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 mu m 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 Al75-80Cu10-12Er3-4Cr7, 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 degrees 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 degrees 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.