High temperature cleavage in Al-Mg alloys

The fracture behaviour of a high strength 5000 series aluminium alloy has been investigated in conditions relevant to industrial reversible hot rolling. To do so, a ductility test implemented on a servo-hydraulic compression machine was used. The material was deformed in the range 490 degreesC to 560 degreesC at a high strain rate (up to 25 s-1). Under these conditions, the material exhibits a ductile to brittle transition. The brittle behaviour is due to both intergranular and transgranular cleavage like fracture. Particular attention was paid on the latter mechanism. Examinations of the fracture surface with the SEM directly after deformation or after etch-pitting the surface were performed. Cross sections were also examined in the optical microscope after anodic oxidation, to confirm the transgranular aspect of fracture. The corresponding mechanism is found to be cleavage, as transgranular planar fracture surfaces are crystallography well defined. Cleavage takes place both on (100) and (110) planes with the same frequency, independently of the deformation conditions in the cleavage range. A possible scenario for the appearance of high temperature cleavage has been proposed. It involves liquid metal embrittlement (LME) at some grain boundaries due to the fusion of deep eutectics. This LME leads to sharp cracks able to initiate cleavage under fast deformation. This scenario is consistent with metallographic observations of local melting as well as with the dependence of the overall features of fracture with temperature and strain rates.