Velikanova, TA; Karpets, MV; Kuprin, VV
High-temperature X-ray diffraction is used to study the evolution of phase states of rapidly solidified Fe64.9Mo19.6Cr14.5C (at.%) alloy during heating from room to solidus temperatures. The alpha- and beta-Mn-like phases (chi and pi, respectively), as well as bcc phases of various composition (alpha(1), alpha(2), alpha), eta-carbon intermetallic (M6C), and R- and fcc phases are identified in the alloy. It is established that the chi-phase is stable in this alloy at high temperatures: it forms during heating in the range 973-1073 K, changes its chemical composition in the range 1073-1273 K, which remains unchanged at 1073-1473 K, and decomposes in the range 1473-1523 K; when it is heated, its relative amount and lattice parameter symbatically change from 20 to 55 wt.% and from 0.9061 to 0.9170 nm, the coefficient of thermal expansion being 16.65a (TM) 10(-6) K-1. The existence of equilibrium regions with participation of the stable chi-phase, gamma + alpha + chi + R, chi + gamma + eta + alpha, chi + eta + gamma + alpha, and chi + alpha + eta + R, has been established in the Fe-Mo-Cr-C phase diagram for the first time. It is shown that the pi-phase, found in the initial spinning alloy, is metastable. This phase abruptly changes its composition near 842 K and completely decomposes in the range 923-1073 K. When heated in the range 293-923 K, its relative amount and lattice parameter vary from 50 to 8 wt.% and from 0.6346 to 0.6401 nm, respectively, the coefficient of thermal expansion being 2.083a (TM) 10(-6) K-1. The coefficients of thermal expansion of the eta- and alpha-phases are 15.257a (TM) 10(-6) and 7.117a (TM) 10(-6) K-1. It is revealed that the temperature of full decomposition of the metastable beta-Mn-like phase and appearance of the stable alpha-Mn-like phase in conditions of high-temperature X-ray diffraction is close to 978 K of alpha-Mn -> beta-Mn polymorphic transformation in pure manganese. The high-temperature X-ray diffraction experiment indicates that pi -> I +/- transformations of metastable phases and gamma -> I +/- transformations of stable phases are diffusionless in this alloy.