Role of decomposition of AlMgFeSi phase in mechanical properties and fracture behaviour of 357 alloys

The φ-AlFeMgSi iron intermetallic phase in Al-Si-Mg alloys is known for its detrimental effect on ductility and strength. An attempt was made to correlate the impact and tensile properties of heat treated Al-7Si-0·55Mg-0·1Fe alloy samples with the changes that occur in the microconstituents such as the eutectic Si and φ phase with prolonged solution treatment. Impact and tensile tests showed that the first 20 h of solution treatment is more sensitive to the changes occurring in the Si particle morphology than that in the φ phase. Decomposition of the φ phase significantly reduces the impact properties after prolonged solution times. Fracture behaviour is controlled mainly by the eutectic silicon morphology. The φ phase particles act as crack initiation sites and facilitate crack propagation in as cast and heat treated alloys. Quality index values show that optimum solution time for Sr modified alloys is 12 h. Impact properties are more sensitive to changes in the φ phase during solution treatment than tensile properties.