The role of plastic deformation on the competitive microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy

Abstract

The role of plastic deformation prior to artificial aging on the microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy designated AF/C 458 was investigated. Induced plastic deformation ranged from a non-stretched or 0% stretch condition to an 8% stretch, with intermediate stretches of 2%, 4% and 6%. Tensile properties, fractography and quantitative precipitate analysis were acquired from specimens that were water quenched from a solution heat treatment, immediately stretched and artificially aged at 150°C. Fractography was investigated through scanning electron microscopy (SEM). Quantitative transmission electron microscopy (TEM) determined the variation in precipitate type, number density, size and volume fraction of the major strengthening precipitates Al₂CuLi (T₁), Al₂Cu (θ″/θ^′) and Al₃Li (δ^′).

Age hardening curves for each level of mechanical stretch illustrated the enhanced aging kinetics of plastically deformed material. Quantitative TEM indicated that increasing amounts of pre-age stretch were found to greatly affect the competitive precipitation kinetics of T₁ and θ″/θ^′ in AF/C 458 augmenting the volume fraction of fine matrix T₁ plates and dramatically decreasing the volume fraction of θ″/θ^′ for isochronal treatments. A quantitative microstructural comparison of specimens exhibiting a given strength demonstrated that the imposed level of cold work dictated the density, size and volume fraction of the competing precipitates. The tensile data indicated a trend of increasing ductility for equivalent yield strengths with the increasing amount of pre-age mechanical stretch and therefore shorter artificial aging times. The quantitative precipitate data were used with a computer simulation for yield strength determination. The theoretical simulation reported calculated yield strengths in good accord with experimental results and can thus be used to predict the optimum microstructural configuration for high strength.