Transformation behaviour and unusual twinning in a NiTi shape memory alloy ausformed using equal channel angular extrusion

Abstract

In this study, the microstructure and phase transformation behaviour of Ti–50.8 at.% Ni alloy severely deformed using equal channel angular extrusion (ECAE) were investigated. The aim of the study was to reveal the effect of severe plastic deformation on the interplay between plastic deformation via dislocation slip and twinning and forward and reverse martensitic transformation. The samples were processed at room temperature, i.e. slightly above the austenite finish temperature, and at 450°C, i.e. well above the austenite finish temperature. Transformation temperatures and multiple step transformation after processing and after low-temperature annealing were studied. The unique findings were: (1) the observation of a mixture of heavily deformed B2 (austenite) and B19′ (martensite) phases in the samples processed at room temperature, although martensite stabilization was expected; (2) the observation of highly organized, twin-related nanograins in the B2 phase of the samples deformed at room temperature, which was attributed to the

(SIM, stress-induced martensitic transformation; SPD, severe plastic deformation) transformation sequence; and (3) the simultaneous observation of B2 austenite and strain-induced B19′ martensite in the samples deformed at 450°C. Strain-induced martensite in NiTi alloys is reported for the first time. The formation of well-organized, twin-related nanograins via severe plastic deformation opens a new opportunity for twinning-induced grain boundary engineering in NiTi alloys, which is believed to improve the cyclic stability and fatigue resistance of these alloys.

Acknowledgements

This work was supported by the National Science Foundation under the contract CMS 01-34554, the Solid Mechanics and Materials Engineering Program, Directorate of Engineering, Arlington, Virginia. Mr. Robert Barber is gratefully acknowledged for his valuable assistance with ECAE processing.