Variant selection and texture inheritance in martensitic steels

This research focuses on the study of a martensitic transformation related phenomenon called variant selection, and on its direct industrial implication in texture inheritance effects. Our approach consists in coupling the crystallographic and mechanical description of the martensitic phase transformation to achieve a better understanding of the transformation mechanisms and texture formation during thermomechanical treatments.
Variant selection and texture inheritance
Because of the presence of an orientation relationship between martensite and its parent austenite, different variants of martensite appear upon transformation. Each variant has a priori the same probability of being formed during quenching. However, in some cases, for example when the material is subjected to stresses, only few of these orientations are created. This phenomenon is called variant selection. If variant selection occurs during the cooling of a textured austenite product, the texture can be partially transferred to the martensite. The texture inheritance can introduce important anisotropy in the mechanical properties of the final product, and can thus be an industrial issue.
Methods: Mechanical testing, characterization techniques (SEM, EBSD, XRD), numerical simulations, atomic-scale modelling.

The research project is funded by the Swiss National Science Foundation
PhD student: Annick Baur
Supervisors: Dr. Cyril Cayron, Prof. Roland Logé.

Formation of a {225}γ thin plate of martensite

Formation of a {225}γ thin plate of martensite according to the martensitic transformation model developed by Cayron. The combination of twin-related variants for the transformation allows a globally invariant {225}γ habit plane. The austenite atoms are represented in green and form an FCC lattice. During the transformation, they turn respectively to blue and red colours. The blue and red atoms are arranged on two twin-related BCC lattices.