Effects of High Magnetic Field and Tensile Stress on Martensitic Transformation Behavior and Microstructure At 4 K in Fe
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up to 220 MPa and kept constant. Then the high magnetic field was increased up to 10 T, and kept constant for 5 min. Then the magnetic field was decreased to 0 and then the stress was decreased to 0 as well. The stress-strain curves were measured during these experiments. The microstructure of the specimens after these experiments were chemically polished by a hydrofluoric acid solution and etched by a 5% nital, and then observed by an optical microscope. The specimens of 2X2X2 mm size were used for the measurement of magnetization curves by VSM in order to determine the magnetic-induced martensitic transformation temperature. The specimens were sealed in evacuated silica capsules and solution-treated at 1473 K for 3.6 ks. One group of specimens were quenched to room temperature by breaking the capsules in brine, and another group of specimens were air-cooled to room temperature to investigate the effect of cooling rate on the transformation temperature because these specimens were smaller than those for the above mentioned tensile tests. RESULTS AND DISCUSSION It was found that more than 50% martensite was formed by applying high magnetic field of 10 T in the experiment indicated in Fig. 1(a). In order to determine the magnetic-induced martensitic transformation temperature, the magnetization curves were measured by VSM as shown in Fig. 2. The curves saturate at around I T in both specimens. There is no effect of cooling rate on the magnetization curves. There is no indication of martensitic transformation on these curves. Therefore, the critical magnetic field is between 7.5 T and 10T. Figures 3 and 4 show the results of experiments indicated in Fig. 1 (b)-(d). Figs. 3 and 4 show the stress-strain curves of Fe-3 INi-0.4C and Fe-27Ni-0.8C, respectively. Figs 3(a) and 4(a) are the stress-strain curves obtained by applying the tensile stress of 220 MPa. Serrations due to the stress-induced martensitic transformation are observed on both curves, more clearly in Fe-27Ni0.8C. Figs 3(b) and 4(b) show the stress-strain curves obtained by applying the tensile stress of 220 MPa in the high magnetic field of 10 T as indicated in Fig. l(c). Serrations due to martensitic transformation are observed in both curves. In the case of Fe-27Ni-0.8C, the transformation proceeds at very low stress level in the early stage of deformation. This is because the potency of nucleation of martensite is increased by applying the high magnetic field. Figs 3(c) and 4(c) show the stress-strain curves obtained by applying the high magnetic field of 10 T under tensile stress of 220 MPa as indicated in Fig. I(d).The left part of curve is formed during the stress-induced martensitic transformation. The right part of curve is formed during the transformation by increasing the magnetic field under tensile stress, and the many clear serrations were generated on the curve because the stress is relieved by the martensitic transformation but the stress is controlled to be constant and increase to 220 MPa. The elongation of the specimen shown in the right
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