Abstract
Al-Mg-Si alloys with total solute contents ranging from 0.8 to 1.4 wt.% were solutionised, quenched and then artificially aged (AA) at 180 {\deg}C, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during preceding ageing. Hardness and electrical resistivity measurements were carried out to complement these measurements. AA was carried out in four different heating media, which allowed for varying the heating rate from 2.4 K/s to 170 K/s. The main result of the study is that there is a competition between vacancy losses and precipitation. Any precipitation taking place during quenching or during heating to the AA temperature helps to prevent vacancies from going to sinks and allows them to assist in solute clustering. Higher solute content, slower heating to 180 {\deg}C and natural pre-ageing before AA were found to have a comparable effect.
Highlights
The technologically important 6XXX series of agehardenable alloys is based on the ternary system Al-MgSi
Unlike during natural ageing (NA), which is driven by excess vacancies for a long time (∼ weeks), during artificial ageing (AA) excess vacancies anneal out much faster and the vacancy site fraction approaches the equilibrium value at the AA temperature, but how fast this happens is not known because vacancy loss is influenced by both vacancy sinks such as dislocation
S1, for how values corresponding to zero natural secondary ageing (NSA) are determined
Summary
The technologically important 6XXX series of agehardenable alloys is based on the ternary system Al-MgSi. After solutionizing and quenching, artificial ageing (AA) at typically 180 ◦C leads to the formation of a series of metastable precipitates that increase strength. The high site fraction of vacancies formed at the solutionizing temperature is partially preserved as nonequilibrium excess vacancies. Unlike during natural ageing (NA), which is driven by excess vacancies for a long time (∼ weeks), during AA excess vacancies anneal out much faster and the vacancy site fraction approaches the equilibrium value at the AA temperature, but how fast this happens is not known because vacancy loss is influenced by both vacancy sinks such as dislocation
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