The constitutive relationships between stress, strain rate, and temperature were analyzed to obtain a unified description of creep and plasticity of aluminum alloys produced by powder metallurgy and of aluminum-based metal-matrix composites. As both classes of materials are characterized by the existence of a threshold stress ( 0 ), a unified description of creep (low strain-rate regime) and plasticity (high strain-rate regime) was obtained by substituting the conventional power-law equation with the sinh relationship, where the applied stress is replaced by the difference between the applied stress and a threshold stress. The stress exponent was n ϭ 3 or 5, and the activation energy was equivalent to the activation energy for self-diffusion or to the activation energy for diffusion of solute elements in the matrix. The model was applied to an unreinforced alloy (2014PM) and a composite (6061 ϩ 20 pct Al 2 O 3 ) tested in tension (under constant load) or torsion (at constant strain rate) in the temperature range between 300 ЊC and 500 ЊC. The results were compared with data available in the literature.