Effect of temperature-deformation regimes of equal channel angular pressing in core-shell mode on the structure and properties of near-equiatomic titanium nickelide shape memory alloy

Abstract

Near-equiatomic titanium nickelide (TiNi) shape memory alloy is subjected to thermomechanical treatment by quasi-continuous equal channel angular pressing (ECAP) in core-shell mode at temperatures of 250, 300, and 400°С at the highest possible accumulated true strain. TiNi bulk samples are successfully deformed for the first time at 300°С – the temperature threshold for dynamic polygonization – at an accumulated true strain e of 2.4. Deformation at low temperatures leads to the premature destruction of TiNi samples in the second pass (e = 1.6), providing high deformation hardening and corresponding strength characteristics. An increase in the deformation temperature to 400°С allows an accumulated true strain of 5.6, while the mechanical and functional properties are inferior to those that occur after deformation at low temperatures. The most balanced combination of properties is achieved after quasi-continuous ECAP in core-shell mode at 300°С and subsequent post-deformation annealing at 400°С for 1 h: dislocation yield stress σy = 1040 MPa, ultimate tensile strength σв = 1166 MPa, and total recoverable strain Ɛrt = 10.3 %.