High-Temperature Phases in the Fe–Mo–Cr–C System

Phase equilibria involving the stable high-temperature quaternary χ_Fe,Cr,Mo,C phase were established in the Fe–Mo–Cr–C phase diagram. The arc-melted alloys were annealed at subsolidus temperatures for 52 h and then quenched in liquid gallium. The solidus temperature of the alloys was determined with the Pirani–Alterthum method. High-temperature X-ray diffractometry was employed to monitor the sequence of changes in the alloy phase composition from room temperature to the solidus temperature. The χ + η + α, χ + η, and χ + σ phase equilibria were directly observed at 973 K < T < 1373 K, 1273 K < T < 1530 K, and 1523 K < T < 1530 K, respectively, in the Fe_52.5Mo_23.5Cr_18.7C_5.3 (at.%) alloy. The χ + M₂₃C₆ + α and χ + σ phase equilibria were directly observed at 973 K ≤ T < 1523 K and 1473 K < T < 1525 K in the Fe_55.5Mo_11.8Cr_28.2C_4.5 (at.%) alloy. It was shown that the two-phase χ + σ equilibrium could be preceded by three-phase χ + η + σ equilibria or a single-phase χ _Fe,Cr,Mo,C equilibrium region (for the Fe_52.5Mo_23.5Cr_18.7C_5.3 alloy in the 1523 K < T < 1530 K temperature range). The quaternary χ _Fe,Cr,Mo,C phase was found in the (51.9–64.9) Fe, (5.4–23.5) Mo, (14.5–35.4) Cr, and (1–10.7) C at.% composition ranges. Primary crystallization regions of the σ _Fe,Cr,Mo,C and α_Fe,Cr,Mo,C phases with solidus temperatures of approximately 1530 K (for the Fe_52.5Mo_23.5Cr_18.7C_5.3 alloy) and 1525 K (for the Fe_55.5Mo_11.8Cr_28.2C_4.5 alloy) were revealed. The linear thermal expansion coefficients for the χ _Fe,Cr,Mo,C, η _Fe,Cr,Mo,C, and α_Fe,Cr,Mo,C phases of different composition observed for different temperature ranges were determined.