KINETICS OF DISCONTINUOUS PRECIPITATION AND COARSENING IN NI-IN ALLOYS

The morphology and growth kinetics of cellular precipitation and disco ntinuous coarsening of cellular precipitates have been studied in two Ni-In alloys containing 4 and 6 at.% Tn in the temperature range 740 t o 1010 K. The data for 2 and 8 at.% In alloys have been taken from ear lier investigations [1, 2]. Growth rate measurements were carried out utilizing optical microscopy. Both optical micros copy and scanning el ectron microscopy were utilized for interlamellar spacing measurements . All alloys were observed to decompose completely by the cellular pre cipitation reaction into a lamellar structure consisting of alternate lamellae of the alpha and Ni3In phases at all aging temperatures. The fine lamellar structure of the primary cells decomposed into a coarse lamellar structure consisting of the same two phases. The primary and secondary cell growth data were analysed by using model after Peterman n and Hornbogen. From the diffusivity values, it has been concluded th at the growth of primary and secondary cells occurs by diffusion of Tn along the grain boundaries. The rate of migration of the reaction fro nt for both cellular precipitation and discontinuous coarsening is inc reased markedly with increasing In content.