On phase stability and phase relations of topologically close packed phases in binary Cr-Os system over the temperature range of 900-1300 C

Osmium (Os) addition has been found to be beneficial in enhancing the creep resistance and microstructure stability of nickel-based single-crystal superalloys. The phase stability and phase relationship of topologically closely packed (TCP) phases in a binary Cr-Os system is thus essential for developing a better understanding of the constitutional effects of Os in multicomponent nickel-based single-crystal superalloys, but are still not fully understood in the literature. In this paper, a series of Cr-Os alloys were prepared, and the phase constituents and the microstructure of both as-cast and annealed alloys over a temperature range of 900-1300 degrees C were analyzed using the X-ray diffraction (XRD) and electron probe microanalysis (EPMA) techniques. The experimental results show that (Cr) solution, (Os) solution and Cr2Os phases are formed during solidification. After long-term annealing at 900-1300 degrees C, the phase constituents and the microstructure of the alloys display obvious changes. Based on the measured data, the phase stability and phase equilibria of TCP between 900 and 1300 degrees C in the binary Cr-Os system are accordingly updated. The stability results for the two TCP phases in the binary CrOs system demonstrate that the Cr3Os phase exists stably, while the Cr2Os phase decomposes into Cr3Os and (Os) at 900-1300 degrees C.