Responses of Pre-transitional Materials with Stress-Generating Defects to External Stimuli: Superelasticity, Supermagnetostriction, Invar and Elinvar Effects

TL;DR

This paper models how stress-generating defects in pre-transitional materials induce equilibrium martensitic embryos, leading to giant reversible strains and effects like superelasticity, supermagnetostriction, Invar, and Elinvar, explained through phase field microelasticity theory.

Contribution

It introduces a new understanding of thermoelastic equilibrium involving nano-sized martensitic embryos caused by defects, explaining giant strains and various ferroic effects in pre-transitional materials.

Findings

Giant reversible strains are induced by defect-stimulated martensitic embryos.

The effects are explained by a thermoelastic equilibrium only with infinite-range defect interactions.

The model accounts for superelasticity, supermagnetostriction, and Invar/Elinvar effects in pre-transitional phases.

Abstract

We considered a generic case of pre-transitional materials with static stress-generating defects, dislocations and coherent nano-precipitates, at temperatures close but above the starting temperature of martensitic transformation, Ms. Using the Phase Field Microelasticity theory and 3D simulation, we demonstrated that the local stress generated by these defects produces equilibrium nano-size martensitic embryos (MEs) in pre-transitional state, these embryos being orientation variants of martensite. This is a new type of equilibrium: the thermoelastic equilibrium between the MEs and parent phase in which the total volume of MEs and their size are equilibrium internal thermodynamic parameters. This thermoelastic equilibrium exists only in presence of the stress-generating defects. Cooling the pre-transitional state towards Ms or applying the external stimuli, stress or magnetic field, results in a shift of the thermoelastic equilibrium provided by a reversible anhysteretic growth of MEs that results in a giant ME-generated macroscopic strain. In particular, this effect can be associated with the diffuse phase transformations observed in some ferroelectrics above the Curie point. It is shown that the ME-generated strain is giant and describes a superelasticity if the applied field is stress. It describes a super magnetostriction if the martensite (or austenite) are ferromagnetic and the applied field is a magnetic field. In general, the material with defects can be a multiferroic with a giant multiferroic response if the parent and martensitic phase have different ferroic properties. Finally the ME-generated strain may explain or, at least, contribute to the Invar and Elinvar effects that are typically observed in pre-transitional austenite. The thermoelastic equilibrium and all these effects exist only if the interaction between the defects and MEs is infinite-range.