Stability of the α-Mn Structure in Rapidly Solidified Fe-Mo-Cr-C Alloys at High Temperatures

Abstract

High-temperature X-ray diffraction is used to study the evolution of phase states of rapidly solidified Fе_64,9Мо_19,6Сr_14,5C (at.%) alloy during heating from room to solidus temperature. The α- and β-Мn phases ((χ and π respectively), as well as bec phases of various composition (α₁, α₂, α), η-carbon intermetallide (М₆С), R- and fee phases are identified in the alloy. It is established that the χ-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 in the range 1073-1473 K, and decomposes in the range 1473-1523 K; when it is heated, its relative amount and lattice parameter simbatically change from 20 to 55 wt.% and from 0.9061 to 0.9170 nm, the coefficient of thermal expansion being 16.65 ·10^-6 K^-1. Existence of equilibrium ranges arias with participation of the stable χ-phase, γ + α+ χ + R; χ + γ + η + α; χ + η +γ + α; and χ + α + η +R, hove been established in the Fe-Mo-Cr-C phase diagram for the first time. It is shown that the π-phase, found in the initial spinning alloy, is metastable. This phase abruptly changes its composition near 842 К 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 2.083 х 10^-6 K^-1. The coefficients of thermal expansion of the η- and &alpha-phases are 15.257 · 10^-6 and 7,117· 10^-6 K^-1. It is revealed that the temperature of full decomposition of the metastable β-Мn- like phase and appearance of the stable α-Mn-like phase in conditions of high-temperature X-ray diffraction is close to 978 К of α-Mn → β-Mn polymorphic transformation of pure manganese. The high-temperature X-ray diffraction experiment is indicating that π → α a transformations of metastable phases and yγ → α a transformations of stable phases are diffusionless in this alloy.