A Dynamic Study of Strain-Induced γ→α′ Martensitic Transformation in an Fe-Cr-Ni Alloy

The formation mechanism of strain-induced γ→α′ martensitic transformation in an Fe-Cr-Ni alloy with a low stacking fault energy has been investigated by in situ deformation with a 3 MV electron microscope. Results obtained are summarized as follows: (1) Formation of α′ martensite crystals always occurs by “double shear mechanism”. (2) α′ crystals are classified into the following two types; one is of “Schiebung” type which has a slow formation speed and the other is of “Umklapp” type which has a rapid formation speed. The former is observed in γ-grains of a single slip system, and the latter in those of multiple slip system or in work-hardened γ-grains. (3) In both cases, γ-twins are always well-formed prior to the formation of α′ crystals. (4) The secondary shear displacement of “double shear” is induced on the interface between γ-twins of the primary slip system and γ-matrix. (5) An α′ crystal grows easily along the primary shear plane.The nucleation and growth mechanisms of α′ crystals are discussed on the basis of the experimental results.