Effect of Quasi-Continuous Equal-Channel Angular Pressing on Structure and Properties of Ti-Ni Shape Memory Alloys

Abstract

The effect of equal-channel angular pressing (ECAP) in quasi-continuous mode on the structure formation and mechanical and functional properties of a near-equiatomic Ti-Ni shape memory alloy (SMA) was studied in this work. ECAP with channel intersection angles of 110° and 120° was carried out at a temperature of 350-450 °C for 2-7 passes. Optimum deformation temperatures of ECAP in quasi-continuous mode are determined as 400 °C for ECAP with a channel intersection angle of 120° and 450 °C for 110°. ECAP with a channel intersection angle of 110° at a temperature of 450 °C yields high values of strength (yield stress σy = 1,090 MPa, ultimate tensile strength σв = 1,150 MPa) and functional (maximum value of completely recoverable strain of 7.5% after ECAP and 8.4% after the addition of post-deformation annealing (PDA) at 400 °C for 1 h) characteristics. With the increase in the deformation temperature of quasi-continuous ECAP with a channel intersection angle of 110° from 350 to 450 °C, structure-morphological transformation in Ti-Ni SMA occurs. The shape of structural elements (grains and subgrains) changes from elongated to equiaxed; the size of the structural elements increases from less than 100 nm to 100-250 nm.