The substructure of (252)_f martensite formed in an Fe- 8Cr- 1 C alloy

Abstract

This work deals with a transmission electron microscopy investigation of plate martensite in an Fe-8Cr-lC alloy, which exhibits a typical (252)_f martensite transformation. It was found that macroscopic martensite plates, such as those used for optical microscopy determination of the habit plane and the shape strain, actually consist of many individual small plates having the same crystallographic orientation and habit plane. Individual plates within a macroscopic plate appear to form by auto-catalytic nucleation. The coalescence plane between individual plates is usually parallel to (011)_b = (lll)_f because the growth of the plates is restricted by stacking faults and twins on (lll)_f, formed as a result of accommodation of the shape strain. Deformation of the martensite at the coalescence sites, mainly on the twin system (112)_b $[\overline {11} 1]$ _b, leads to a complex substructure of the macroscopic plates. Stacking faults and twins in the austenite may be inherited in the martensite, which leads to further complexity. Prior to the coalescence, the substructure of individual martensite plates is simple, and consists of a low density of irregularly distributed twins on (112)_b $[\overline {11} 1]$ _b and dislocations having Burgers vector a_b/2$[\overline {11} 1]$. These defects are probably caused by accommodation deformation. The martensite/austenite interface contains a set of parallel dislocations having a spacing of about 13 A. These dislocations are likely to be screw dislocations with Burgers vector a_b/2$[\overline {11} 1]$ = a_f/2$[\bar 101]$, which accomplish the complementary shear in the phenomenological crystallographic theory.