above 1200" C., causing the uniting of both limited solid solutions a t 1700" C.
Table 11. Melting Points for Cr-Mo Alloys Composition of Samples Original Chemical weighing analysis ' Atomic % M o 0 0. 15.5 15 35.1 35 41.0 40 53.0 50 62.1 60 77.2 75 85.4 80 100 100
Melting Point O
c.
1770 1700 1730 1800 1825 1910 2080 2200
2520
Table 111. Lattice Constants of Cr-Mo Alloys Composition of Samples Atomic % M o
Annealing Temperatures
0
10 20 30 40 50 60 70 80 90 100
Table IV. Composition of Samples Atomic % W 0 10
