Interaction between molybdenum and manganese to form effective dispersoids in an Al-Si-Cu-Mg alloy and their influence on creep resistance

The present work investigates the effect of adding Mo and Mn to an Al-Si-Cu-Mg alloy on the formation of thermally stable dispersoids. Mo and Mn in combination formed a large volume fraction of uniformly distributed dispersoids within the Al matrix, because of the opposite partitioning behavior (kMo>1vs.kMn<1) during the non-equilibrium solidification. Mo (without Mn) formed coherent α-Al(Fe,Mo)Si dispersoids with a BCC lattice, which were mostly located within intradendritic regions. Subsequent addition of Mn increased the amount of the dispersoids by substituting the Fe atoms. The combined addition also led to a more uniform dispersoid distribution by eliminating the interdendritic dispersoid-free zones (DFZs). Consequently, creep resistance in the temperature range of 300-350 °C improved significantly. At 300 °C and 30 MPa, the minimum creep rate decreased, and creep time-to-fracture and strain-to-fracture each increased with increasing Mn up to 0.5 wt%. Further addition of Mn resulted in increased presence of the eutectic intermetallics in the interdendritic regions, which deteriorated the ductility of the alloy leading to a shorter creep-life time.