THE EFFECT OF EXTERNAL STRESS ON THE DISCONTINUOUS PRECIPITATION IN AN AL-ZN ALLOY AT HIGH AND LOW-TEMPERATURES

The effect of external tensile stress on continuous precipitation (DP) has been investigated in an Al-21.8 at.% Zn alloy at high (215-degrees-C) and low (75 and 50-degrees-C) temperatures. The ratio of the macroscopic lattice diffusivity, D, to the DP boundary velocity, upsilon, (D/upsilon) is estimated to be larger than the interatomic spacing, lambda, at the high temperature, and smaller than lambda at the low temperatures. Under tensile stresses, the DP rates are enhanced at the grain boundary segments oriented transverse to the stress direction and suppressed at those oriented parallel to it at both high and low temperatures. Furthermore, Yi and Park show the DP rate changing continuously with temperature over the range where D/upsilon increased from values much smaller than lambda to those much larger. These results show that the diffusional coherency strain is the major driving force for DP even at low temperatures where, with D/upsilon < lambda, no solute diffusion is usually assumed to occur in front of the moving DP boundaries.