Papers by Keyword: Martensitic Phase Transition

Abstract: Two phase-field models for solid-solid phase transitions driven by configurational forces are reviewed, and the formulation of the models is briefly presented for the non-conserved case, as an example. Applications include martensitic phase transformations in, e.g., shape memory alloys and sintering in ceramic producing. They are compared with two other famous models for phase transitions that are driven by mean curvature.

Abstract: High damping capacity is one of the prominent properties of NiTi shape memory alloy (SMA), having applications in many engineering devices to reduce unwanted vibrations. Recent experiments demonstrated that, the hysteresis loop of the stress-strain curve of a NiTi strip/wire under a tensile loading-unloading cycle changed non-monotonically with the loading rate, i.e., a maximum damping capacity was obtained at an intermediate strain rate (ε^._critical). This rate dependence is due to the coupling between the temperature dependence of material’s transformation stresses, latent-heat release/absorption in the forward/reverse phase transition and the associated heat exchange between the specimen and the environment. In this paper, a simple analytical model was developed to quantify these thermo-mechanical coupling effects on the damping capacity of the NiTi strips/wires under the tensile loading-unloading cycle. We found that, besides the material thermal/mechanical properties and specimen geometry, environmental condition also affects the damping capacity; and the critical strain rate ε^._critical for achieving a maximum damping capacity can be changed by varying the environmental condition. The theoretical predictions agree quantitatively with the experiments.

Abstract: Neutron diffraction technique is used to identify the crystal structures and study the phase transformation in a ternary Ni55Fe20Al25 alloy, prepared in the ordered and site-disordered states by annealing and rapid-melt-quenching techniques. We report the observation of ‘step-like’ wavy structures in the thermomagnetic curves taken on ordered samples. No such features are observed in the site-disordered counterparts. Negative magnetoresistance at 10 K and in a magnetic field of 80 kOe in the disordered sample is nearly two times larger than that in the annealed sample. A tentative explanation for these phenomena is given.

Abstract: Recent experiment [1] revealed many new characteristics of the domain patterns in a superelastic polycrystalline NiTi tube during tensile loading, such as domain wall instability and branching, dynamic topology transition of domain patterns. In this paper, we use the continuum mechanics approach and model the polycrystal as a phase-transformable continuum described by non-local nonlinear elasticity [2]. We simulate the equilibrium macroscopic domain patterns and their evolution in the tubes under tensile loading by the nonlocal Finite Element Method (FEM). It is revealed that the loading path dependence and dynamic topology transition of domain patterns are mainly due to thermodynamic metastability of the tube system. Our simulations capture all the key features of the domain patterns observed in the NiTi polycrystalline tubes.