Transformation toughening is the result of the interaction of the crack-tip triaxial stress state with austenitic dispersions in the microstructure of the steel. The most important prerequisite is that dispersed austenite should exhibit high stability. Chemical and size stabilization are the usual routes to achieve high stability limits. On the other hand, the obtained toughening is directly proportional to the transformation volume change. It is known that martensitic transformation is accompanied by dilation of the order of 4%. This dilatation contributes to toughening by the associated pΔV term, where p is the triaxial pressure ahead of the crack tip. Therefore, higher ΔV values lead to higher toughness. The transformation volume change can be influenced through alloying additions. Magnetic changes, such as ferromagnetism and the INVAR effect, become important in austenite and reduce the volume change. It is therefore important to select alloy compositions, which maximize austenite stability and promote a large volume change. In this work computational alloy thermodynamics and the **CALPHAD** method were applied to describe the compositional dependence of the lattice parameters in austenite and martensite. The so-called **two-gamma states model** was applied. The states γ_{1} and γ_{2} are two distinct electronic states of γ-iron separated by a specific energy difference and in equilibrium with each other. The γ_{1} low-T state is antiferromagnetic with a low magnetic moment and the γ_{2} high-T state is ferromagnetic with a high magnetic moment. Alloying additions of Ni, Co and Cr were considered in a matrix typical of the AF1410 UHS steel. It was found that compared to the reference material 14Co-10Ni, which exhibits a volume change of 2.2%, the composition 14Co-15Ni-2Cr exhibits a higher austenite stability and at the same time a higher volume change of 3.3%, leading to a higher fracture toughness, which arise from TRIP interactions in the crack tip.

More info:

G.N. Haidemenopoulos, M. Grujicic, G.B. Olson, and Morris Cohen, On the Optimization of Precipitated Austenite for the Transformation Toughening of Steels, CALPHAD, Vol.13, No.3, p.207-216, 1989.

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