Structural Changes during Aging in Ni–Cu–Si Modulated Structure Alloys

Abstract

Structural changes during aging of Ni–Cu–Si alloys containing various amounts of Si were investigated by means of transmission electron microscopy and X-ray diffraction. Results obtained are as follows; In an early stage of aging, a periodic phase decomposition occurred along the [100] direction in several Ni–Cu–Si alloys (Ni-40 at%Cu-3∼1.2 at%Si) which had been held at suitable temperatures. The wavelength of the periodic decomposition was observed to be constant for a given composition and temperature during the initial aging.For high Si alloys or during the low temperature aging, γ′ (Ni3Si) particles were nucleated coherently on each zone at the end of the constant wave period. These particles increased in size and interparticle spacing as aging progressed and had the regular periodicity and alignment. When the Si content was reduced or after the high temperature aging, typical tweed structures were formed after the [100] phase decomposition. The γ′ particles nucleated on the tweed structure showed two types of distribution, that is, a random distribution and a periodic distribution.