Effects of Magnetic Field and Hydrostatic Pressure on Martensitic Transformations in Some Shape Memory Alloys
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martensitic transformation start temperature, MS , and the validity of a newly proposed equation by our group to evaluate the relation between M, and critical magnetic field, Hc, for inducing martensitic transformation. (ii) the effect of magnetic field on magnetoelastic martensitic transformation in an ausaged Fe-Ni-Co-Ti shape memory alloy, which occurs only while a magnetic field is applied and disappears when the magnetic field is removed. (iii) the effect of magnetic field on morphology and arrangement of martensites in Fe-Ni alloy single crystals. (iv) the effect of hydrostatic pressure on martensitic transformation start temperature and the validity of a newly proposed equation by our group to evaluate the relation between M. and hydrostatic pressure. (v) the morphology of martensite induced by a hydrostatic pressure. (vi) the effects of magnetic field and hydrostatic pressure on the martensitic transformation process. RESULTS AND DISCUSSION Effect of Magnetic Field on Martensitic Transformation Temperature The specimens used were three invar Fe-Ni alloys[4], disordered and ordered Fe-Pt invar alloys[5], non-invar Fe-Ni-C[6] and Fe-Mn-C alloys[7. Their structural change associated with martensitic transformation are basically those from fcc to bcc. High field magnetization measurements were performed at Research Center for Materials Science at Extreme Conditions, Osaka University, the magnetic field being a pulsed one with its maximum strength of 31MA/m. Details of the ultra high magnetic field instrument have been reported elsewhere[8]. Here we show the typical result of an Fe-31.7at%Ni alloy exhibiting a non-thermoelastic martensitic transformation (M. is 164K) and that of an ordered Fe-24.Oat%Pt alloy exhibiting a thermoelastic martensitic transformation ( M M, AS, and Af are 153, 123, 139 and 177K, respectively, and the degree of order, S, is about 0.8). Figure 1 shows typical magnetization curve (M(t)-H) for the invar Fe-31.7at%Ni alloy, where AT represents the temperature difference between setting temperature, T, and M. ( AT=T-M, ). In the figure, an abrupt increase in magnetization is recognized at a certain strength of magnetic field ( indicating with an arrow ). The strength of magnetic field at the abrupt increase in magnetization corresponds to the critical one, H,, for inducing the martensitic transformation at T, inversely meaning that the setting temperature, T, corresponds to the martensitic transformation start temperature under the strength of magnetic field of HC, M.'. The relation thus obtained between the critical magnetic field and the shift of M, AM., (=M,'-M,) is shown in Figure 2 a with solid squares for the Fe-31.7at%Ni alloy, and is shown in Figure 2 (b) for the Fe-24.0att alloy with S- 0.8. It is known from the figures that the shift of M, increases with increasing magnetic field for both the alloys irrespective of non-thermoelastic and thermoelastic martensitic transformation. Recently we have proposed the following equation[9] to estimate the relation between the critical magne
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