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Al-Mg-Si alloys are widely used as age-hardenable alloys. The main hardening contribution comes from the precipitates formed in the material. As the diffusion processes depend on the vacancy amount present in the system, it is important to have some insight into the vacancy concentration in order to simulate the precipitation kinetics in these systems. In case of aluminium alloys, the high cooling rates result in the considerable amount of the excess (frozen-in) vacancies so that taking the equilibrium concentration of vacancies for the kinetic simulation may lead to serious errors in the predictions of the precipitation state of the material.

In this example, the effect of quenching process on the natural ageing process in Al-Mg-Si alloy is discussed. The alloy is solution annealed and, afterwards, quenched in two ways. The first way is a standard water-quench to room temperature (treatment P1). The other way is to quench in a low-melting alloy at 160°C, hold it at this temperature for two minutes and, subsequently, quench in water to room temperature (treatment P2). Figure 1 shows schematically the treatment features.


Figure1: Heat treatments investigated in this example. Treatment P1 ends with the water-quenching to room temperature. Treatment P2 includes an isothermal hold for two minutes at 160°C before reaching the room temperature.

 

Differences in the precipitation kinetics were observed for these treatments. The resistivity measurements (Fig. 2) show a clear change of the sample state treated with P1, while no such behavior is observed for the sample treated with P2. The atom probe tomography (APT) analysis shows also differences in the amount of clusters found (Fig. 3). The cluster distribution of P2 sample is identical with the random distribution (except of a single precipitate which seems to be found by chance), while the sample P1 shows indicates clustering.


Figure2: Results of the resistivity measurement of the samples stored at room temperature after various heat treatments. Resistivity increase after treatment P1 was observed. For the sample after treatment P2, the resistivity retained its value.
 


Figure3: Results of the Atom Probe Tomography measurement of the samples stored at room temperature after various heat treatments. The cluster analysis in the sample after treatment P1 (a) shows a remarkable deviation from the random solution model. The results of this analysis in the sample after treatment P2 (b) confirm the random distribution of the solute atoms with an exception of a single heterogeneous precipitate was found. 

 

The way in which these heat treatments result in the different behavior is revealed when the evolution of the vacancy concentration is analyzed. On one hand, the equilibrium vacancy concentration at room temperature is the same for both of the samples and it does not provide any explanation for the different outcomes. Nevertheless, the FSAK model which is implemented in MatCalc, calculates the actual vacancy concentration and it predictions (shown on Fig. 4) allow some understanding of the processes occurring in these samples. For treatment P1, the straight quenching results in the high concentration of the excess vacancies. These had not enough time during cooling to reach the sinks at which the vacancy annihilation occurs. Hence, these vacancies are available for the diffusion of the solute atoms of Mg and Si which tend to form clusters at room temperature. In case of treatment P2, the isothermal holding at 160°C is in the region when the vacancy mobility towards sinks is still considerable and the amount of the frozen-in ones is over two orders of magnitude lower than at the end of treatment P1. In result, the diffusion of solute atoms at room temperature is inhibited and the cluster formation is prevented during the measured timescale.

Figure4: Evolution of the equilibrium and actual (frozen-in) vacancy concentration in the samples after treatment P1 (a) and P2 (b). While the predicted equilibrium concentration is the same in both samples, the concentration of the current ones is over two orders of magnitude larger after treatment P2 than after treatment P1.