<I>In-situ</I> Observation of Strain-Induced γ→ε→α′ and γ→α′ Martensitic Transformations in Fe–Cr–Ni Alloys

Abstract

Strain-induced γ→ε→α′ and γ→α′ martensitic transformations in Fe–Cr–Ni alloys have been investigated by in-situ deformation with a high voltage electron microscope. The results obtained are summarized as follows:[A] γ→ε→α′ transformation: (1) Two sets of ε-crystals are formed along the primary and the conjugate slip planes in γ-crystals oriented for multiple slip systems. Fine α′-crystals are frequently formed at the intersections between these ε-crystals. (2) In γ-crystals oriented for a single slip system α′-crystals are also formed. In this case, cross-slip accompanying a stair-rod dislocation frequently occurs in local regions, and stacking faults corresponding to the secondary shear displacement are induced to form α′-crystals. (3) These α′-crystals grow to make coalescence with each other, and large α′-crystals are formed.[B] γ→α′ martensitic transformation: (1) γ→α′ transformation also occurs by “the double shear mechanism”. (2) Formation processes of α′-crystals are classified into slow “Schiebung” type and rapid “Umklapp” type. The former is observed in γ-grains oriented for a single slip system, and the latter in those oriented for multiple slip systems or in work-hardened γ-grains. (3) In both cases, γ-twins are always well formed prior to the formation of α′-crystals, and the secondary shear displacement is initiated at the interface of these γ-twins. (4) Each α′-crystal grows easily along the γ-twin plane.Nucleation and growth mechanisms of α′-crystals in these transformations are discussed on the basis of the experimental results.