Effects of W and Si microadditions on microstructure and the strength of dilute precipitation-strengthened Al-Zr-Er alloys

The effect of single or combined micro-additions of tungsten (0.03 at.%) and silicon (0.02-0.40 at.%) on the evolution of Al-3(Zr,Er) (Ll(2)) nanoprecipitates in a cast Al-0.11Zr-0.005Er (at.%) alloy is investigated utilizing isochronal (200-600 degrees C) and isothermal (400, 425 and 450 degrees C) aging treatments. Atom-probe tomography measurements reveal that the slow-diffusing W, upon aging, partitions to the Ll(2)-nanoprecipitates (average composition of 0.11-0.37 at.% W; partitioning ratio of 5-12), without altering their crystal structure or sphe-roidal morphology, as confirmed by transmission electron microscopy. First-principles calculations indicate that W occupies the Al sublattice sites of Al3Zr(Ll(2)), leading to a small increase in the lattice parameter of the nanoprecipitates. In the low-Si (0.02 at.%) alloys, the precipitation and coarsening kinetics of nanoprecipitates are unaffected by W additions. A small enhancement in nanoprecipitate coarsening resistance is achieved, however, by W additions in the high-Si (0.4 at.%) alloys. The W-modified alloys exhibit significant improvements in compressive creep resistance at 300 degrees C; the threshold stress increases from 11 MPa in the W-free alloy to 15 and 16 MPa in the low-Si and high-Si alloys with W additions, respectively. This is explained by an increased lattice parameter mismatch between the Ll(2)-nanoprecipitates and the matrix, due to Er enrichment in the nanoprecipitates in the presence of W. Silicon additions, in alloys with and without W, lead to a higher peak microhardness (Delta HV similar to 90-110 MPa) and higher creep threshold stresses (Delta sigma th-2-4 MPa) by increasing the Ll(2) nanoprecipitate number density, but at the expense of their long-term coarsening resistance.