Modification of preferred martensitic variant distribution by high magnetic field annealing in an Ni–Mn–Ga alloy

The preferred martensitic variant distribution in Ni53Mn25Ga22ferromagnetic shape memory alloy (FSMA) samples annealed without and with a high magnetic field of 12 T applied during the annealing process was investigated by electron backscatter diffraction. It is revealed that the high magnetic field applied during annealing enhances the regular arrangement of martensitic variants from the morphological point of view and effectively modifies the preferred orientation distribution of martensitic variants without changing the misorientation between them from the crystallographic point of view. Only one texture component, \{ 1{\overline 1}0\} \langle 33{\overline 2}\rangle, exists in the sample annealed without a magnetic field, whereas two additional texture components, \{ 4{\overline 6} 3\} \langle 31{\overline 2}\rangle and \{ 1\overline 1 0\} \langle 110 \rangle, are developed in the sample annealed in a high magnetic field. The new finding that the preferred martensitic variant distribution can be efficiently modified by introducing a high magnetic field during the annealing process will shed light on the development of high-performance polycrystalline FSMAsvianovel processing techniques.

New approach to twin interfaces of modulated martensite

In Ni–Mn–Ga ferromagnetic shape memory alloys, the crystallographic nature of martensitic variant interfaces is one of the key factors governing the variant reorientation through field-induced interface motion and hence the shape memory performance. So far, the crystal structure studies of these materials – conducted by means of transmission electron microscopy – have suffered from uncertainties in determining the number of unit cells of modulated superstructure, and consequently improper interpretations of orientation correlations of martensitic variants. In this paper a new approach is presented for comprehensive analysis of crystallographic and morphological information of modulated martensite, using automated electron backscatter diffraction. As a first attempt, it has been applied for the unambiguous determination of the orientation relationships of adjacent martensitic variants and their twin interface characters in an incommensurate 7M modulated Ni–Mn–Ga alloy, from which a clear and full-featured image of the crystallographic nature of constituent twin interfaces is built up. Certainly, this new approach will make it feasible not only to generalize the statistical analysis of martensitic variant distributions for various materials with modulated superstructure, but also to give insight into the crystallographic characteristics of martensitic variant interfaces and the variant reorientation mechanism of new advanced materials for interface engineering.

A Multi-Scale Model of Martensitic Transformation Induced Plasticity at Finite Strains

A physically-based multi-scale model for martensitic transformation induced plasticity is presented. At the fine scale, a model for one transforming martensitic variant is established based on the concept of a lamellae composed of a martensitic plate and an austenitic layer. Next, the behaviour of 24 potentially transforming variants is homogenized towards the behaviour of an austenitic grain. As a simple example, the model is applied to deformation and transformation of a single austenitic grain under different deformation modes.