Abstract
Formation and growth of modulated structure with aging were investigated for Ni-37 at%Cu-8 at%Si and Ni-30 at%Cu-5 at%Si alloys which show a typical modulated structure, by means of transmission electron microscopy, X-ray diffraction and electrical resistivity measurements. The results obtained are as follows: (1) For short time aging, the periodic phase-decomposition of [100] direction was observed and its spacing did not change during short time aging. The solute concentration increased rapidly during this aging period. Thus the behavior of phase decomposition may support Cahn’s spinodal theory. (2) For longer time aging, zone-spacing increased in proportion to t1⁄4. The growth of zone-spacing takes place in the following way. The zones which are formed by the waves of three [100] directions during short time aging assume the simple cubic lattice type. With a progress of aging the simple cubic structure is changed to the fcc type which is formed by extinction of zones existing in the nearest neighbor site in the simple cubic structure. On further aging, through the same extinction mechanism, the fcc type modulated structure changes into the simple cubic structure in which the spacing of the remaining zones is twice the original simple cubic structure. It is considered that an excellent periodicity and alignment of the zones in the modulated structure is formed through this growth mechanism.
