Improvement of high temperature strength of 2219 alloy by Sc and Zr addition through a novel three-stage heat treatment route

Abstract

Significant improvement of high temperature strength of 2219 alloy was achieved by addition of equal atom percentages (at%) of Sc and Zr and using a novel three-stage heat treatment route. Processing of the alloy involved casting in a water-cooled copper mould with a cooling rate in the range of 102 to 103 K/s. This was followed by direct ageing of the cast alloy at 375 ℃ to form thermally stable L12 type coherent Al3(Sc, Zr) precipitates in the matrix. Subsequently, the alloy was solutionized at 535 ℃ and further aged at 200 ℃ which caused the formation of highly dense θ′′ (Al3Cu) and θ′ (Al2Cu) precipitates. From microstructural characterization it was found that the high number density of these precipitates is due to heterogeneous nucleation on pre-existing Al3(Sc, Zr) precipitates. Presence of Al3(Sc, Zr) precipitates together with high number density of θ′′ and θ′ precipitates results in remarkably high 0.2% proof stress of 456 MPa at room temperature, 295 MPa at 200 °C and 227 MPa at 250 °C along with 5% ductility. It was found that Al3(Sc, Zr) precipitates also suppress the coarsening of θ′′ and θ′ precipitates thus making them thermally stable at high temperature.