Unusual lattice parameters in two-phase systems after annealing

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Abstract
The lattice parameter observed at room temperature for the aluminium-rich phase in fully precipitated AlSi alloys has been found to be significantly greater than the expected equilibrium value. This effect increased with silicon content and with annealing temperature. Two possible causes were considered: (i) the difference between the atomic volumes for silicon in the precipitates and in the aluminium matrix; and (ii) the difference between the thermal expansion coefficients of the matrix and the precipitates. A quantitative description is based on the theory of Eshelby developed originally for the case of elastic distortions in a crystal by point imperfections. Distortions due to cause (i) vanished during the anneal. For cause (ii) the theoretical prediction obtained agreed fairly well with the experimental data. The validity of the Eshelby model in this case was discussed. For precipitation studies where the change in lattice parameter is followed, a procedure is proposed to obtain correct solid solubilities.