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In general, with the addition of solutes, the yield strength of alloys isexpected to increase and this phenomenon is known as solid solutionstrengthening. However, reports on the ``anomalous'' softening with alloyingadditions are not uncommon; for example, it is known that the heterogeneousnucleation of dislocations at the sites of solute atoms can lead to softening.In this study, using Molecular Dynamics (MD) simulations, we show anomaloussoftening in Cu-Al alloys deformed at 300 K; specifically, not only the yieldstress but also the magnitude of stress drop at yield decrease with increasingAl content. We calculate the stress needed for the homogeneous nucleation ofpartial dislocation loops using a continuum model. One of the key inputs to thecontinuum model is the stacking fault energy (SFE). We carry out thethermodynamic integration to evaluate the free energies in crystals with andwithout the stacking faults and hence calculate the SFE as a function of Alcontent at 300 K. Using the SFE values thus obtained we show that softening isa consequence of the reduction of SFE values with the addition of Al in asystem where yielding is controlled by the nucleation of dislocation loops.Specifically, the results of continuum model are in good agreement with MDsimulation results of pure copper. However, in the Cu-Al alloys, the drop inyield strength can only be explained using a combination of the continuum model(which assumes homogeneous nucleation), and the reduction in heterogeneousnucleation barrier (which is a function of the ratio of the unstable and stablestacking fault energies). We show that the decrease in stress drop can berationalised in terms of the stored energy available at yielding -- the storedenergy at yielding decreases with increasing Al, and, as a result, the maximumdislocation density decreases with increasing Al content.

Stacking fault energy induced softening in the nucleation limited plasticity regime: a molecular dynamics study on Cu-Al alloys