The effect of external tensile stress on discontinuous precipitation (DP) has been investigated in an Al-21.8 at.% Zn alloy at high(215℃) and low (75 and 50℃) temperatures. The ratio of the macroscopic lattice diffusivity D to the grain boundary velocity, υ, (D/υ) is estimated to be larger than the interatomic spacing, λ, at the high (215℃) temperature, and smaller than λ at the low temperatures. Under tensile stresses, the DP rate is 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 $Park^{(22)}$ show the DP rate changing continuously with temperature over the range where D/υ increased from values much smaller than λ to those much larger. These results show that the diffusional coherency strain is the major driving force for DP even at low temperatures where, D/υ<λ, no solute diffusion is usually assumed to occur in front of the moving DP boundaries.
DP growth rates were quantitatively analyzed with diffusional coherency strain theory. The analyzed results were consistent with that predicted by composition change of DP in quantitative sense. Finally. It was possible to show the diffusional coherency strain energy plays the major role in DP at any times, but it was impossible to eliminate the possibility of minor contribution of other driving forces to DP completely.
