Abstract
The present work deals with structural change during aging for Fe-W-Co alloys containing additional elements, Cr and V.
The results obtained are as follows:
(1) The super-saturated solid solutions of the alloys were decomposed into two phases; W-rich and W-poor zones in the early stages of aging. Age-hardening of the alloys is mainly due to this structural modulation. The W-rich zone transforms to a meta-stable intermetallic fct η′-phase continuously and finally to a stable precipitate, (FeCo)7W6. In the W-poor zone the superlattice of FeCo or Fe3Co may be formed, and the superlattice gives some influence on the age-hardening.
(2) The wave length and the amplitude of the modulated structure during aging increase very slowly. The fact that the softening rate of the alloys during over-aging is very small may be due to the small growth rate of the modulated structure.
The activation energy for the growth of the wave length in the modulated structure is nearly equal to an energy for volume diffusion of W atoms in alpha iron. Therefore, this suggests that the growth of the modulated structure is controlled by the diffusion rate of W atoms.
