On the Secondary Reaction Associated with Eutectoid Transformation in Cu-Be Alloy

The behaviour and kinetics of cell growth in a secondary coarse lamellar pearlite reaction in Cu-22.6 at%Be alloy have been studied principally by metallographic observations. The variations in cell radius, interlamellar spacing and volume fraction of γ phase in pearlite cells with ageing time have been determined by quantitative metallographic measurements of the specimens aged at temperatures from 673 to 823 K.The results obtained are as follows:(1) After initial, fine lamellar pearlite reaction has almost completed, coarse lamellar pearlite reaction occurs wherein the fine pearlite is replaced by the coarse pearlite.(2) Cell growth rate of coarse pearlite decreases, and the interlamellar spacing increases with ageing time.(3) Interlamellar spacing of coarse pearlite is 4∼5 times larger than that of fine pearlite in every ageing time.(4) Be concentration of α phase in fine pearlite is supersaturated at the initial ageing stage, then decreases with ageing time and attains an equilibrium value prior to the occurrence of a coarse pearlite reaction. Then it is considered that there are no chemical free energy changes during the cell growth of coarse pearlite.(5) Grain boundary diffusivities, evaluated from the discontinuous coarsening model proposed by Livingston and Cahn are reasonable. Therefore, it can be concluded that the cell growth of coarse pearlite occurs by the discontinuous coarsening mechanism.