Effects of Sb micro-alloying on precipitate evolution and mechanical properties of a dilute Al-Sc-Zr alloy

An Al-Sc-Zr aluminum alloy with Sb micro-addition (Al-0.066Sc-0.050Zr-0.021Sb at%) is cast and heat-treated to study the effects of Sb on the nucleation, growth, coarsening kinetics and precipitate morphology, and resulting mechanical properties at ambient and elevated temperatures. When isochronally aged, the Sb-containing alloy exhibits a peak microhardness (607±12 MPa) at 475 °C, which is greater than that of a comparable Sb-free alloy at the same temperature (549±17 MPa), and a smaller rate of decrease of microhardness values due to precipitate coarsening for aging temperatures >475 °C. When isothermally aged, the Sb-containing alloy achieves larger peak microhardness values at 300 °C for more than a month (~80 MPa difference) and 400 °C for ~8 h (~200 MPa difference) than the Sb-free alloy. Atom-probe tomography of the peak-aged Sb-containing alloy demonstrates that Sb partitions to the precipitates, and is enriched in the Zr-rich shell (up to 0.35 at% Sb). For creep testing at 300 °C, the Sb-containing alloy exhibits smaller steady-state strain-rates than the Sb-free control alloy at applied stresses >15 MPa. It is hypothesized that the effects of Sb micro-alloying (partitioning to precipitates, enhanced precipitate coarsening and higher creep resistance) are linked with the following mechanisms: (i) enhanced Zr diffusion in the matrix due to attractive Sb-Zr interactions; (ii) reduction in matrix/precipitate interfacial free energy, when Sb is present; and (iii) an increase in precipitate/matrix lattice parameter mismatch resulting in stronger elastic interactions with dislocations.