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

Abstract

Abstract-Methods of X ray diffraction, transmission and scanning electron microscopy, and selected area electron diffraction (SAED) have been used to study the phase and elemental composition and structure of alloys close to the stoichiometric Ti 50 Ni 25 Cu 25 alloy. Based on the method of rapid quenching of the melt (free jet melt spinning), alloys of the quasi binary TiNi-TiCu section have been prepared, which in the ini tial as cast state exhibited the thermoelastic martensitic transformations B2 ↔ B19 and related shape mem ory effects. The chemical composition of the Ti 50 + x Ni 25 Cu 25 -x alloys was varied by changing titanium and copper concentrations within x ≤ ±1 at % (from Ti 49 Ni 25 Cu 26 to Ti 51 Ni 25 Cu 24 ). It has been established that quenching at a cooling rate equal to 10 6 K/s leads to the amorphization of all the alloys under consideration. Heating to 723 K and higher leads to the devitrification of the alloy with the formation of a nanocrystalline or submicrocrystalline structure of the B2 austenite. The mechanical properties of these alloys have been measured in the initial amorphous state and in the polycrystalline martensitic state. It has been shown that, depending on the extent of the deviations of the alloy composition from the stoichiometry, which cause the decomposition of the alloys in the process of nanocrystallization, regular changes are observed in their mechanical properties and in the shape memory effects. The kinetics of the processes of the devitrification of the alloys, as well of the forward and reverse martensitic transformations, have been studied, their charac teristic temperatures have been determined, and a diagram of the dependence of the characteristic tempera tures on the chemical composition of the alloys has been constructed.Keywords: Ti 2 NiCu based alloys (Ti 50 Ni 25 Cu 25 ), rapid quenching of alloys by free jet melt spinning, nanoc rystals, thermoelastic martensitic transformations, shape memory effects

Design and Development of Ti–Ni, Ni–Mn–Ga and Cu–Al–Ni-based Alloys with High and Low Temperature Shape Memory Effects

“…The influence of the alloy decomposition at temperatures of TMTs is a known important fact noted already in the discussion on the B2 alloys Ti–Ni–Cu [44,45]. At the same time, it is obvious that not only the mechanical properties of the alloys were changed, but also the temperature characteristics of the TMTs decreased.…”

“…Adding Cu up to 15 at.% decreased all martensite temperatures of TMTs B2 ↔ B19′, but slightly increased the temperatures of the TMTs B2↔B19 (Table 2). Starting from Cu 25 at.%, the Ti 50 Ni 50−x Cu x alloys undergo eutectoid decomposition with the formation of equiaxial and lamellar dispersed particles of the B11-TiCu phase [44,45].…”

“…In commercial synthesis of multicomponent materials it is difficult to obtain large-volume ingots with high-precision chemical composition and at the same time to avoid its uncontrolled deviation. Of course, it is necessary to take into account this fact in the design and development of new alloys, for example, because of a possible decomposition that determines significant changes in their physical and mechanical properties and, especially, ductility, in nonstoichiometric TiNi-based alloys [32,38,42,43,44,45,46]. In the alloys with high-temperature SMEs, the effect of homogeneous and/or heterogeneous decomposition is also responsible for changes in the properties during their exploitation [47,48,49,50].…”

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].…”

Specific features of the electronic properties of Ti50Ni50–x Cu x alloys with the shape memory effect

“…It can be due to that a Ti-Cu compound was not isomorphic to a Ti-Ni compound and had the trigonal B11 lattice. Because of this, incomplete reversibility of ρ(T) in these alloys, in which the precipitation of the B11phase particles in the TiNi-TiCu quasi-binary section is determined by the solubility limit at copper content x ≈ 25 at %, can be due to the formation of metastable clusters or dispersed phases enriched in excess elements, because of the narrow homogeneity region of the B2 solid solution with respect to variations of their chemical compositions in titanium and nickel or titanium and copper [16,17].…”