Effects of θ' precipitates on the mechanical performance and fracture behavior of an Al-Cu alloy subjected to overaged condition

In this work, the effects of shear-resistant plate-shaped theta' (Al2Cu) precipitates on mechanical performance and fracture behavior of a heat-treatable Al-Cu alloy were studied. The precipitates, dislocations and micro-voids around grain boundaries were observed using transmission electron microscope and scanning electron microscope equipped with electron backscatter diffraction. The Al-Cu alloy was overaged to ensure that the theta' phase was the only precipitate in the material. The results show that concentration of local strain and pile-up of dislocations occurred along grain boundaries during tensile deformation. A method was developed to calculate the total dislocation density nearby the grain boundaries in the deformed alloy, from which a quantitative relation was established between precipitate number density and void areal density on intergranular fracture surface. The effect of precipitates on the mechanical properties of Al-Cu alloy was also discussed qualitatively. The precipitate number density of the over-aged Al-Cu alloy acted as the dominating factor that influenced the dislocation density and void density on grain boundaries, which thus determined the mechanical properties and fracture behaviors of the alloy.