  We study the effect of Stacking Fault Energy (SFE) on the deformation
behaviour of copper and copper-aluminium alloys using Molecular Dynamics (MD)
simulation. We find that both yield stress and the magnitude of stress drop at
yield decrease with increasing Al content. This "anomalous" softening behaviour
is explained on the basis of nucleation controlled yielding behaviour. Further,
the decrease in stress drop is rationalised in terms of the stored energy
available at yielding - we show that this decreases with increasing Al. As a
result, the maximum dislocation density is found to decrease with increasing Al
content. Finally, we show that the yield stress calculated using a continuum
model of homogeneous nucleation of partial loops agrees well with the yield
stress seen in the simulations.
