Ni-9.6 at%Ti合金の2相分離に関する磁気的研究

Abstract

The saturation magnetization at 77 K, (Ms)77, and the magnetization at high and low fields of 23.9 and 1.19 kA/m over a temperature range of 77∼273 K, MH(T) and ML(T), were measured for the present alloy aged at 723∼888 K. From the MH(T) curves, the Curie temperature, θc, and the steepest gradient of the curve, (−dM⁄dT), were estimated. Details of the M(T) curves were interpreted on the basis of the conception of magnetic mass balance. The configurations of composition variation and their changes with aging were analysed.Aging behaviour of the alloy was quite different below 793 K and above 813 K. For the specimens aged at 723 K, for example, (Ms)77 and θc, increased continuously with aging time up to 105 s. During the aging, the value of (−dM⁄dT) was relatively small, and the tails of the ML(T) curves shifted to higher temperature side. These results indicate that in these aged specimens composition fluctuations are present and grow in amplitude without any delay, where the composition of the Ti-poor regions decreases continuously with aging. For the specimens aged at 823 K, on the other hand, (Ms)77 and θc increased after an incubation time of 6 min, and became saturated after a long aging time of 30 h. For the incubation or the later stage of aging, M(T) decreased sharply at near θc. For the intermediate stage of aging where (Ms)77 and θc increased, ML(T) decreased rather discontinuously with temperature. These results suggest that some nuclei are formed during the incubation time. Thereafter, a three-phase structure is produced and the Ti-poor regions grow by enchroaching the homogeneous matrix, finally resulting in two-phase structure. It is concluded that the Ni-Ti alloy is decomposes by spinodal mechanism below 793 K and by nucleation-growth mechanism above 813 K.