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 ( k M o > 1 v s . k M n < 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.