Influence of Heat Treatment and Deformation on the Structure, Phase Transformation, and Mechanical Behavior of Bulk TiNi-Based Alloys

Abstract

We present a brief overview of the structural and phase transformations and mechanical properties of bulk binary TiNi shape memory alloys, which demonstrate attractive commercial potential. The main goal of this work was to create a favorable microstructure of bulk alloys using both traditional and new alternative methods of thermal and thermomechanical processing. It was found that the implementation of an ultrafine-grained structure by different methods determined an unusual combination of strength, ductility, reversible deformation, reactive resistance of these alloys to subsequent tensile or torsion tests at room temperature, and, as a consequence, the highly reversible effects of the shape memory and superelasticity. It is shown that the alloys Ti49.8Ni50.2 and Ti49.4Ni50.6 are incapable of aging, and, after being subjected to ECAP, were characterized by their high strength (σu up to 1200 MPa) and ductility (δ up to 60–70%). A combined treatment of multi-pass rolling and HT of the Ti49.5Ni50.5 and Ti49Ni51 alloys prone to aging have provided even greater strength (σu up to 1400–1500 MPa) with slightly lower ductility (25–30%). The microstructure, phase composition, and martensitic transformations in Ti-Ni alloys with varying Ni concentrations ranging from 50 to 51 wt.% were investigated by TEM, SEM, and X-ray methods. The mechanical behavior of the alloys was studied during tensile and torsion tests.