Microstructural features of intergranular brittle fracture and cold cracking in high strength aluminum alloys

Intergranular brittle fracture has been mainly observed and reported in steel alloys and precipitation hardened Al-alloys where intergranular precipitates cover a major fraction of the grain boundary area. 7xxx series aluminum alloys suffer from this problem in the as-cast condition when brittle intermetallics cover almost all the interdendritic spaces and grain boundaries. Brittle intermetallics formed during the non-equilibrium solidification upon direct chill (DC) casting provide favorable sites for crack initiation and propagation at different stages of solidification and cooling. Such micro-cracks may propagate rapidly in the presence of residual thermal stresses and lead to catastrophic failure of DC-cast billets that is called 'cold cracking'. In this research work fractographic investigations were performed on as-cast AA7050 specimens failed during tensile tests. In-situ tensile tests were also performed using an Environmental Scanning Electron Microscope (ESEM) to follow the propagation path of cracks at critical temperatures. They revealed that the alloy loses its ductility below 300 deg. C and becomes extremely vulnerable to cracking and failure. Comparison of the fracture surfaces of tensile samples with real cracked billets helps to understand the mechanisms of crack formation and catastrophic failure.