Effect of oxide dispersoids on precipitation-strengthened Al-1.7Zr (wt %) alloys produced by laser powder-bed fusion

Binary Al-1.7 wt % Zr (Al-0.5 at % Zr) alloys, with and without 1 wt % (0.7 vol %) Al2O3 nanoparticle additions (80-100 nm), are fabricated by laser powder-bed fusion (L-PBF) from blends of Al, Zr, and Al2O3 powders. Elemental Zr, which is similar to 80 % dissolved in the melt pool, forms primary L1(2)-Al3Zr precipitates upon solidification and is also retained in a supersaturated solid solution in the Al matrix. The primary L1(2)-Al3Zr phase takes three forms: (i) discrete, micron-sized precipitates that nucleate fine Al matrix grains, (ii) elongated "strings " of submicron precipitates forming in highly enriched Zr regions, and (iii) < 100 nm precipitates within fine Al-grains. The Al2O3 nanodispersoid additions are either lost to slag or entrapped within Al-grains as oxide dis-persoids, resulting in an oxide concentration of 0.46 wt % for the modified ODS alloy. Al2O3 nanoscale dis-persoids (0.28 wt %) are also observed in the unmodified alloy, and are assumed to originate from (i) native oxides from the surface of the Al powder particles, and/or (ii) reaction with trace amounts of oxygen in the processing environment. The hardness upon aging at 400 C reaches a peak value of-750-850 MPa after 1.5 h, consistent with precipitation of secondary L1(2)-Al3Zr nanoprecipitates. Upon overaging, hardness slowly de-creases to 500 MPa (the as-processed value) after 1,500 h at 400 C, with no difference observed between the Al -Zr and Al-Zr-Al2O3 alloys. Strengthening mechanisms combines Hall-Petch strengthening from micron-sized grains, Zr solid-solution strengthening, and secondary L1(2)-Al3Zr precipitate strengthening, with slow precipi-tate coarsening and effective grain pinning by Al3Zr precipitates and oxide dispersoids located at grain bound-aries. Creep resistance is comparable for both alloys, which show threshold stresses of ~15 and 6 MPa, at 300 and 400 C respectively. Oxygen content analysis is recommended for all Al-based alloys fabricated via L-PBF due to the potential for non-reproducible nanoscale oxide inclusions (from native oxide on Al powders and from reaction with residual oxygen), which may affect the mechanical properties of the alloys, but are difficult to observe via SEM or TEM analysis.