The temperature dependence of the mechanical properties of aluminum alloys containing low-melting-point inclusions

Abstract

The energy absorbed during high temperature impact testing of binary aluminum alloys containing low-melting-point inclusions of Bi, Cd or Pb has been found to drop suddenly in a narrow temperature range; below the transition temperature, the specimens bend without breaking, while above it they fracture intergranularly. This embrittlement is clearly associated with the presence of the liquid inclusions. Classical liquid metal embrittlement (LME) of a 6061 aluminum alloy where its external surface was wetted by the ternary BiCdPb eutectic confirmed that the brittle intergranular failures observed in AlBi, AICd and AlPb are a manifestation of LME. It was found that a fracture surface energy model for LME is able to account for the result that embrittlement is most severe in alloys containing liquid Bi inclusions and least severe in alloys containing liquid Pb inclusions. The reduction in the fracture surface energy when the inclusions melt facilitates crack initiation and crack propagation. However, the finite amount of embrittling liquid produced by each inclusion is capable of facilitating crack propagation over short distances only, which may account for the observation that a minimum concentration of grain boundary inclusions are necessary to cause embrittlement in these alloys.