Ambient- and elevated-temperature strengthening by Al3Zr-Nanoprecipitates and Al3Ni-Microfibers in a cast Al-2.9Ni-0.11Zr-0.02Si-0.005Er (at.%) alloy

Abstract

Strengthening mechanisms at ambient and elevated temperatures are studied in a cast Al-0.11Zr-0.02Si-0.005Er (at.%) alloy with a 2.86 at.% Ni addition, containing: (i) incoherent Al3Ni microfibers formed during eutectic solidification; and (ii) coherent, equiaxed Al3Zr (L12-structure) nanoprecipitates created on subsequent aging. Strengthening contributions from microfibers and nanoprecipitates are cooperative at ambient temperature, over the full range of Al3Zr precipitation during under-, peak-, and over-aging states. In contrast, during compressive creep testing at 300 °C, the binary eutectic Al-Al3Ni alloy is not further strengthened by the Al3Zr nanoprecipitates, reflecting their lower number density (5.8 × 1022 m−3) in the regions between Al3Ni microfibers, where load transfer and/or microfiber/dislocation interactions provide strengthening. Also, when the Al-0.11Zr-0.02Si-0.005Er (at.%) alloy is modified with very low Ni concentrations of 0.07 at.%, without Al3Ni microfiber formation, the precipitation kinetics of Al3Zr(L12) are unaffected and negligible amounts of Ni are measured in the nanoprecipitates. The binary Al-2.86Ni at.% alloys with Al3Ni eutectic microfibers, with and without Al3Zr nanoprecipitates, are significantly more creep resistant at 300 °C than dilute Al-Sc or Al-Zr alloys strengthened solely by Al3Zr or Al3Sc nanoprecipitates. Unlike Al-Zr alloys, their upper service temperature is, however, limited to ∼400 °C, above which Al3Ni coarsening becomes rapid.