Crystallography of Lath Martensite in Steels

Abstract

The double lattice invariant shear version of the phenomenological theory, has been used to explain all the known crystallographic features of lath martensite. When the lattice invariant shear systems are taken to be rational slip systems in martensite with slip directions along [11\bar1]B and [1\bar1\bar1]B, the habit plane solutions converge to an (hhl)F habit plane that coincides with, or is close to, (557)F. Up to 35 different combinations of these rational lattice invariant shear systems lead to (hhl)F habit planes that lie 9° to 14° from (111)F. The theory correctly predicts the habit plane, the orientation relationship, (including the observed spread in the angle between the close packed directions [1\bar10]F and [11\bar1]B), the shape strain, the internal substructure of dense dislocations with a⁄2[11\bar1]B dislocations dominant, and the presence of an array of a⁄2[1\bar1\bar1]B dislocations approximately 3.5 nm apart in the austenite/martensite interface. The theoretical explanation of lath martensite also indicates that approximately twin-related laths can form with the same habit plane, and that this arrangement of laths in a packet will provide a significant degree of mutual accomodation of the shape strain.