Departmental Seminar - Materials Science and Engineering: Prof. Dirk Raabe

Metastability Alloy Design

Dr. Dierk Raabe

Professor, Director

Max-Planck-Institut für Eisenforschung

Germany

Abstract:

Efficient bulk alloy design for high strain hardening and damage tolerance can be achieved by designing matrix metastability in such a way that with increasing mechanical loading athermal transformation mechanisms are activated, providing additional strain hardening reserves at higher deformation.

Ideally, these additional deformation mechanisms, such as formation of stacking faults, mechanical twinning and martensitic transformations, are not activated at the same load but subsequently when the preceding hardening effects are getting exhausted.

When applied to metallic alloys with face centered cubic structure this approach enables the design of a number of ultrahigh strength steels and high entropy alloys based on planar slip, mechanical twinning (TWIP steels) and on martensitic transformations (TRIP steels).  In these materials matrix metastability is carefully adjusted through adequate alloying to tune the desired stacking energy.

Recently this metastability design concept has in steels, high entropy alloys and beta titanium also been applied to the activation of athermal transformation effects occurring primarily in confined microstructure regions such as at interfaces and dislocations. In such cases local transformation and the associated confined strain hardening equip metallic alloys with enhanced damage tolerance.