Formation of nanocrystalline structure in the amorphous Ti50Ni25Cu25 alloy upon severe thermomechanical treatment and the size effect of the thermoelastic martensitic B2 ↔ B19 transformation

• Published in: Physics of metals and metallography 2012-03, Vol.113 (3), p.271-282
• Main Authors: Pushin, V. G., Kuranova, N. N., Pushin, A. V., Valiev, E. Z., Kourov, N. I., Teplykh, A. E., Uksusnikov, A. N.
• Format: Article
• Language: English
• Subjects: Chemistry and Materials Science; Diffusion; Materials Science; Metallic Materials; Phase Transformations; Structure
• ISSN: 0031-918X; 1555-6190
• DOI: 10.1134/S0031918X1203012X

The results of the comparative analysis of the Ti 50 Ni 25 Cu 25 -alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain-induced amorphization even with the retention, even after 5–15 rev, of a large number of highly dispersed nanocrystals less than 3–4 nm in size with a distorted B 2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low-temperature nanocrystallization during subsequent annealing starting from 250–300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B 2-type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B 2-austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B 2 alloy has been established.