Effect of magnetic fields on magnetoelastic martensitic transformation in an ausaged Fe-Ni-Co-Ti shape memory alloy

Abstract

Magnetoelastic martensitic transformation in an ausaged Fe-31.9Ni-9.8Co-4.1Ti (at%) shape memory alloy, which is induced only while a magnetic field is applied and is transformed back to the austenite when the magnetic field is removed, has been examined in detail by carrying out electrical resistivity, differential scanning calorimetry (DSC) and magnetization measurements, applying pulsed ultra high magnetic fields up to about 30MA/m. As a result, the following characteristics were found: Transformation temperatures, Ms and Af, and the latent heat of transformation in the ausaged alloy under no magnetic field were 127K, 159K and 334.4J/mol, respectively. Both the austenitic and martensitic states were ferromagnetic in the measured temperature range between 4.2 and 31 OK, and the spontaneous magnetization in the austenitic state was smaller than that in the martensitic one. Martensites were induced in the alloy under a pulsed magnetic field higher than a critical one, Hc, and when the magnetic field was removed, they were transformed back to the austenite completely or incompletely depending on the setting temperature, T. That is, in case of T > Af, all the induced martensites disappeared completely, while in case of Ms < T > Af, a part of them was retained. However, the retained martensites were transformed back completely when the specimen was heated up beyond Af. These behavior are similar to those of stress-induced martensitic transformations and are reasonablly explained thermodynamically by considering the hysterisis of transformation temperatures of the alloy. The amount of the induced martensites was determined by magnetization measurements, and it increased with increasing magnetic field in the range higher than Hc, irrespective of the formation temperature.