Damage of the martensite interfaces as the mechanism of the martensite stabilization effect in the NiTi shape memory alloys

Abstract

The aim of the paper was a comprehensive experimental study of the martensite stabilization effect (MSE) in NiTi alloys under various conditions to find the mechanism behind this phenomenon. The MSE was studied in the quenched Ni50Ti50 and Ni51Ti49 alloys subjected to pre-deformation to various strains in three ways: deformation in the martensite state, cooling under stress through a temperature range of the forward transformation and deformation in the austenite state due to the stress-induced martensite formation. It was found that after pre-deformation by the martensite reorientation, the MSE was observed even after a small pre-strain. After pre-deformation by cooling under stress or by stress-induced transformation, the MSE was small if the pre-strain was less than 5%; otherwise, the value of this effect was significant. A new mechanism of the martensite stabilization effect was considered. It was assumed that the damage to the martensite boundaries during the pre-deformation (without plastic strain) decreased their mobility and a larger thermodynamic force needed to be provided to move the interfaces during the reverse transformation.