Development of a coarsening resistant microstructure in precipitation strengthened aluminium alloys with Zr, Ta and Hf
The work herein aims at developing precipitation-strengthened aluminium-copper (Al-Cu) alloys that can meet the current challenges of the aeronautical and automobile sectors to increase the fuel efficiency where operating temperatures are above 200 ℃. The present study shows the effect of micro-additions of Zr, Ta and Hf in designing the newer generation of high strength Al-Cu alloys by microstructural engineering using a three-step heat treatment route. Prior ageing (400-450 ℃) before solutionizing is effective to form L12 ordered coherent precipitates. However, discontinuous precipitation is a challenge that can be avoided by micro-additions of Si with Hf. Further, ageing at 190 ℃ resulted in a dense distribution of strengthening θ"/θ' plates, nucleated heterogeneously on these L12 ordered precipitates resulting in a significant improvement in mechanical properties. The synergistic coupling of high and low-temperature strengthening phases resulted in the slower growth of θ"/θ' plates. The higher number density of θ"/θ' plates along with L12 ordered precipitates in the α-Al matrix rationalize the observed higher yield strength in Zr, Ta and Hf modified Al-Cu alloys. The reduction in the coarsening rate of θ' plates in the modified Al-Cu alloys was observed during a long time exposure at 250 ℃. Atomic-resolution composition analysis reveals the partitioning of slow diffusing elements to the θ' plates, which delays their coarsening. It induces a high-temperature stable microstructure with 250 MPa of yield strength at 250 ℃.