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The concept of the dual-energetic martensite form (CODEM) for the background of divers phenomena in shape memory alloys

by V. Prieb

The review of my intimate and creative life with shape memory alloys during the last 45 years (1974-2019) and the saying goodbye to them. It is now time to renounce on that and to move on to my new muses Erato and Melpomene. May 2019

Abstract:
The concept of the dual-energetic martensite form (CODEM) is presented, based on the experimentally proven fact that the martensitic transformations and associated phenomena in SMAs be determined not only by the generally defined and assumed polydomain form of martensite, resulting from the 1st accommodation mechanism, but also real and essential by its monodomain form without substructure.

A martensite polydomain has higher free energy than that of a martensite monodomain due to contribution of stacking fault energy . The whole energetic spectrum of martensite states between the both martensite forms is determined by the stacking fault density .

The equilibrium temperature of austenite and martensite monodomain is much higher than that of austenite and martensite polydomain. The first one is a fundamental quantity determined only by the SMA composition, while the second one is very variable and can be influenced by several metallurgical factors. The austenite is metastable in the temperature range between the both equilibrium temperatures. This metastable state is characterized by well-known premartensitic phenomena such as softening of the elasticity modulus, formation of fluctuative intermediate shear structures, superelasticity along the Clausius-Clapeyron equilibrium line.

The CODEM considers different effects of the elastic energy of AM-phase boundaries and of internal MM-martensite boundaries on the free energy of the two-phase system: the first contribution equally increases the free energies of austenite and martensite and thus of the whole system without affecting the two-phase equilibrium, the second one only increases the free energy of the martensite and thus changes the two-phase equilibrium.

The energy increasing of the whole two-phase system causes the need for continuous supercooling to drive the forward martensitic transformation (athermic transformation kinetics in the thermoelasticity model). The energy increasing of only the martensite due to the coalescence of martensite variants (2nd accommodation mechanism) leads to a decreasing of the equilibrium temperature during the transformation.

The decreasing of stacking fault density in the martensite phase by the ferroelastic deformation cycles or in individual martensite crystals by changing the stress field in their environment, consiedered in the CODEM as martensite monodomainization, causes true stabilization of the martensite in sense of increasing its reverse transformation temperature in the temperature range beetween the both equilibrium temperatures. This explains RPM (return point memory) effects in partial cycles of ferroelastic deformation and thermo-induced reverse transformation (also known as SMART- effect or TAME).

The martensite monodomainization leads to the degradation of the memory properties of SMA-actuators after several working cycles. The stabilization by the plastic deformation at that does not take place. The blocking of martensite boundaries by dislocations occurs only through plastic deformation of the martensite. These two stabilization mechanisms can be clearly recognized by aging at different temperatures.

Many experimental results obtained so far at the SMAs were no longer justifiable by the thermoelasticity model and some even directly contradict its main postulates. The CODEM provides comprehensive and plausible explanations for most of the phenomena known in SMAs.

More:
"The concept of the dual-energetic martensite form (CODEM)
as a background of various phenomena in shape memory alloys"
- pdf-file