Fe-Co-Ni steels constitute the basis of UHS steels with UTS>2GPa and KIC>150MPa m1/2. A known example in this class of steels is CarTech AerMet 100 marketed by Carpenter Technology https://en.wikipedia.org/wiki/Aermet
The extraordinary strength-toughness combination stem from the operation of Transformation-Induced Plasticity (TRIP) effect arising from dispersed austenite, which forms during the tempering treatment of the steel. A high stability is required for the high triaxiality stress state in front of a crack tip, in order to achieve high fracture toughness. The stability of austenite can be tuned with certain alloying additions. The Figure below depicts contours of the free energy change for martensitic transformation (stability parameter) and superimposed isothermal section of the Fe-Ni-Co system at the tempering temperature of 510oC. For a given composition Xo, the associated austenite composition is Xγ, which then defines the corresponding stability parameter. It is thus possible to select the alloy composition, which maximizes the austenite stability for maximum TRIP effect and fracture toughness.
Further details can be found in: G.N. Haidemenopoulos, M. Grujicic, G.B. Olson, and M. Cohen, Thermodynamics-Based Alloy Design Criteria for Austenite Stabilization in the Fe-Ni-Co System, Journal of Alloys and Compounds, Vol.220, p.142-147, 1995 .
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