Effects of Zn and Cr additions on precipitation and creep behavior of a dilute Al–Zr–Er–Si alloy

Abstract

The effects of adding 0.08 at.% Cr or 1 at.% Zn to a dilute Al-0.11Zr-0.005Er-0.02Si at.% alloy are studied in terms of the precipitation behavior of Al3Zr (L12-structure) nanoprecipitates and the resulting alloy's creep resistance. Although Cr and Zn additions do not affect measurably the precipitation kinetics or coarsening resistance, the modified alloys exhibit changes in dislocation creep resistance at 300 °C: the creep threshold stress is decreased by 17% (2 MPa) in the Zn-modified alloy and increased by 25% (3 MPa) in the Cr-modified alloy. This is attributed to a modification of the lattice parameter of Al3Zr(L12), which affects the ease with which matrix dislocations climb over the nanoprecipitates. The Zn-modified alloy exhibits Al3Zr(L12) nanoprecipitates containing 6–7 at.% Zn, as determined by atom-probe tomography, which reduces the lattice parameter by 0.17% as a result of Zn substituting for Al on its sublattice, as calculated utilizing density functional theory. The Al3Zr(L12) nanoprecipitates in the Cr-modified alloy contain 0.10–0.20 at.% Cr (1.2–2.5 times more than the matrix) and 0.28–0.51 at.% Er (a 60- to 100-fold enrichment). Erbium, which increases the lattice parameter misfit of Al3Zr(L12) with the Al matrix, is confirmed to be particularly potent in increasing the creep resistance of aluminum alloys containing L12 nanoprecipitates. An alloy design methodology for creep resistance is also validated, whereby precipitate compositions measured by APT are input to DFT calculations to determine their effects on lattice parameter misfit.