Role of twin boundary mobility in performance of the Ni-Mn-Ga single crystals

Abstract

Ni-Mn-Ga alloys have attracted great interest for more than ten years from the scientific community because of the exceptional properties of their twinned martensitic microstructure, combined with their magnetic properties such as high anisotropy. The high mobility of their martensitic twin variant boundaries enables the magnetic shape memory actuation and the high mechanical vibration damping capacity, to name just a few applications. Their material properties are in several ways unique, such as the large magnetic-field-induced strain of several per cent operating at frequencies of several hundred Hz. The most mobile twin boundaries have been found in high-quality single crystals. However, the twin boundary mobility of different crystals often varies significantly. It may even vary in the same crystal, which is not favorable for their practical use. In this work, twin mobility of several types of Ni-Mn-Ga single crystals is studied under monotonic uniaxial and shear loading as well as in dynamic loading at different temperatures. Furthermore, the performance of the material is studied by mechanical and magneto-mechanical cycling of the twin boundaries to reveal changes in the material properties. The results show that the stress needed for the twin mobility in both uniaxial and shear mode can be very low. In the shear mode the twin boundary motion can start even at 0.07 to 0.23 MPa stress. However, the stress onset for the twin boundary motion in the single variant state can be more than a decade higher than in the state with existing twin boundaries. As demonstrated in this Thesis by 2 × 109 cycles at 2 % strain peak-to-peak, 10M Ni-Mn-Ga has potentially a long fatigue life, however several reasons which may reduce their long-term performance are confirmed or proposed.