SUMMARY

Shape memory alloys (SMAs) are widely used in a broad variety of applications in multiscale devices ranging from nano-actuators used in nano-electrical-mechanical systems (NEMS) to large energy absorbing elements in civil engineering applications. This research introduces a multiscale analysis for SMAs, particularly Nickel-Titanium alloys (NiTi). In macroscale, a phenomenological constitutive framework is adopted and developed by adding the effect of phase transformation latent heat. Analytical closed-form solutions are obtained for modeling the coupled thermo-mechanical behavior of various large polycrystalline SMA devices subjected to different loadings, including uniaxial loads, torsion, and bending. Thermo-mechanical responses of several SMA devices are analyzed using the introduced solutions and the results are validated by performing various experiments on some large SMA elements. In order to study some important properties of polycrystalline SMAs that the macroscopic phenomenological frameworks cannot capture, including the texture and intergranular effects in polycrystalline SMAs, a micromechanical framework is used. Generalized coupled thermo-mechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples the effects of texture and the tension-compression asymmetry on the thermo-mechanical response of polycrystalline SMAs are studied. The interaction between the stress state, number of grains, and the texture on the thermo-mechanical response of polycrystalline SMAs is also studied. For studying some aspects of the thermo-mechanical properties of SMAs that cannot be studied neither by the phenomenological constitutive models nor by the micromechanical models, molecular dynamics simulations are used to explore the martensitic phase transformation in NiTi alloys at the atomistic level. The martensite reorientation, austenite to martensite phase transformation, and twinning mechanisms in NiTi nanostructures are analyzed and the effect of various parameters including the temperature and size on the phase transformation at the atomistic level is studied.