Effects of Magnetic Field and Hydrostatic Pressure on Martensitic Transformations in Some Shape Memory Alloys

ABSTRACTThe recent works carried by the author's group on the effects of magnetic field and hydrostatic pressure on martensitic transformation are reviewed, which mainly concerned with some shape memory allovs, such as Fe-Pt, Fe-Co-Ni-Ti, Ti-Ni and Cu-Al-Ni alloys. The works clarify the effects of magnetic field and hydrostatic pressure on martensitic transformation temperature, magnetoelastic martensitic transformation and morphology and arrangement of martensites and transformation process of athermal transformation. That is, transformation start temperatures in Fe-Pt and Fe-Ni alloys examined increase with increasing magnetic field, but are not affected in Ti-Ni and Cu-Al-Ni alloys. On the other hand, transformation start temperature decreases with increasing hydrostatic pressure in the Fe-Ni-Co-Ti alloy, but increases in Cu-Al-Ni alloys. The magnetic field and hydrostatic pressure dependences of the martensitic start temperature are in good agreement with those calculated by the equations proposed by our group. In the work on the ausaged Fe-Ni-Co-Ti alloy, the appearance of magnetoelastic martensitic transformation is newly found. In addition, several martensite plates grow nearly parallel to the direction of applied magnetic field in the specimen of an Fe-Ni alloy single crystal. Moreover, we found that in the Cu-Al-Ni alloys exhibiting an athermal martensitic transformation, isothermal holding at a temperature above Ms makes martensitic transformation to start and the incubation time increases with increasing ΔT = T − Ms (T represents holding temperature). The above results show that the magnetic field and hydrostatic pressure effectively control not only the transformation temperature but also the morphology and distribution of martensites induced, as in the case of uniaxtial stress and compression.