Dynamical properties of premartensitic NiTi

Physical metallurgy of Ti–Ni-based shape memory alloys

“…The TA 2 branch of a binary Ti-Ni alloy determined by Tietze et al [175] suggests that the 1/3 TA 2 phonon reaches zero energy (i.e., complete softening) at transformation temperature. However, this seems to be inconsistent with the fact that R-phase transformation or B19 0 transformation is of first order [181], for which complete mode-softening cannot be expected.…”

“…Elastic constant measurements [166][167][168][169][170][171][172]72] always show a softening of elastic constant c 0 prior to martensitic transformations; Neutron scattering experiment in most cases [173][174][175][176][177][178] confirm a low-lying TA 2 acoustic phonon which further softens with approaching transformation temperature. Elastic neutron scattering in most cases reveals a central peak [174,175], and X-ray diffraction reveals diffuse scattering and extra diffuse spots [179,104,97,107]. These suggest the existence of static martensite-like structure above the transformation temperature.…”

“…Such information is indispensable for a microscopic understanding of the instability of the parent phase, and for the identification of the order parameters of the martensitic transformation [199]. Due to the difficulty of preparing single crystals of Ti-Ni-based alloys, so far there has been only a limited number of reports on the phonon dispersion relation of Ti-Ni binary alloys [175][176][177] and ternary alloys Ti-Ni-Fe [173,174] and Ti-Ni-Cu [178,211]; some of them are incomplete. Fig.…”

Probing the Fermi surface by positron annihilation and Compton scattering

“…Inelastic neutron scattering in the hightemperature phase found a temperature-dependent softening of the TA 2 [110] phonon branch near a q vector of 1/3 [110] 2p/a, 93,94 this being a nesting vector in the Fermi surface calculated by Zhao and Harmon 95 (and which gives rise to an instability in their calculated phonon spectrum). By measuring Compton profiles along 28 different crystallographic directions in a single crystal of Ni 0.515 Ti 0.485 , and then reconstructing them using a direct Fourier method, 96 Shiotani et al were able to infer the Fermi surface.…”

Composition Dependences of Entropy Change and Transformation Temperatures in Ni-rich Ti–Ni System

“…5a, b together with reference data [6]. First, an anomalously small H D in the P phase is observed, thought to be the result of inherent lattice instability, which is referred to as the resultant of the softening of a TA 2 [110] phonon branch at the position q * 1/3[110]2p/a [23][24][25]. Qualitatively, a larger c suggests a larger electronic density of states (DOS) at the Fermi energy level (Dðe F Þ).…”