Calorimetric and acoustic emission study of martensitic transformation in single-crystalline Ni2 MnGa alloys

Abstract

The jerky character of austenite-martensite phase transformation in Ni2MnGa single crystals (with 10M martensite structure) has been investigated by thermal cycling using a differential scanning calorimeter (DSC) and by detection of acoustic emissions (AEs) at low cooling and heating rates (0.1 K/min and below). It is illustrated that, besides the low cooling and heating rate, mass and surface roughness are also important parameters in optimizing the best signal/noise ratio in order to obtain individual peaks suitable for statistical analysis. Three types of samples, differing in the twin structure and twin boundary behavior, were investigated with and without surface roughening made by electro-erosion. The statistical analysis, carried out for both (thermal and acoustic) types of signals, provided power-law behavior. In calorimetric measurements the energy exponents, obtained in cooling, were the same within the experimental errors (=1.7±0.2) for the three samples investigated. In acoustic emission experiments the energy and amplitude, α, exponents were determined both for cooling and heating. The exponents for cooling and heating runs are slightly different. They are larger for heating for both α and , in accordance with the asymmetric acoustic activity: we observed higher acoustic activity (higher number of hits) during cooling. The effect of the surface roughness is negligible in the exponents (but higher acoustic activity corresponds to higher roughness) and the following values were obtained: =1.5±0.1 and α=2.1±0.1 for cooling as well as =1.8±0.1 and α=2.6±0.1 for heating. Our results are in accordance with the results of Gallardo [Phys. Rev. B 81, 174102 (2010)PRBMDO1098-012110.1103/PhysRevB.81.174102] obtained in Cu based alloys: the exponents of the energy distributions, for both DSC and AE signals, were the same within the experimental errors. Furthermore, our exponents obtained from the AE measurements are close to the values obtained by Ludwig et al. (α=2.6±0.1 and =1.75±0.1) [App. Phys. Lett. 94 121901 (2009)APPLAB0003-695110.1063/1.3103289] and Niemann et al. (=1.9±0.1) [Phys. Rev. B 86, 214101 (2012)PRBMDO1098-012110.1103/PhysRevB.86.214101] in Ni2MnGa alloys with similar 10M martensite structure.