Effect of Cr Addition on Microstructure and Mechanical Properties at Elevated Temperature of Al–Mn–Mg–Si Alloys

Recently, the remarkable strengthening effect of α-Al(Mn, Fe) Si dispersoids at both ambient temperature and elevated temperature were found by several researches. In AA3xxx alloys, a large amount of dispersoids can form by applying suitable heat-treatment. In the present work, the influences of Cr addition on mechanical properties and microstructures of the Al–Mn–Mg–Si alloys were investigated. The mechanical properties at ambient temperature were evaluated by micro-hardness and yield strength. Yield strength at 300°C and creep resistance at 300°C were used to evaluate materials’ mechanical properties at elevated temperature. Moreover, the microstructures in as-cast and heat-treated conditions were quantitatively analyzed by optical and transmission electron microscopes. Results revealed that the addition of Cr increased area percentage of Mn-containing intermetallic particles. It also indicated that solubility of Mn element decreased due to Cr addition. Very little amount of Cr were detected in Mn-containing intermetallic particles and dispersoids. The distribution of dispersoids was not influenced by Cr addition. Number density and volume fraction of dispersoids decreased because of Cr. Electrical conductivity decreased significantly because of 0.30% Cr addition which indicated that a large amount of Cr were still in solid solution condition. Micro-hardness and yield strength at ambient temperature increased with the increasing content of Cr, and the Cr addition increased yield strength at elevated temperature as well. Moreover, creep resistance at 300°C improved dramatically with the increasing content of Cr.