MICROANALYTICAL STUDY OF THE DISCONTINUOUS PRECIPITATION REACTION IN A CO-13 AT-PERCENT AL-ALLOY

High spatial resolution energy dispersive X-ray (EDX) microanalysis ha s been used to determine the changes of the Al concentration accompany ing the formation of alpha and beta lamellae during discontinuous prec ipitation (DP) and discontinuous coarsening (DC) reactions in a Co-13 at.% Al alloy. The detected step-like change of the Al across the reac tion front (RF) has confirmed that the solute redistribution during DP and DC is essentially limited to the RF. The values of the grain boun dary diffusivity triple product, S delta D-b determined from the profi les of the Al concentration across the alpha lamellae yielded the conv incing evidence that the DP reaction in the Co-13 at.% Al alloy is gov erned by the grain boundary diffusion along the moving reaction front. The EDX analysis has also shown that the Al profiles within the singl e alpha lamella can vary with increasing distance from the reaction fr ont. This was caused either by the variation of the alpha lamella thic kness preceding re-nucleation of the gamma lamella or by the stop-and- go motion of the cell boundary. Two kinds of discontinuous coarsening (denoted as DC I and DC II) occurring simultaneously after termination of DP reaction have been identified. They differ from each other not only by the a lamellae thickness but also the residual saturation in A l content which suggests the various surface and chemical contribution s to the total driving force of DC I and DC II. (C) 1997 Acta Metallur gica Inc.