The Effect of Transition Metal Additions (Mo and W) on Double Oxide Film Defects in an Al-Si-Mg Alloy

Double oxide film defects in Al alloys have been associated with reduced mechanical properties and increased variability in properties in cast Al alloys. This paper explores the effects of the addition to an Al-7Si-0.3Mg alloy (2L99) of around 0.4 to 0.5wt.% of the transition metals Mo or W. The variability of tensile properties was significantly reduced, resulting in an approximate doubling of the values of the Weibull Modulus of the Ultimate Tensile Strength, and an increase of 10 to 20% in the Weibull Modulus of the %Elongation. Scanning Electron Microscopy EM examination of the fracture surfaces revealed the presence of oxide films, as expected, but also found AlN. The mechanism(s) by which Mo or W additions improve mechanical properties has not been established but the presence of AlN suggested the accelerated consumption of the internal atmosphere of the double oxide film defects, leading to a reduction in their size and hence their deleterious effects.