Thermodynamics-based alloy design criteria for austenite stabilization and transformation toughening in the Fe---Ni---Co system

Transformation toughening has been widely applied in metastable austenitic steels. Recently this toughening mechanism has been extended to ultrahigh strength secondary-hardening martensitic steels, bearing suitable austenitic dispersions. The resulting dispersed-phase transformation toughening depends on the stability of the austenitic dispersions. The stability of dispersed austenite depends on various factors including the chemical composition and size of austenite particles, the stress state and the yield strength of the matrix. A single-parameter characterization of the stability of the austenitic dispersion is provided by the M_s^σ temperature and a functional form relating that temperature with the above-mentioned factors is developed. The microstructural requirements for dispersed-phase transformation toughening are then derived in terms of the austenite particle size and chemical enrichment in stabilizing solutes. Compositional effects on austenite stability have been studied by performing thermodynamic calculations using the Thermo-Calc software. The free-energy change ΔG^ch = G^b.c.c. − G^f.c.c. for martensitic transformation (a measure of austenite stability) has been evaluated as a function of composition in the ternary Fe---Ni---Co system. This information, when superimposed on isothermal sections at the tempering temperatures of interest, provides a way for selecting alloy compositions that maximize the thermodynamic stability of dispersed austenite.