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