Crystal Structure Defects in Titanium Nickelide after Abc Pressing at Lowered Temperature

Abstract

The experimental results regarding the effect of warm (573 K) abc pressing with an increase in the specified true strain, e, up to 9.55, on the microstructure and crystal structure defects (dislocations, vacancies) of the Ti49.8Ni50.2 (at %) alloy are presented. It is shown that all samples (regardless of e) have a two-level microstructure. The grains–subgrains of the submicrocrystalline scale level are in the volumes of large grains. The average sizes of both large grains and subgrain grains decrease with increasing e to 9.55 (from 27 to 12 µm and from 0.36 to 0.13 µm, respectively). All samples had a two-phase state (rhombohedral R and monoclinic B19’ martensitic phases) at 295 K. The full-profile analysis of X-ray reflections of the B2 phase obtained at 393 K shows that the dislocation density increases from 1014 m−2 to 1015 m−2 after pressing with e = 1.84 and reaches 2·1015 m−2 when e increases to 9.55. It has been established by positron annihilation lifetime spectroscopy that dislocations are the main type of defects in initial samples and the only type of defects in samples after abc pressing. The lifetime of positrons trapped by dislocations is 166 ps, and the intensity of this component increases from 83% in the initial samples to 99.4% after pressing with e = 9.55. The initial samples contain a component with a positron lifetime of 192 ps (intensity 16.4%), which corresponds to the presence of monovacancies in the nickel sublattice of the B2 phase (concentration ≈10−5). This component is absent in the positron lifetime spectra in the samples after pressing. The results of the analysis of the Doppler broadening spectroscopy correlate with the data obtained by the positron annihilation lifetime spectroscopy.

Effect of Severe Torsion Deformation on Structure and Properties of Titanium–Nickel Shape Memory Alloy

“…The deformation of titanium nickelide via ECAP and abc-pressing under similar deformation-temperature conditions (T = 450 • C, e = 5-10) resulted in the formation of a B2-austenite structure with an average grain/subgrain size of 200-600 nm [36][37][38][39][40]. This finding correlates well with the results obtained and indicates the effectiveness of the STD mode in terms of obtaining an ultrafine-grained structure.…”

Structure and Properties of TiNi Shape Memory Alloy after Quasi-Continuous Equal-Channel Angular Pressing in Various Aged States

“…The deformation of samples after pre-aging for 1 h allows the formation of a somewhat more finely dispersed structure with an average size of its elements (grain and subgrain) of 92 nm, as compared to that of samples after pre-aging for 5 h (115 nm). It should be noted that, in itself, obtaining a mixed nanograined and nanosubgrained structure in titanium nickelide bulk samples is an important scientific and technological result [27][28][29]. During pre-aging for 1 h, the growth of Ti 3 Ni 4 phase particles proceeds less intensively than that after pre-aging for 5 h [30][31][32].…”