Influence of quasi-continuous ECAP with various channel intersection angles on the structure formation, mechanical and functional properties of Ti-Ni shape memory alloys

Abstract

Influence of quasi-continuous ECAP with channel intersection angles of 110 and 120° on the structure formation and properties of Ti-50.1 at% Ni shape memory alloy was studied and compared. The structure was studied using X-ray diffraction analyses and transmission electron microscopy. The mechanical properties were determined by the uniaxial tensile and hardness tests. The maximum completely recoverable strain and temperatures of reverse martensitic transformation were estimated by a thermomechanical method using a bending mode for strain inducing. After ECAP with channel intersection angle of 120° for 3 passes at 400 °C a mixed ultrafine-grained structure with high density of free dislocations and incompletely equiaxed structural elements size of about 130-150 nm is formed. In comparison, after ECAP with channel intersection angle of 110° an ultrafine-grained structure with deformation bands elongated in the direction perpendicular to the sample axis, consisting of the less uniform equiaxed and ellipsoid structural elements with the size of about 50-150 nm is obtained. After both studied ECAP regimes samples have relatively high values of hardness (220 HV) and strength characteristics (σy = 850/1000 MPa, σB = 1040/1020 MPa, 120/110°). Maximum completely recoverable strain of 7.1 % is obtained after ECAP with channel intersection angle of 110°, and post-deformation annealing at 400 °C, 1 h.