In-situ SEM study on damage evolution at coarse secondary particles in 6061 aluminum alloy

In-situ SEM observations of fracture toughness tests have been used to characterize microscopic damage evolution at coarse particles around a main crack in a 6061 aluminum alloy. About 13% of the particles coarser than 1 μm in diameter is identified as Mg₂Si and the remaining is α-AlFeSi. All of the coarse Mg₂Si particles suffer from damage within several hundred micrometers from a crack-tip. Meanwhile, some of the α-AlFeSi particles remain intact. The coarser the particles, the more considerably they are damaged. Fracture of the particles is predominantly observed for the coarser particles, while the tendency of interfacial debonding increases with decreases in the particle size. In addition, in-situ fracture strength values of the two kinds of particles are estimated as a function of particle size. Stronger dependency of the strength on the size is shown in the α-AlFeSi particles than in the Mg₂Si particles. It is concluded that the fracture toughness of the alloy is scarcely affected by the damage at the coarse particles. However, much more considerable effects on strength and ductility are predicted due to the very low strengths of the particles.