The paper reports on an experimental study of inelastic strain recovery, due to superelasticity and shape-memory effects, and plastic deformation under torsion and bending in a binary TiNi alloy containing 50.9 at % of Ni. It is found that the material under torsion and bending displays an anomalously high superelasticity of 16.4% and 13.5% and total recovery strain of 24.6% and 16.4% which exceed the theoretical limit of recoverable strain equal to 10.3% in the alloy involved in a cubic B2 to monoclinic B19’ phase transformation. The correlation between the high recovery strains and the intensity and mechanisms of plastic deformation with increasing total strain is discussed.

The regularities of the ultrafine-grained structure formation in the binary TiNi-based alloy under stepwise warm (673–573 K) abc pressing are presented. The appearance of a mixed type microstructure based on the submicrocrystalline and nanograined components is established. The preferential formation of nanograins inside the volumes of microbands of localized deformation is observed. The phase composition and martensitic transformations at 295 K in the titanium nickelide samples produced by the warm abc pressing are studied.

Shape Memory Alloys: Properties, Technologies, Opportunities: Mechanisms of Microstructure Evolution in TiNi-Based Alloys under Warm Deformation and its Effect on Martensite Transformations

The effect of temperature on the ultimate tensile strength (σuts) and fracture elongation (δ) under tension of titanium nickelide (50.2 at.% Ni) with initial coarse-grained structure and structures after abc-pressing at 873K (alloy 1, true deformation е = 2.2) and after “step-by-step” pressing at 873K→673K (alloy 2, е = 4.2) are studied. The grain dimensions in the samples of alloy 1 (2-40 μm) are smaller than ones in initial samples (20-70 μm). The structure of alloy 2 is submicrocrystalline (grain/subgrain dimensions are 100-700 nm). It was found that the martensitic transformations (MT) B2←→B19ʹ had been observed under cooling and heating of initial samples. The sequences of В2→R→В19ʹ→В2 MT were observed in alloys 1 and 2 (R – the rhombohedral martensitic phase). The finish of B19ʹ→B2 MT take place in the temperature range 340-355 K. The tension tests were carried out at temperatures 293K, 343K and 473K (or 523 K), i.e. in the B19ʹ phase, pretransition temperature range and B2 phase, accordingly. The “stress-strain” dependences obtained at these temperatures are quantitavely the same for all samples. The martensitic plateau connected with the reorientation and/or generation of the B19ʹ martensitic crystallites are observed under tension of samples at 293K and 343K. No the generation of stress-induced B19ʹ martensite was observed under tension at 473K (or 523K). The increase of σuts was found after abc-pressing. The maximum values of σuts are observed under tension of samples of alloys 1 and 2 at 293K and 343K (in B19ʹ phase). The alloy 1 demonstrates the highest plasticity (δ about 80%) at temperature range 293-343 K. The σuts and δ decrease essentially (about 2 times) with the temperature increase to 473K (or 523K).