Decay kinetics of nonequilibrium Al-Si solid solutions

The decay kinetics and nonequilibrium state parameters of supersaturated su bstitutional solid solutions Al1-xSix (x<20 at. % Si) obtained by high-pres sure synthesis have been studied comprehensively by resistivity relaxation and differential scanning calorimetry. It was shown that the decomposition process can be divided into two main stages. The first one consists of nucl eation, growth, and coarsening processes usually observed during first-orde r phase transformations in metals and alloys. On the second stage the exper imental data for isothermal relaxation and isochronal heat release can be e xplained assuming a nontraditional exp(-t/tau)(alpha) kinetics with exponen t alpha approximate to 1/3. The drastic enhancement of the heat-release con tribution at the latest stage of the phase separation is observed for x app roximate to 6-8 at. % Si. Further increase of Si concentration in the face- centered cubic (fcc) lattice of an Al-based solid solution leads to the for mation of fee Si clusters and initiates the splitting of the first stage of the decomposition process into three different branches. For any initial S i content, the second stage of the Al1-xSix phase transformation can be con sidered as a structural transition of fee Si to tetrahedral Si inside of la rge Si clusters embedded in the AI-rich matrix.