Effect of Chromium on Microstructure and Mechanical Properties of Al–Cu–Er–Zr Alloy

Introduction of 0.3% Cr into Al–Cu–Er–Zr alloy leads to significant modification of the grain structure and almost complete elimination of a dendritic structure. The average grain size of 40 μm corresponds to the size of a dendritic cell. The good modifying capacity is explained by formation of intermetallic compounds of the phase first identified of approximate composition Al_75–80Cu_10–12Er_3–4Cr₇, which is a substrate for nucleation of primary aluminum. After quenching and aging, slight hardening in the alloy is achieved due to L1₂ –Al₃(Zr,Er) dispersoids with an average diameter of 40 nm and q’(Al₂Cu) precipitates with a diameter up to 100 nm and a thickness up to 5 nm. No particles containing chromium are found within the matrix. According to thermodynamic calculations, hot rolling of AlCuErZrCr alloy at a temperature of 440°C should proceed in the ((Al)+Al₃Zr+Al₇Cr) phase region. However, analysis of the fine structure in an annealed condition after rolling showed presence of only L1₂ –Al₃(Zr,Er) dispersoids. Differences in alloy hardness on annealing up to 300°C may be explained either by the influence of dissolved chromium atoms or by formation of chromium-containing particle clusters, which could not be detected.