Austenite–Martensite Interface Dislocations in Stainless Steel

Abstract

WHEN 18 chromium : 8 nickel austenitic stainless steel is plastically deformed at room and sub-zero temperatures, e (hexagonal closest packing) and α′ (body centred cubic) martensites are formed. The e phase forms as the result of the propagation of Shockley partial dislocations on every second {111} γ plane and appears in the form of thin disks having a {111} γ habit1. The mechanism of the γ → α′ transformation in this steel is not yet entirely understood. Venables1 suggested that α′ martensite nucleates from the e phase and grows by the motion of a parallel grid of screw dislocations such as that postulated by Frank2. Bogers and Burgers3 showed, by geometrical arguments, that the shears necessary for the transformation correspond to ‘well-known’ (that is, Shockley partial) dislocations and also suggested the possibility that the transformation proceeded by way of the motion of a parallel array of screw dislocations. Lagneborg4 found that deformation induced α′ nucleated preferentially at the intersections of two e disks or at the intersections of active slip planes with e disks, and proposed that the martensite-like region existing near the centre of a Shock-ley partial dislocation wras a favourable nucleation site. He did not discuss a mechanism for the subsequent growth of α′. To our knowledge, no direct evidence for the postulated dislocation interfaces has been observed. During a recent investigation of the effects of shock loading on 18 chromium : 8 nickel stainless steel, an experimental observation was made that showed a matrix-martensite interface composed of an array of dislocations. This communication presents this observation and discusses its relevance to the previously postulated transformation mechanisms.