EUTECTOID DECOMPOSITION IN AG-GA

The kinetics and mechanisms of eutectoid decomposition in alloys near Ag-15.3 wt pct Ga (Ag-21.8 at. pct Ga) were investigated. Isothermal d ecomposition of the parent beta phase exhibits C-shaped time-temperatu re-transformation (TTT) curves below the eutectoid temperature. Pearli te forms from the eutectoid temperature (380 degrees C) to at least 85 degrees C below this temperature. TWO distinct types of pearlite form : a coarse lamellar structure at higher reaction temperatures and a fi ne lamellar structure with characteristically faceted boundaries. The fine pearlite forms in competition with coarse pearlite below 315 degr ees C, and the interlamellar spacings of the two pearlites differ by a n order of magnitude. The constituents of coarse pearlite are the equi librium alpha and beta' phases with fee and hP9 crystal structures, re spectively. Fine pearlite is composed of alpha and a metastable phase, beta '', with a crystal structure identified as 9R (structure type hR 3). Both types of pearlite undergo coarsening by a discontinuous, or c ellular, reaction.