A quench sensitivity study on the novel Al–Li–Cu–X alloy AF/C 458

Abstract

The quench sensitivity of a novel Al–Li–Cu–X alloy, designated AF/C 458, was investigated through employing cooling rates of 1.8, 68 and 290 °C/s from the solution heat treatment temperature. Two versions of this alloy were investigated; an unrecrystallized temper consisting of equiaxed subgrains approximately 10 μm in diameter, with the second having recrystallized grains, absent of substructure. Subsequent mechanical testing, fractography and microstructural characterization were used in order to investigate the degree of quench sensitivity for naturally and artificially aged specimens. Varying levels of pre-age plastic deformation were also implemented for the artificially aged specimens.

The slowest quench rate from the solution heat treatment correlated with the highest strength in the as-quenched condition but with subsequent minimal gains from natural aging. The as-quenched strength is attributed to the precipitation of large Al₂CuLi(T₁) plates during cooling. Conversely, the rapidly quenched specimens were free of coarse precipitation and exhibited superior gains during natural aging at ambient temperature due to the enhanced nucleation and growth of Al₃Li(δ^′). Despite the presence of the coarse T₁ precipitation in the slowly cooled specimens, the strength did not vary by more than 10% between the three quench rates for a given heat treatment. However, there was a consistent trend of enhanced ductility with increasing quench rate from the solution heat treatment temperature for each aged condition. Samples with the recrystallized grain structure had lower strengths in the as-quenched and naturally aged conditions when compared to samples with an unrecrystallized grain structure. Different levels of pre-age deformation did not affect the quench sensitivity for the under-aged conditions investigated in this study.