Geometry of Adaptive Martensite in Ni-Mn-based Heusler alloys

TL;DR

This paper explores the geometric structure of adaptive martensite in Ni-Mn-based Heusler alloys, clarifying the relations between different unit cells and their implications for phase boundary compatibility and twin boundaries.

Contribution

It provides an analytic geometric framework for understanding adaptive martensite, linking various modulated phases through precise equations based on phenomenological theory.

Findings

Derived exact relations between unit cells of modulated phases

Analyzed compatibility at habit planes in Ni-Mn alloys

Discussed the impact of modulation on lattice constants

Abstract

Modulated martensites play an important role in magnetic shape memory alloys, because all functional properties are closely connected to the twin microstructure and the phase boundary. The nature of the modulated martensites is still unclear. One approach is the concept of adaptive martensite, which regards all modulated phases as nanotwinned microstructures. In this article, we use the Ni-Mn-based shape memory alloys as an example to show the geometric rationale behind this concept using analytic equations based on the phenomenological theory of martensite. This could enhance discussions about the implications of the adaptive martensite by showing the exact relations between the various unit cells used to describe the structure. We use the concept to discuss the compatibility at the habit plane, the nature of high-order twin boundaries and the dependence of the lattice constants on the different types of modulation.