Mechanism of the shape memory effect in martensitic alloys: an assessment

Abstract

The (one-way) shape memory effect is a phenomenon that when a martensitic alloy is deformed in a martensitic state it recovers its original shape upon heating to the parent phase. This is a universal effect for certain martensitic alloys. We will assess the mechanism of the effect critically and select the essential factors which govern the effect. We try to understand it from a unified view, invoking the group–subgroup symmetry relation between the parent and martensite phase, along with analysis of reversible twinning modes in martensite. By such an assessment, we will show why typical shape memory alloys, such as Ti–Ni, Cu–Al–Ni etc., exhibit good shape memory characteristics, while others, such as ferrous alloys, do not. Thus, we will show that most of the shape memory characteristics of various martensitic alloys can be understood consistently from such an approach.

Keywords:

• shape memory alloys
• mechanism of shape memory effect
• martensitic transformation
• twinning
• lattice correspondence
• group–subgroup relation
• ferroics
• ferrous martensite

Acknowledgements

The authors are grateful to many colleagues for stimulating discussions on the present topic: Professors Yunzhi Wang, M. Tokonami, T. Suzuki, T. Tadaki, T. Ohba, S. Morito, Y. Murakami and Y. Nakata. One of the authors (KO) also acknowledges the support of Foundation of Advancement of International Science in Tsukuba. Part of this work (AS) was supported by the US Department of Energy.