Elastic properties of B2-NiTi and B2-PdTi

Origin of the phase separation into B2 and L21 ordered phases in X-Al-Ti (X: Fe, Co, and Ni) alloys based on the first-principles cluster variation method

The phase separation behaviors from a single B2 ordered phase into two separate B2 and L2₁ ordered phases in X-Al-Ti (X: Fe, Co, and Ni) alloys are analyzed utilizing the cluster variation method (CVM), based on interaction energies derived from electronic band structure calculations. A cubic approximation of the CVM is adopted for X₂Al_2-x Ti _x ([Formula: see text]) alloys limiting the interchange between Al and Ti atoms on the [Formula: see text]- and [Formula: see text]-sublattices of an L2₁ ordered structure with X atoms fixed on the [Formula: see text]-sublattice. The phase stabilities of the B2 and L2₁ structures are examined, and phase diagrams at the pseudo-binary section, XAl-XTi, are determined. The two-phase regions of the B2 and L2₁ phases (i.e. phase separation behavior) are successfully produced in Co- and Ni-Al-Ti alloy systems, and no phase separation is observed in the Fe-Al-Ti alloy. The origins of phase separation in the Co- and Ni-Al-Ti alloys are mechanical instability and a combination of mechanical instability and chemical repulsions of unlike pairs, respectively.

An in situ synchrotron study of the localized B2↔B19′ phase transformation in an Ni–Ti alloy subjected to uniaxial cyclic loading–unloading with incremental strains

High-resolution in situ synchrotron X-ray diffraction was applied to study a cold-drawn and solution-treated 56Ni–44Ti wt% alloy subjected to uniaxial cyclic loading–unloading with incremental strains. The micro-mechanical behaviour associated with the partial and repeated B2↔B19′ phase transformation at the centre of the sample gauge length was studied with respect to the macroscopic stress–strain response. The lattice strains of the (110)B2 and different B19′ grain families are affected by (i) the transformation strain, the load-bearing capacity of both phases and the strain continuity maintained at/near the B2–B19′ interfaces at the centre of the gauge length, and (ii) the extent of transformation along the gauge length. With cycling and incremental strains (i) the elastic lattice strain and plastic strain in the remnant (110)B2 grain family gradually saturate at early cycles, whereas the plastic strain in the B19′ phase continues to increase. This contributes to accumulation of residual strains (degradation in superelasticity), greater non-linearity and change in the shape of the macroscopic stress–strain curve from plateau type to curvilinear elastic. (ii) The initial 〈111〉B2 fibre texture transforms to [120]B19′, [130]B19′, [150]B19′ and [010]B19′ orientations. Further increase in the applied strain with cycling results in the development of [130]B19′, [102]B19′, [102]B19′, [100]B19′ and [100]B19′ orientations.

Compton profiles and nesting of Fermi surfaces for B2-TiNi and B2-TiPd

The band structures of the shape memory alloys B2-TiNi and B2-TiPd are calculated by the full potential linearized augmented plane wave method with the local density approximation. The theoretical Compton profiles for B2-TiNi and B2-TiPd are calculated. In addition, the three-dimensional (3D) occupation number densities obtained by Lock-Crisp-West (LCW) analysis are presented for the first time. These 3D occupation number densities are in good agreement with the Compton scattering experiment for TiNi. Both shape memory alloys are based on martensitic transformation, which is caused by soft phonons. The charge-density wave is created by nesting of Fermi surfaces, which leads to phonon softening. To examine the nesting vectors quantitatively, we calculate the generalized susceptibility χ(q). χ(q) shows peaks at 0.315[110]2π/a and 0.4[111]2π/a for TiNi and at 0.275[110]2π/a and 0.395[111]2π/a for TiPd. Although the nesting vector in the [110] direction agrees with that from the phonon experiment, the nesting vector in the [111] direction differs from that in the experimental results.