High temperature, high strain rate embrittlement of Al-Mg-Mn alloy: Evidence of cleavage of an fcc alloy

The fracture behaviour in tension of an Al-Mg -Mn alloy has been investigated. At high temperatures and strain rates, intergranular brittle fracture is observed along with cleavage fracture. Intergranular fracture is related to local melting at the grain boundaries. Cleavage occurs in equal proportions on the {100} and {110} crystallographic planes. The area fraction of cleavage facets on the fracture surface has been quantified. Their initiation is shown to be related both to the liquid metal embrittlement of the grain boundaries and to the presence of brittle Mn containing particles at the grain boundaries. Cleavage fracture in an aluminium alloy also requires an inhibition of plastic flow which prevents plastic blunting at the crack tip. It is proposed that this modification of the plastic behaviour is provided by the decrease in stacking fault energy at high temperatures in Al-Mg alloys.