Thermodynamic assessment of the Cr–Mo–Si ternary system

Abstract

Based on the experimental isothermal sections at 1000 and 1200 °C and the liquidus projection from the measurements by the present authors, the Cr–Mo–Si ternary system is thermodynamically assessed using the CALculation of PHAse Diagram method (CALPHAD). The parameters of the thermodynamic descriptions of all the phases in the Cr–Mo–Si ternary system are optimized on the basis of the reported thermodynamic data and the constitution models of the Cr–Mo, the Cr–Si and the Mo–Si terminal binary systems. Two solution phases, liquid and bcc_A2, were described as substitutional solutions. The intermetallic phases, CrSi, CrSi2 and MoSi2, were modeled by two sublattices, (Cr,Mo):Si, (Cr,Mo):Si2 and (Mo,Cr):Si2, respectively, while βCr5Si3 was modeled by three sublattices (Cr,Mo,Si)2:(Cr,Si)3:Cr3. Cr3Si and Mo3Si have the same crystal structure (cP8, Pm-3n) and present a continuous intermetallic phase M3Si with the two sublattice model (Cr,Mo,Si)3:(Cr,Si)1. Similarly, αCr5Si3 and Mo5Si3 with the same crystal structure (tI32, I4/mcm) form a continuous intermetallic phase M5Si3 with the three sublattice model (Cr,Mo)4:(Cr,Mo,Si)1:(Mo,Si)3. Since the very small solubilities of Cr and Mo, the Diamond-Si phase was treated as a pure element phase. Using the current optimized parameters, the isothermal sections, the liquidus projection and the reaction sequence chart are calculated. The comparison between the calculated results and the experimental investigations, including the isothermal sections, the liquidus projection, and the solidification processes of the as cast alloys, shows that the present calculation can well reproduce the experimental equilibria. The present modeling can finely describe the experimental information of the Cr–Mo–Si ternary system.