The influence of various thermomechanical treatment schemes, which include rotary forging and severe plastic deformation by equal-channel angular pressing, on the structure and the mechanical and func

The effect of severe plastic deformation by equal-channel angular pressing (ECAP) under normal and quasi-continuous regimes on the structure and the mechanical and functional properties of a Ti–50.2 at % Ni shape-memory alloy (SMA) has been studied. ECAP was carried out at an angle of intersection of channels of 120° in the normal regime with heating between passes at 450°C for 20 passes and in the quasi-continuous regime at the temperature of 400°C for three, five, and seven passes. The hot screw rolling with subsequent annealing at 750°C for 30 min and cooling in water was used as a control treatment (CT). A mixed submicrocrystalline and nanosubgrained structure was formed. The increase in the number of passes from three to seven led to a decrease in the average size of structural elements from 115 ± 5 to 103 ± 5 nm and to an increase in the fraction of grains/subgrains having a size less than 100 nm. After ECAP (seven passes) and post-deformation annealing at the temperature of 400°C for 1 h, a completely recoverable strain was 9.5%; after normal ECAP, 7.2%; after CT, 4.0%.

The effect of post-deformation annealing (PDA) of nickel-enriched Ti-50.9 at.% Ni shape memory alloy deformed in various initially deformation-induced structure states on the structure and properties was studied. The phase composition, temperature ranges of martensitic transformations and structure were studied using X-ray diffractometry and TEM. Mechanical and functional properties were determined using Vickers hardness tests and thermomechanical method using a bending mode for recovery strain inducing. The PDA at 430 °C (1 h) of the nickel-enriched Ti-Ni SMA with the dynamically recovered, dynamically polygonized or dynamically recrystallized structures after compressing deformation leads to the precipitation of finely dispersed (nanosized) Ti₃Ni₄ particles. The most significant increase of the completely recoverable strain (from 8.8 to 11.8%) and shape recovery rate (from 88 to 100%) as compared to the reference treatment is observed after PDA of the alloy with the dynamically polygonized structure.

Ti-Ni shape memory alloys were subjected to a rotary forging (RF) from the diameter of 20 or 12 mm to 5 mm at relatively low temperatures of 450 and 350 °C. The structure was studied using X-ray diffraction analysis and transmission electron microscopy. Mechanical properties were determined by uniaxial tensile and Vickers hardness tests, and the maximum completely recoverable strain by a thermomechanical method. The obtained results showed that RF at low temperatures allows manufacturing 3 m-long Ti-Ni rods. A mixed submicrocrystalline and nanosubgrained structure with an average size of structural elements of 118 ± 7 nm after forging from 20 mm, and 126 ± 7 nm after forging from 12 mm at 350 °С provides the highest mechanical (σ_в = 1113 MPa after forging from 20 mm, and 1065 MPa after forging from 12 mm) and functional (maximum completely recoverable strain of 8.0 %, after post-deformation annealing at 400 °С, 1 h) properties as compared to the initial state before RF (700 MPa, and 4.4 %, respectively).