Online Lecture by prof. Cyril Cayron

A partial overview on the crystallography of martensitic transformations and twinning

Laboratory of Thermomechanical Metallurgy (LMTM), PX Group Chair, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

Title:
A partial overview on the crystallography of martensitic transformations and twinning

Abstract:
Deformation twinning and martensitic transformations are important plastic deformation mechanisms in shape memory alloys and in modern steels and titanium alloys. Both are classified a “shear” mechanisms; the parent/daughter interface is the shear plane.
This simple shear assumption, quite easy to accept for type I deformation twins, is however impossible from a crystallographic point of view for the type II twins and for most of martensites. Metallurgists solved this issue for type II twins by assuming that the interface plane is irrational, and for martensite by assuming that the martensite lath or plate is a composite made of A) two variants, or B) one variant and regular sets of dislocations forming a Lattice Invariant Strain (LIS).

This is the basis of the Phenomenological Theory of Martensite Crystallography (PTMC) proposed in the 1950’s. In the PTMC-A, the variants in the pairs are linked by specific relationships called “transformation twins” that are outputs of the calculations. In the PTMC-B, the LIS slip systems are inputs arbitrarily chosen. The knowledge of the lattice parameters (metrics) of the parent and daughter phases is required for both A and B versions. The PTMC explains many crystallographic features of martensite. However, it remains phenomenological, not completely predictive, mesmerized by polar decompositions, quite heavy in its calculations, clumsy in its way to treat the symmetries, and mute on the atomic paths. In this presentation, a simple alternative approach will be proposed in which the interface plane is not necessarily fully invariant.

Thanks to this additional degree of freedom, the atomistic paths, variant selection and texture changes can be calculated and predicted. The habit planes and the variants favored by external stresses are predicted uniquely from the distortion matrix. Since this matrix is not a simple shear or an Invariant Plane Strain (IPS), the Schmid or Patel-Cohen factors classically used in the PTMC had to be replaced by a more general parameter, the “Interaction Work” (IW). The transformation twins and their junction planes are determined without solving any equation, from a careful consideration of the symmetries and by using the inter-correspondences between the variants. It appears clearly that shear plane for the type I twins and the shear direction for the type II twins are generic, i.e. they do not depend on the metrics of the parent and daughter phases. Another type of twin called “weak twin” was introduced to describe the transformation twins in which the pairs of variants not linked by a two-fold symmetry. Some odd deformation twins reported in recent literature are also probably weak twins.

During this presentation, the theory will be compared to EBSD experiments on martensite in steels and in NiTi shape memory alloys, and on deformation twins in pure magnesium.
The talk will end by the description of an experiment in a AuCu alloy showing that the order-disorder phase transformation is both diffusive and displacive, and that the coupling between the two mechanisms creates a “magic” effect called TADA for Thermally Activated Distortion with Amplification.

This session was chaired by prof. Erik Offerman (TU Delft).