Steels: martensitic transformation, part 2. Lecture 2 of 12

This lecture explains why the orientation relationship between martensite and austenite is irrational, in terms of the combination of the Bain strain and rigid-body rotation required to generate an invariant line. This is followed by an explanation of the crystallographic theory of martensite, the most elegant theory that explains completely the crystallography of the austenite to bcc martensite transformation. The lattice invariant deformation is introduced and the irrational habit plane explained. The fcc-hcp (austenite to hexagonal close packed) transformation is explained next, including the volume change. Finally, the thermodynamics of the martensite transformation, including stored energy terms and the estimation of the martensite-start temperature, the calculation of the size of martensite plates. The information is then used to design a novel steel with a strength in excess of 2 GPa, huge toughness, ductility, impact toughness and weldability. So after two lectures, it is possible to use the information on the atomic mechanism to design useful steels that can be mass-produced. Associated teaching materials can be found on: http://www.phase-trans.msm.cam.ac.uk/... H. K. D. H. Bhadeshia