Quantified contribution of β″ and β′ precipitates to the strengthening of an aged Al–Mg–Si alloy

It is generally believed that β″ precipitates, rather than β′ precipitates, are the major strengthening precipitates in aged Al–Mg–Si alloys. The reason for this difference is not well understood. To clarify this, two samples of the same Al–Mg–Si alloy but with different aging states were prepared. The under-aged sample only contains nano-precipitates of the β″ type, while the peak-aged one contains nearly equal volumes of β″ and β′ precipitates. We have, for the first time, separated the strengthening effect of the contribution from βʺ and βʹ precipitates, respectively, by an indirect approach based on high-precision measurements of volume fractions, number densities, sizes, proportions of the precipitates, their lattice strains, the composition and grain size of the matrix. The β′ precipitates, which take 45.6% of the total precipitate volume in the peak-aged sample, contribute to the entire precipitation strengthening by only 31.6%. The main reason why they are less useful compared to β″ precipitates has been found to be associated with their smaller lattice strains relative to the matrix, which is 0.99% versus 2.10% (for β″).