Stability concepts for thermally-induced martensitic transformations in crystals

Ryan Elliott, University of Michigan

Shape memory alloys (SMAs), such as equi-atomic NiTi, have become an important area of research in recent years due to the potential for novel applications of the shape memory effect and pseudoelasticity. The shape memory effect is the ability of the material to erase relatively large mechanically-induced strains (up to 8%) by moderate increases in temperature (≈ 10 - 20°C).Pseudoelasticity refers to the ability in a somewhat higher temperature regime to accommodate these strains during loading and recover upon unloading (via a hysteresis loop).

The underlying mechanism is a reversible martensitic (displacive) transformation between solid-state phases, often occurring near room temperature. The transformation can be induced by changes in temperature or by changes in stress due to the strong thermo-mechanical coupling.

A nano-mechanical model of temperature-induced phase transitions is presented. The model is based on temperature-dependent interatomic potentials and finite strain continuum mechanics. It is used to explicitly determine stress-free equilibrium paths in temperature-strain space. Ideas from continuum mechanics as well as lattice dynamics are used to investigate the local and global stability of the equilibrium paths obtained for a prototypical B2 cubic crystal that employs temperature-dependent pair-potentials.

Abstract Author(s): Ryan S. Elliott<br />John A. Shaw<br />Nicolas Triantafyllidis