Effect of Isochronous Annealings on the Microstructure and Mechanical Properties of the Ti49.8Ni50.2 (at.%) Alloy after abc Pressing at 573 K

Abstract

The regularities and features of the evolution of the grain-subgrain structure, phase composition and mechanical properties in Ti49.8Ni50.2 (at.%), depending on the temperature of isochronous annealings at 573-973 K are herein studied. The state of the Ti49.8Ni50.2 (at.%) alloy samples after abc pressing at T = 573 K with the given true strain e = 9.55 was taken as the initial state. It is shown that the grain-subgrain structure of the samples after annealing for 1 h in the temperature range of 573-673 K changes slightly. In samples annealed at 673 K, regions with the microband structure similar to the microstructure of a fast-frozen turbulent liquid flow were found. It has been established that during annealing at 773 K the beginning of an active recrystallization process is realized; the size of grains does not exceed the submicrocrystalline scale (~200 nm). At 873 K, the recrystallization process occurs in the entire volume of the samples; the grains with an average size of 2 ± 0.5 µm are almost equiaxed. The microstructure of the samples after annealing at 973 K (with average grain sizes of 5 ± 0.5 µm) is qualitatively similar to the microstructure of the samples after annealing at 873 K. It was found that the phase composition of the samples as a result of isochronous annealing at 573-973 K changes from R and B19' immediately after abc pressing to a three-phase state: B2, R and B19' phases. It is shown that the highest values of yield stress σy, ultimate tensile strength σUTS (1043 MPa and 1232 MPa, correspondingly) and low ductility (the deformation to fracture εf = 48%) are observed in the initial samples. Increasing the temperature of post-deformation annealing and, correspondingly, the development of recrystallization, led to a decrease in σy, σUTS and an increase in εf to the values of these characteristics in the coarse-grained samples (σy = 400 MPa, σUTS = 920 MPa and εf = 90%).