Interaction of dislocations with nm-sized precipitates is one of the most effective hardening mechanisms in metallic alloys. Nevertheless, precipitation hardening is strong in some metals (Al) and weak in others (Mg) and there is not a clear explanation for this behaviour that can lead to the design of stronger alloys through precipitation hardening. To understand these differences, precipitation hardening was analysed in two model alloys: Al-4 wt.% Cu and Mg- 4wt.% Zn. The alloys were manufactured by casting, followed by a solid solution and ageing at high temperature to reach the different precipitate distributions. The critical resolved shear stress was determined by means micropillar compression tests in single crystals oriented for single slip (basal and pyramidal slip in the case of Mg) from the polycrystalline alloys. Moreover, the dislocation/precipitate interaction mechanisms were carefully analysed by means of transmission electron microscopy. It was found that q'' shearable [1] or q' impenetrable [2] precipitates lead to strong hardening in Al alloys. precipitates are overcome by Orowan loops by pyramidal dislocations [3] but are easily sheared by basal dislocations [4] in Mg alloys, leading to strong hardening in the former and weak hardening in the latter.

In order to understand the different behavior of Al and Mg alloys, precipitate strengthening was analyzed by means of dislocation dynamics and molecular dynamics. It was found that the combined contribution of solution hardening and either coherency strains or large Peierls stresses to shear the precipitates were responsible for the high strength of the Al-Cu alloys. The Orowan mechanism was responsible for precipitate strengthening of Mg alloys deformed by pyramidal slip while precipitate shearing by basal dislocations was favoured because the Mg basal plane was parallel to one crystallographic plane of the precipitate. Possible strategies to enhance precipitation hardening in Mg alloys were discussed.

[1] R. Santos-Güemes, L. Capolungo, J. Segurado, J. LLorca. Dislocation dynamics prediction of the strength of Al-Cu alloys containing shearable q'' precipitates. Journal of the Mechanics and Physics of Solids, 151, 104375, 2021.

[2] R. Santos-Güemes, B. Bellón, G. Esteban-Manzanares, J. Segurado, L. Capolungo, J. LLorca. Multiscale modelling of precipitation hardening in Al-Cu alloys: dislocation dynamics simulations and experimental validation. Acta Materialia, 188, 475-485, 2020.

[3] R. Alizadeh, J.-Y. Wang, J. LLorca, Precipitate strengthening of pyramidal slip in Mg-Zn alloys. Materials Science and Engineering A, 804, 140697, 2021.

[4] R. Alizadeh, J. LLorca. Interaction between basal dislocations and precipitates in Mg-4Zn alloy: Mechanisms and strengthening. Acta Materialia, 186, 475-486, 2020.