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Habit plane variants (HPVs) are traditionally used as the fundamental microstructure unit in micromechanical models of shape memory alloys. Recently, an approach using Correspondence Variants (CVs) has emerged. Previous research has shown that HPVs cannot completely explain superelastic deformation of Nitinol, especially beyond initiation of the martensitic phase transformation that gives rise to superelasticity. In this work, a statistical comparison of both modeling approaches is presented for the case of superelastic Nitinol subjected to uniaxial tension and uniaxial compression. Specifically, volume fractions of variants that contribute to deformation were calculated using the two models and then compared. The relative differences were found to be about 30% in tension and 10% in compression at high strains. Allowing detwinning to occur in the HPV model reduced the relative differences in CV volume fractions to about 20% in tension. [ABSTRACT FROM AUTHOR]

Plastic deformation of B2 Nickel Titanium is usually attributed to { 110 } 〈 001 〉 slip and { 114 } 〈 221 〉 deformation twinning. The most commonly observed hot-worked texture of these alloys, is defined by a { 111 } 〈 u v w 〉 gamma fiber and { h k l } 〈 110 〉 partial alpha fiber. A Visco-Plastic Self Consistent (VPSC) model was used to establish relationships between microscopic slip and twin activity with the observed macroscopic hot-rolling texture. This knowledge will better aid in modeling NiTi austenite plasticity. Since the primary slip modes in NiTi do not have five independent slip systems, and deformation that can be accommodated by twinning is limited, multiple deformation modes must contribute to NiTi ductility. It is shown that { 110 } 〈 001 〉 slip, { 100 } 〈 001 〉 slip and, { 114 } 〈 221 〉 twin deformation modes need to be active simultaneously to explain the observed textures. The relative CRSS ratios and hardening parameters were varied to study the effect of the deformation modes on the various texture components. Textures observed below 723 K and at less than 80% rolling reduction were simulated with deformation primarily accommodated on the { 110 } 〈 001 〉 slip mode and { 114 } 〈 221 〉 twinning mode. Textures observed at temperatures greater than 903 K and greater than 80% rolling reductions were captured in the simulations that included all three deformation modes. Activity on the { 100 } 〈 001 〉 slip mode strongly correlated with the { 110 } 〈 110 〉 texture component observed at high temperatures. [Display omitted] • Single deformation modes do not explain hot-deformed textures in NiTi. • Textures are explained only if {110} slip, {100} slip and {114} twin modes act simultaneously. • The {112} 〈 110 〉 texture component correlates to {110} slip with a Pearson coefficient of 0.78. • Textures observed above 903 K and 90 % reduction strongly correlate to {100} slip. • {100} slip strongly correlates to the {110} 〈 110 〉 texture component with a Pearson coefficient of 0.91. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical investigation of the transformation temperature and transformation strain behaviors of a promising new Ni50.3Ti29.7Hf20 high-temperature shape memory alloy was conducted. Actuation behavior of single crystals with loading orientations near [001]B2, [1¯10]B2, and [111]B2, as well as polycrystalline material in aged and unaged conditions was studied, together with the superelastic, polycrystalline torsion response. These results were compared to analytic calculations of the ideal transformation strains for tension, compression, and torsion loading of single crystals as a function of single crystal orientation, and polycrystalline material of common processing textures. H-phase precipitates on the order of 10-30nm were shown to increase transformation temperatures and also to narrow thermal hysteresis, compared to unaged material. The mechanical effects of increased residual stresses and numbers of transformation nucleation sites caused by the precipitates provide a plausible explanation for the observed transformation temperature trends. Grain boundaries were shown to have similar effects on transformation temperatures. The work output and recoverable strain exhibited by the alloy were shown to approach maximums at stresses of 500-800MPa, suggesting these to be optimal working loads with respect to single cycle performance. The potential for transformation strain in single crystals of this material was calculated to be superior to binary NiTi in tension, compression, and torsion loading modes. However, the large volume fraction of precipitate phase, in part, prevents the material from realizing its full single crystal transformation strain potential in return for outstanding functional stability by inhibiting plastic strain accumulation during transformation. Finally, calculations showed that of the studied polycrystalline textures, [001]B2 fiber texture results in superior torsion performance, while [011]B2 fiber texture results in superior tensile behavior, and both [011]B2 and random textures will result in the best possible compression performance. [ABSTRACT FROM AUTHOR]

Abstract: Mechanical responses and texture developments were observed in situ during the creation of multiaxial stress states in polycrystalline NiTi parallelepiped specimens, achieved via sequential compression along unique principal axes. For all of the compression stages, regardless of initial texture, the two major texture components behaved similarly: (100) poles aligned with, while (011) poles oriented perpendicular to, the loading direction. The effective critical resolved shear stress needed to induce significant reorientation, however, was reduced through prior loading. In the macroscopic responses, prior transverse direction loading resulted in widening of the reorientation plateau, a reduction of the effective Young’s modulus, and substantial alteration of effective Poisson’s ratios during axial direction straining. Additionally, these empirical results are presented in a manner conducive to the verification of shape memory alloy micromechanics and continuum mechanics constitutive models. [Copyright &y& Elsevier]