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

Abstract

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 B2 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 B2 type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2 austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.

Stability of an Amorphous TiCuNi Alloy Subjected to High‐Pressure Torsion at Different Temperatures

“…Analysis also demonstrates that in the MS Ti 50 Ni 25 Cu 25 alloy, HPT processing and subsequent annealing produce markedly finer grains than annealing of the initial MS alloy . For instance, the grain size in the MS Ti 50 Ni 25 Cu 25 alloy after annealing at 450 °C for 10 min reaches 1.5 µm (with martensite plate observed within the grains) (Figure a), whereas in the MS Ti 50 Ni 25 Cu 25 alloy subjected to HPT and subsequent annealing at 450 °C the grain size is below 100 nm (Figure b).…”

“…For example, in the case of Nd–Fe–B MS amorphous alloys, HPT processing leads to a separation of elements and nanocrystallization of α‐Fe‐nanocrystals (with a size of ≈20 nm) in the amorphous phase . At the same time, HPT processing of the melt‐spun amorphous Ti–Ni–Cu alloy does not lead to intensive nanocrystallization . In this connection, of great interest are more detailed studies of the evolution of the amorphous structure in this Ti–Ni–Cu alloy during HPT processing and its influence on the alloy's properties.…”

Evolution of the amorphous structure in melt-spun Ti50Ni25Cu25 alloy subjected to high pressure torsion deformation

“…It should be noted that the glass transition temperature of MS TiNiCu amorphous alloy is about 320 С, the crystallization temperature is 450 С [24,25], which is noticeably higher than the highest employed temperature of HPT processing (150 С). However, heating during HPT processing can play in important role [26,27].…”

Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

“…According to neutron diffraction data, the degree of long range atomic order η in this alloy is equal to 0.90 ± 0.05 [22]. No diagrams of phase equilibria in these ternary alloys have been revealed in the literature, except for the TiNi-TiCu quasi binary section.…”

“…The necessity of obtaining substantially higher strength properties of the alloys in the form of small dimension semiproducts with the retention or even improvement of a complex of functional parameters of the SME has initiated one of the approaches, prima rily in the cases of ternary Ti-Ni-Cu alloys, that is based on the use of methods of ultrarapid quenching from the melt (RQM), in particular, the method of free jet melt spinning, to produce very long, very thin ribbons with an SME [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”