Martensite modulus dilemma in monoclinic NiTi-theory and experiments

Correct assessment of the NiTi martensite moduli and its significance are the subject of this paper. Current experiments and published experimental data reveal the considerable difference between the macroscopic elastic moduli for twinned martensite and detwinned martensite (single crystal state). Although this difference is significant, it is not explained adequately, and often neglected in modeling. In this study, based on the atomistic simulations, we establish Voigt and Reuss bounds and Hill's estimate for macroscopic moduli for both the internally twinned states and single crystal (detwinned) states. The predicted elastic moduli in the twinned state compare favorably to the experimental thermally induced martensite (with internal twin arrangement) moduli. The single crystal moduli predictions conform to the experimental moduli for deformed martensite or stress-induced martensite in tension and compression showing asymmetry. We draw attention to the need for correct interpretation of the elastic moduli for understanding a wide range of phenomena in shape memory alloys, such as the transformation stress-strain response, the fatigue behavior, the stored elastic energy, the stress hysteresis and others.