Effect of aging on scale-dependent plasticity in aluminum alloy 2024

The effect of aging on strain-gradient plasticity (SGP) in aluminum alloy 2024 is quantified using nanoindentation. The measured hardness decreases with increasing depth for all conditions; however, the characteristic depth over which the decrease occurs changes. An indentation model which incorporates SGP correlates for penetration depths greater than ∼300 nm, and enables estimation of material length scale ( ℓ * ) that is a constitutive parameter in gradient plasticity theory. This ℓ * is 50–200 nm for alloy 2024, increasing through a maximum with increasing artificial aging time, and is always smaller than values obtained for pure metals. It is suggested that the specific ℓ * values depend on dislocation interactions with atomic clusters through precipitates produced by artificial aging. The implication of the measured values of ℓ * , and other constitutive parameters, on the stress elevation at crack tips in ductile materials is briefly discussed.