Strength and stability through variable micro segregation behaviour of Ta and Zr solutes at intermetallic interfaces in Al-Cu alloys

A small amount of Zr and Ta are added to aluminium-copper (Al-Cu) alloys, and the evolution of strengthening phases with multi-step heat-treatment and their stability after prolonged thermal exposure at 250 °C has been studied. The L12 ordered (Al,Cu)3(Zr,Ta) precipitates were formed in the Al-matrix, followed by preferential nucleation of Cu-rich θ"/θ' plates on the L12/matrix interfaces. Transmission electron microscopy and atom probe tomography studies established the segregation of Zr and Ta at θ' plate/matrix in the aged alloys. It is shown that Zr segregates at coherent broad interfaces of θ' plate and inhibits the growth of these interfaces. Segregation of Ta primarily occurs at the growing θ' plate interface in the lengthening direction (semi-coherent interface) along with Zr and restricts the lengthening of the plate. The restricted growth triggers sympathetic nucleation of newer plates at the plate edges to sustain the growth process. A yield strength of ∼475 MPa at room temperature is recorded due to the presence of ordered precipitates and the growth restriction of θ' plate. Segregation of slow diffusing elements (Zr and Ta) also enhances the thermal stability of the microstructure.