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The interaction between dislocations and nano-precipitates during deformation directly influences hardening response of precipitation-strengthening metals such as Al-Mg-Si alloys. However, how coherent and semi-coherent nano-precipitates accommodate external deformation applied to an Al alloy remains to be elucidated. In-situ tensile experiments in a transmission electron microscope (TEM) were conducted to study the dynamic process of dislocations cutting through coherent needle-like β″ precipitates with diameters of 3∼8 nm. Comprehensive investigations using in-situ, ex-situ TEM and atom probe tomography uncovered that β″ precipitates were firstly sheared into small fragments, and then the rotation of the fragments, via sliding along precipitate/matrix interfaces, destroyed their initially coherent interface with the Al matrix. In contrast, semi-coherent βʹ precipitates with sizes similar to β″ were more difficult to be fragmented and accumulation of dislocations at the interface increased interface misfit between βʹ and the Al matrix. Consequently, βʹ precipitates could basically maintain their needle-like shape after the tensile deformation. This research gains new insights into the interaction between nano-precipitates and dislocations. [Display omitted] [ABSTRACT FROM AUTHOR]

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″). [ABSTRACT FROM AUTHOR]