THESIS
2014
xii, 105 pages : illustrations ; 30 cm
Abstract
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;
Post a Comment