Martensite variants generated by the mechanical transformation of precipitated interlath austenite

Intercritically tempered (5-9)Ni steels are toughened by the introduction of islands of precipitated austenite along the boundaries of dislocated martensite laths. The interlath islands of precipitated austenite transform to martensite under the stress field ahead of a cleavage crack. Since the fresh martensite is a different crystallographic variant than that of the surrounding packets, it efficiently refines the effective grain size for transgranular cleavage. A straightforward calculation shows that this behavior is dictated by the need to minimize the elastic strain energy of the fresh martensite.