Precipitation of silicon in a solid quenched aluminium-silicon (1.3 at%) alloy studied by positron annihilation

Doppler broadening of the positron annihilation line was measured for quenched and aged specimens of an aluminium-silicon (1.29 at% Si) alloy. One set of specimens was aged at room temperature (set A) and one set was isochronally (t = 30 min) aged at temperatures ranging from 347 to 884 K (set B). The lineshape parameter S measured in the as-quenched condition was larger than that of well-annealed pure aluminium. S decreased with time of ageing at room temperature, which might be ascribed to the formation of vacancy loops decorated with silicon atoms. After 1320 h at room temperature the specimens of set A were aged for 30 min at 353 K in vacuo. Then, the lineshape parameter S decreased further, which is interpreted as due to a further disappearance of the earlier formed vacancy loops. The behaviour of the lineshape parameter S for set B can be described as follows: 1. 350 to 450 K. Constant value of S, about the same as found for pure aluminium. 2. 450 to 580 K. Increase of S, ascribed to precipitation of vacancies near the Si aggregates/matrix interface relieving transformation strains. 3. above 580 K. Decrease of S to the pure-aluminium value. This efect is ascribed to the growth of silicon precipitates accompanied by annihilation of vacancies. The misfit between the silicon precipitates and the Al-rich matrix is accomodated by dislocations. (author)