<em>In Situ</em> Characterization of the Elasticity and Stress-Induced Phase Transformation of NiTi Shape-Memory Alloy

Abstract

In the presented paper, a sample of polycrystalline shape-memory NiTi alloy is studied under compression up to 5% by the means of laser-excited and laser-detected ultrasound waves. The evolution of a propagation velocity of the surface acoustic wave is measured in situ during mechanical loading. An inverse method based on the Ritz-Rayleigh numerical approach is then used to obtain the development of elastic properties of the sample. This process enables an analysis of the evolution of stress-induced transformation from the austenitic to the martensitic phase with the possibility to describe several stages of such transformation, i.e., the transformation to full R-phase, its reorientation causing strong anisotropy of the polycrystalline sample, and consecutive gradual transition to martensite.

Non-Contact Characterization of Acoustoelastic Parameters of Advanced Materials by Laser-Ultrasound

“…Detailed context with microstructural processes is possible to discuss after evaluation of elastic properties by inversion of acoustic data in [8]. In this paper, the described example illustrates potential of the laser-based ultrasonic method for in situ material characterization during mechanical testing.…”

“…This problem can be solved by evaluation of second-order elastic constants as a function of the applied stress. It means that in each loading step, the inverse procedure for determination of complete elastic tensor [8] will be applied, and hence softening phonon modes can be identified. Secondly, the time of flight measurement of wave velocity by the contact approach is influenced by the change of sample size and shape due to applied strain.…”