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 austen itic steels. Recently this toughening mechanism has been extended to u ltrahigh strength secondary-hardening martensitic steels, bearing suit able austenitic dispersions. The resulting dispersed-phase transformat ion toughening depends on the stability of the austenitic dispersions. The stability of dispersed austenite depends on various factors inclu ding the chemical composition and size of austenite particles, the str ess state and the yield strength of the matrix. A single-parameter cha racterization of the stability of the austenitic dispersion is provide d by the M(s)(sigma) temperature and a functional form relating that t emperature with the above-mentioned factors is developed. The microstr uctural requirements for dispersed-phase transformation toughening are then derived in terms of the austenite particle size and chemical enr ichment in stabilizing solutes. Compositional effects on austenite sta bility have been studied by performing thermodynamic calculations usin g the Thermo-Gale software. The free-energy change Delta G(ch)=G(b.c.c .)-G(f.c.c.) for martensitic transformation (a measure of austenite st ability) has been evaluated as a function of composition in the ternar y Fe-Ni-Co system. This information, when superimposed on isothermal s ections at the tempering temperatures of interest, provides a way for selecting alloy compositions that maximize the thermodynamic stability of dispersed austenite.