Six phases with structures isomorphous with the iron – chromium sigma phase have been reported in binary alloys of the transition elements of the first long period1–3. In addition, Goldschmidt4 has shown that the phase FeMo, stable between 1,180° and 1,540° C., has the same structure. In the binary system cobalt - molybdenum, a similar phase Co2Mo3 exists between 1,250° and 1,620° C.5; recent work in this Laboratory has shown that Co2Mo3 has a sigma-phase structure. The interplanar spacings of the alloy containing 61 atomic per cent molybdenum are very similar to those of the alloy with 50 atomic per cent FeMo given by Goldschmidt, except that they are slightly smaller, as might be expected when the slightly smaller cobalt atom replaces iron. Multiplying the Co2Mo3 d-values by 0.981 gives values which agree well with those for FeCr given by Goldschmidt4.
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