Nucleation and precipitation in an Al‐Si(1 at.%) alloy investigated by positron annihilation

Abstract

AbstractIn AlSi alloys incoherent precipitates containing the pure solute element and having diamond cubic crystal structure are formed during a post‐quench annealing without formation of any metastable phase (Mondolfo). Quenched‐in vacancies have a strong effect on rates of precipitation from solid solution by transporting Si atoms as well as by their influence on the nucleation process. One suggestion is the precipitate formation by homogeneous nucleation of small Si clusters rich of vacancies (see for instance Beller). Another picture is the heterogeneous precipitation.Positron annihilation can be expected to be very useful in studying small precipitate nuclei and the role of vacancies in the nucleation process. Positrons provide information of the concentration, configuration and internal structure of such lattice defects in solids, like dislocations, vacancies and vacancy‐clusters (Hautojärvi). The method is also very fruitful in studying small coherent precipitations (Dlubek et al. 1981). The aim of the present work is to study the nucleation and precipitation processes going on in an AlSi (1 at.%) alloy between room temperature and 300 °C by measuring the positron lifetime spectra.The spectra of pure, well annealed metals without any localisation sites are one‐component exponential decay spectra. The time constant is called mean positron lifetime (in Al:τ = 165 ps). It is a measure of the reciprocal electron density. If positrons are localized at a certain kind of traps (such like vacancies or dislocations) a second exponential decay component will appear. The lifetime of the second, longer component is characteristic of the kind of the trap, the intensity of this component is connected with the trap density. It is for instance possible to distinguish clearly between monovacancies, divacancies and small vacancy clusters on the basis of the τ2 values (Hautojärvi).The investigated AlSi(1 at.%) alloy was prepared from 4 N materials. After homogeneisation at 560 °C for some hours the samples were quenched into water at room temperature. Isochronal annealing treatment of samples was performed in 20 K steps (0.5 hours). After each annealing step the samples were slowly cooled down from the annealing temperature to room temperature within 5 minutes. Figure 1 shows the results of the isochronal annealing experiment.