Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys

High-strength products made from atomized Al-Zn-Mg-Cu-Co alloy powders have good combinations of strength, ductility, resistance to stress-corrosion cracking and fracture toughness. Powder Metallurgy (PJM) methods produce fine metallurgical structures and compositions which cannot be produced by Ingot Metallurgy (IJM) methods. Fine structures result from very rapid solidification and from the effect of fine dispersoids in restricting grain growth. Stress-corrosion cracking (SCC) performance is favored by grain morphology of PJM products. Co₂Al₉ particles in PJM products are 0.02 to 2.0 μm spheroids occurring frequently on grain boundaries where they may serve several functions in slowing SCC attack. Oxide particles are irregular shapes, 0.01 to 0.04 μm in size, occurring in clusters at grain boundaries and in grain bodies. Some of the oxide particles are magnesium oxide and alter the environment in a SCC crack to arrest attack. Porosity is not a significant factor in the structure of PJM products made by a vacuum compacting process. P/M wrought products have superior combinations of high strength and stress-corrosion cracking resistance compared to IJM 7075 and 7050 alloys. While equaling the fracture toughness of 7075 alloy, the PJM products at present have somewhat lower fracture toughness than 7050 alloy, due in part to a larger amount of second-phase particles in the form of Co₂Al₉ and oxide.