Next Lecture
Wednesday, October 5, 2016, 10:00
Experimental study and simulation on localization
of phase transformation in shape memory alloys
Ing. Petr Sedlák, Ph.D.
Institute of Thermomechanics of the CAS, v. v. i
Lecture outline:
The remarkable properties of shape memory alloys (SMA) that are utilized in a number of useful applications are due to a phase transformation between austenite and martensite. In many situations, the transformation does not proceed homogenously, but in the form of macroscopic transformation bands, i.e. in a highly localized manner. Since these inhomogeneities influence the mechanical response and reduce the fatigue performance of SMA products, the roots and mechanisms of localization have been investigated by material scientists and engineers for many years. In a unique experiment utilizing the advanced technique of three-dimensional X-ray diffraction (3D-XRD), complete strain and stress states of the polycrystalline grains close to the macroscopic transformation band front have been resolved on a grain-by-grain basis for the first time. Results show substantial heterogeneity of stress between grains – implied by anisotropy of both elastic and transformation properties – and a striking redistribution of macroscopic (homogenized) stress near the interface. Analysis of the experimental data allowed the team to adapt an established constitutive model tailored for NiTi SMA so that non-local, gradient effects could be included. Consequent numerical simulation of the propagating transformation band demonstrated how the internal stress redistributes close to the phase interface within the wire causing the macroscopic localization to occur.
