Magnetic and magneto-mechanical properties of Ni-Mn-Ga magnetic shape memory alloys

摘要

Ni-Mn-Ga alloys close to stoichiometric Ni50Mn25Ga25 (at. %) composition have recently gained considerable interest due to the possibility of rearrangement of their martensite microstructure in magnetic field. The rearrangement is accompanied by large strains of up to 10%. This effect is different from ordinary magnetostriction and it is referred to as magnetic shape memory effect (MSME).The Thesis presents the first attempt to study the temperature limits of irreversible and reversible MSME by exploiting a theoretical model and experimentally determined temperature dependences of magnetic and other material properties governing the existence of MSME. The obtained predictions are compared with direct observations of MSME. Extraordinary magneto-mechanical effects in Ni-Mn-Ga alloys, not discussed previously, are investigated and compared with theoretical models in this Thesis. These effects include reversible MSME with strain close to 6%, magnetic field controlled superelasticity with strain close to 6%, and up to 30% changes of magnetization during loading in static magnetic field. Unique simultaneous measurements of strain and magnetization on Ni-Mn-Ga alloys are presented for various experiments such as, e.g., during MSME and reversible MSME.The broad spectrum of experiments presented in the Thesis corroborates the important role of magnetic anisotropy, twinning stress and temperature for existence and reversibility of MSME. The simultaneous measurement of strain and magnetization in various experiments confirms experimentally the close relation between martensite microstructure and its magnetic properties and demonstrates the interplay between martensite microstructure and magnetic field. Good agreement of all presented experimental results with the used theoretical model supports validity of the model and shows that the model is suitabile for predicting temperature and stress limits of MSME or reversible MSME, and for modelling of magnetic-field induced superelasticity. Some of the presented experiments can additionally be considered as application examples. The original findings presented in this Thesis broaden the general scientific understanding of MSME and can serve as informative source when considering possible engineering usage of Ni-Mn-Ga alloys as actuators, sensors, or intelligent material.