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

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 10(2) to 10(3) K/s. This was followed by direct ageing of the cast alloy at 375 degrees C to form thermally stable Ll(2) type coherent Al-3(Sc, Zr) precipitates in the matrix. Subsequently, the alloy was solutionized at 535 degrees C and further aged at 200 degrees C which caused the formation of highly dense theta (Al3Cu) and theta' (Al2Cu) precipitates. From microstructural characterization it was found that the high number density of these precipitates is due to heterogeneous nucleation on preexisting Al-3(Sc, Zr) precipitates. Presence of Al-3(Sc, Zr) precipitates together with high number density of theta and theta' precipitates results in remarkably high 0.2% proof stress of 456 MPa at room temperature, 295 MPa at 200 degrees C and 227 MPa at 250 degrees C along with 5% ductility. It was found that Al-3(Sc, Zr) precipitates also suppress the coarsening of theta and theta' precipitates thus making them thermally stable at high temperature.