When the grain size of metallic materials is reduced to the nanometer range, the materials exhibit phenomenal properties and the physical mechanisms underlying behind them differ significantly with those active in coarse-grained counterparts. In this thesis, the effects of grain size on the phase transition properties of nanostructured superelastic NiTi are investigated. Superelastic NiTi with wide range of grain size from 10 to 1500 nm were manufactured using severe cold-rolling and nanocrystallization heat treatments. We have identified a critical grain size of about 60 nm below which the hysteresis loop area, the temperature dependence of the transformation stress, and the latent heat of transformation rapidly decrease and tend to vanish. Using Landau-Devonshire theory of pha...[ Read more ]

When the grain size of metallic materials is reduced to the nanometer range, the materials exhibit phenomenal properties and the physical mechanisms underlying behind them differ significantly with those active in coarse-grained counterparts. In this thesis, the effects of grain size on the phase transition properties of nanostructured superelastic NiTi are investigated. Superelastic NiTi with wide range of grain size from 10 to 1500 nm were manufactured using severe cold-rolling and nanocrystallization heat treatments. We have identified a critical grain size of about 60 nm below which the hysteresis loop area, the temperature dependence of the transformation stress, and the latent heat of transformation rapidly decrease and tend to vanish. Using Landau-Devonshire theory of phase transition we show that such reductions are basically caused by gradual dominance of interfacial energy terms over the bulk energy together with introduction of intergranular pressures caused by grain size reduction. Moreover, the classical Clausius-Clapeyron equation breaks down to explain the temperature dependence of the transformation stress due to lack of two-phase coexistence. We further found that such reductions in the above properties have significant effects on the rate-dependent thermomechanical response of the material. While the reduction of latent heat causes the smaller temperature variations during loading/unloading, the reduction of hysteresis loop area brings about smaller heat accumulation with the number of cycles during high frequency cyclic loading. Such smaller heat effects together with the decreased temperature dependence of the transformation stress bring about weaker rate-dependent sensitivity and unprecedented thermomechanical stability in the response of the material under high frequency cyclic loading. In-situ X-ray diffraction experiments under mechanical loading/unloading show that in contrast to the coarse-grained superelastic NiTi that the elastic strains are constant and transformation strain is complete in the stress plateau regime, with grain size reduction the contribution of elastic strains gradually increase and the transformation strain changes continuously with applied strain. In another word, the macroscopic applied deformation is mainly carried by transformation strain for grain size>60 nm while for grain size<60 nm the elastic deformation of the remnant austenite also plays an important role as deformation carrier. It is such competition between transformation and elastic strains controlling the overall macroscopic response.

Keywords: Shape memory alloys; Grain size; Hysteresis loop area; Temperature dependence of transformation stress; Latent heat; Hysteresis heat, rate-dependent response; cyclic loading; In-situ X-ray diffraction; Deconvolution; Continuous phase transition; deformation mechanism; transformation matrix; elastic strains;