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 decomposition of the parent β phase exhibits C-shaped time-temperature-transformation (TTT) curves below the eutectoid temperature. Pearlite forms from the eutectoid temperature (380°C) to at least 85°C below this temperature. Two distinct types of pearlite form: a coarse lamellar structure at higher reaction temperatures and a fine lamellar structure with characteristically faceted boundaries. The fine pearlite forms in competition with coarse pearlite below 315°C, and the interlamellar spacings of the two pearlites differ by an order of magnitude. The constituents of coarse pearlite are the equilibrium α and β′ phases with fee and hP9 crystal structures, respectively. Fine pearlite is composed of α and a metastable phase, β″, with a crystal structure identified as 9R (structure type hr3). Both types of pearlite undergo coarsening by a discontinuous, or cellular, reaction.