EFFECTS OF HIGH MAGNETIC-FIELD AND TENSILE-STRESS ON MARTENSITIC-TRANSFORMATION BEHAVIOR AND MICROSTRUCTURE AT 4.2-K IN FE-NI-C ALLOYS

Effects of high magnetic field and tensile stress on martensitic trans formation behavior and microstructure at 4.2 K have been studied in Fe -31Ni-0.4C and Fe-27Ni-0.8C (wt%) shape memory alloys. In order to mak e cleat the difference of the effects of high magnetic field and tensi le stress on the amount of martensite, two types of experiments were c onducted, that is, the magentic field of 10 T was applied first and th en the tensile stress of 220 MPa was applied In constant magnetic fiel d in one type of experiment and the tensile stress of 220 MPa was appl ied first and then the magnetic field of 10 T was applied under consta nt tensile stress in the other type if experiment, The amount of marte nsite formed in these two experiments were compared. It was found that the critical magnetic field to induce martensitic transformation is b etween 7.5 T and 10 T. In the case of Fe-27Ni0.8C, martensitic transfo rmation is stress-induced at lower level of stress in magnetic field t han in the case no magnetic field is applied. The amount of martensite formed by increasing the magnetic field under constant stress is larg er than that formed by increasing the stress in the constant magnetic field, which shows that the effect of tensile stress on the number den sity of martensite is larger than that of magnetic field in this exper imental condition.