Modeling and measurements of creep deformation in laser-melted Al-Ti-Zr alloys with bimodal grain size

Elemental powders of Al, Ti, Sc, and Zr are blended to create binary Al-0.5Ti and Al-0.25Ti-0.25Zr (at%) alloys via laser powder-bed fusion (L-PBF), with Al-1Ti and Al-0.25Sc-0.25Zr alloys produced for comparison. The Al-Ti alloys formed primary D023-Al3Ti precipitates upon solidification, whereas the Al-(Sc/Ti)-Zr alloys formed primary L12-Al3(Sc,Ti,Zr) precipitates which nucleated bands of fine grains. Upon aging, the Al-Ti alloys do not harden (consistent with most Ti remaining in solid solution), unlike the Al-(Sc,Ti)-Zr alloys, as expected from precipitation of secondary L12-Al3(Sc,Ti,Zr) nanoprecipitates. Upon compressive creep testing at 300-400 degrees C, Al-0.25Ti-0.25Zr exhibits steady-state strain rates comparable to those of Al-0.5Zr at low stresses (up to-14 MPa), but about an order of magnitude faster at higher stresses. The effects of bands of fine grains (formed by L12- Al3(Sc,Ti,Zr) primary precipitates at the bottom of the melt pools) on hardness and creep resistance is investigated via finite element modeling of simple slab models, with varying geometries approximating experimental melt pool geometries. The degree of connectivity of coarse-and fine-grain regions is a greater factor in the resulting creep strain rates than the volume fraction or spatial orientations of these two regions. This suggests that reducing the concentration of Zr (in Al-0.5Zr), or partially substituting with a less expensive element such as Ti (as for Al-0.25Ti-0.25Zr), may maintain the added L-PBF processability benefits of Zr and reduce the volume fraction of fine grains without sacrificing creep resistance, allowing for further tailoring of custom microstructures.