Grain boundary decohesion in body-centered cubic Mg-Li-Al alloys

Body-centered cubic (BCC) Mg-Li alloy can be effectively strengthened by Al addition. However, brittle fracture degrades the ductility. Further improvement in mechanical properties requires a deep understanding of the fracture mechanism. In this work, we investigate the fracture behaviour of BCC Mg-Li-Al alloys and probe underneath mechanisms. Three factors are likely responsible for the grain boundary decohesion: narrow precipitation free zones provide dislocations initiation and slip pathway; shearing of intragranular precipitates by planar slip will cause large stress concentration to GBs at the end of slip bands; grain boundary precipitates of Al-rich ordered phases weaken the interatomic bonding among grains. Once the stress locally exceeds GB cohesion, cracking nucleates and propagates along/through GBs and intergranular/cleavage fracture takes place.