A study of the structure of G-P zones in Ti-rich TiNi shape memory melt-spun ribbons

Abstract

A hybrid structure composed of ordered C11 b Ti 2 Ni and Ti 5 Ni 3 substructures for the G-P zones precipitated in Ti-rich Ti-Ni shape memory ribbons is proposed to explain the appearance of extra diffuse scattering streaks in transmission electron microscopy observations. The point defects of Ni vacancies and Ti antisites, resulting in the lattice fluctuant displacement, also cause the incommensurate extra diffuse scattering streaks. IntroductionThin films/ribbons of Ti-rich TiNi-based shape memory alloys (SMAs) fabricated by sputtering or melt-spinning have been the subject of much recent research, since their intrinsic large recovery strain and force can be used as powerful microactuators in microelectromechanical systems. TiNi-based SMAs are known to undergo B2→B19′ martenstitic transformation upon cooling. However, an intermediate R-phase (rhombohedral) transformation can be induced easily by the stress field around precipitates, which will depress the B19′ transformation temperature simultaneously. When the R-phase transformation is induced, TiNi SMA undergoes a two-stage B2→R→B19′ transformation. As a result, an intermediate R-phase transformation may occur prior to B19′ martensitic transformation when the stress field is established around the precipitates [1]. Thin coherent plate-like Ti-rich Guinier-Preston (G-P) zones and the nano-scale disc-like Ti 2 Ni precipitates are found in either sputtered Ti-rich TiNi films heated at various temperatures [1][2][3] or in melt-spun ribbons with or without annealing [4]. Studies using high-resolution transmission electron microscopy (HRTEM) have shown that the plate-like G-P zones contain few epitaxial layers and have perfect coherent interfaces with the B2 matrix [5,6]. The existence of Ti-rich G-P zones in Ti-rich TiNi films/ ribbons is quite interesting since they increase the yield stress for slip deformation, because the coherent strain fields around them resist the dislocation movements, resulting in good shape memory properties [6,7]. The structure of G-P zones is presumed to be a body-centred tetragonal (BCT) lattice, equivalent to the C11 b ordered structure for

On the nature of point defect and its effect on electronic structure of rocksalt hafnium nitride films

“…4d). The present authors find that the Burgers circuits drawn around these core of void spaces are completely closed, indicating that the observed void spaces do not originate from certain dislocation, but more possibly from V Hf and their cluster [40,41]. These findings from HRTEM images agree well with GIXRD and Raman measurements, consistently proving that, in over-stoichiometric d-HfN x films, the dominant point defects are V Hf .…”

Coherent Precipitates as a Condition for Ultra-Low Fatigue in Cu-Rich Ti53.7Ni24.7Cu21.6 Shape Memory Alloys

“…However, they likely contribute to the increased background between 38°and 44°2h for the austenite state and increased background at the position where typically the B2 phase is present for the martensite phase. It was pointed out by several authors that the tetragonality of the GP zones can vary strongly and thereby also their lattice parameter, which contributes to this diffuse scattering [33].…”