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 degrees C has been studied. The L1(2) ordered (Al,Cu)(3)(Zr,Ta) precipitates were formed in the Al-matrix, followed by preferential nucleation of Cu-rich theta ''/theta ' plates on the L1(2)/matrix interfaces. Transmission electron microscopy and atom probe tomography studies established the segregation of Zr and Ta at theta ' plate/matrix in the aged alloys. It is shown that Zr segregates at coherent broad interfaces of theta ' plate and inhibits the growth of these interfaces. Segregation of Ta primarily occurs at the growing theta ' 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 similar to 475 MPa at room temperature is recorded due to the presence of ordered precipitates and the growth restriction of theta ' plate. Segregation of slow diffusing elements (Zr and Ta) also enhances the thermal stability of the microstructure.