Thermal stability and high-temperature shape memory characteristics of Ti–20Zr–10Ta alloy

The microstructure, martensite transformation behavior, thermal stability and shape memory behavior of Ti–20Zr– 10Ta high temperature shape memory alloy were investigated. The Ti–20Zr–10Ta alloy exhibited a reversible transformation with the high martensite transformation temperature of 500oC and good thermal stability. The alloy displayed the elongation of 15% and a maximum recovery stain of 5.5% with 8% pre-strain.     

Incommensurate–commensurate transition and nanoscale domain-like structure in iron doped Ti–Ni shape memory alloys

Precursor phenomena of displacive transformation have been studied by optical and transmission electron microscope observation and X-ray diffraction of Ti–(50???x)Ni–xFe (x?=?2,?4,?6,?8 in at.%) alloys. We found that a Ti–44Ni–6Fe alloy exhibits a second-order-like incommensurate–commensurate transition without latent heat and discontinuity in lattice parameters. In other words, diffuse scatterings appear in an electron diffraction pattern at an incommensurate position on cooling; they move gradually towards 1/3? 110? as the temperature decreases and lock into the commensurate position at 180?K. The commensurate phase is not expanded along one of the ? 111? directions, unlike the R-phase formed by a first-order transformation in Ti–48Ni–2Fe and Ti–46Ni–4Fe alloys. In addition, the commensurate phase shows a nanoscale domain-like structure, which is inherited from the incommensurate state of the parent phase. Thus, the anomalies in physical properties observed in the incommensurate state are most likely the precursor phenomena of the commensurate phase in the Ti–44Ni–6Fe alloy. In the case of a Ti–42Ni–8Fe alloy, the incommensurate state remains even at 19?K.     

Differences between the R-phase and the commensurate phase in iron-doped Ti–Ni shape memory alloys

A Ti–44Ni–6Fe (at. %) alloy was recently reported to exhibit a second order-like incommensurate–commensurate transformation. Whether or not the commensurate (C) phase and the R-phase in Ti–Ni alloys are the same phase has now been investigated. Transformation behaviour was examined in a series of Ti–(50 ? x)Ni–xFe (at. %) alloys with x = 2.0, 4.0, 5.0, 5.5, 5.7, 6.0. Transformation temperature, entropy change and the transformation strain decrease continuously as x increases up to x = 5.7, where the product phase is apparently the R-phase. However, there is an obvious discontinuity in these values between x = 5.7 and x = 6.0. In addition, electron diffraction experiments have revealed that the intensity of satellite reflections of the C-phase is obviously weaker than that of the R-phase. These results strongly suggest that the C-phase is different from the R-phase.     

Atomistic modeling of ternary additions to NiTi and quaternary additions to Ni–Ti–Pd,Ni–Ti–Pt and Ni–Ti–Hf shape memory alloys

The behavior of ternary and quaternary additions to NiTi shape memory alloys is investigated using a quantum approximate method for the energetics. Ternary additions X to NiTi and quaternary additions to Ni–Ti–Pd, Ni–Ti–Pt, and Ni–Ti–Hf alloys, for X=Au, Pt, Ir, Os, Re, W, Ta,Ag, Pd, Rh, Ru, Tc, Mo, Nb, Zr, Zn, Cu, Co, Fe, Mn, V, Sc, Si, Al and Mg are considered. Bulk properties such as lattice parameter, energy of formation, and bulk modulus of the B2 alloys are studied for variations due to the presence of one or two simultaneous additives.     

Structure and thermoelastic martensitic transformations in ternary Ni–Ti–Hf alloys with a high-temperature shape memory effect

The effect of alloying by 12–20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi–NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic B2 ? B19’ martensitic transformations, which occur in the temperature range 320–600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the B2 and B19’ phases are measured, and the microstructure of the B19’ martensite is analyzed.