Relative phase stability of L12 and DO22/DO23 structures in Al3Nb, Al3Zr and Al3V compounds

Abstract

The relative stability of the different tri-aluminide (Al₃M) phases in three binary systems (M = Zr, Nb and V) was assessed for their potential to form fine cubic L1₂ precipitates in additively manufactured alloys. Supersaturated thin films of Al-(8–30) at% M were sputtered and heat treated during in-situ x-ray diffraction (XRD) measurements to observe the temperature ranges of stability for each phase. As-sputtered films were then processed with laser tracks simulating additive manufacturing solidification conditions, and the formation of phases in the laser tracks was correlated with density functional theory (DFT) and nucleation rate calculations. We found that the metastable L1₂ structure is highly competitive with the stable DO₂₃ structure in the Al-Zr system, but much less stable than the DO₂₂ structure in the Al-Nb system, and both the DO₂₂ and Al₈V₅ structure in the Al-V system. These experimental results were found to be in good agreement with the DFT and kinetic calculations, as we determined that the metastable L1₂ in Al-Zr only requires a small amount of undercooling to favor its nucleation over the stable DO₂₃, suggesting additive manufacturing can be a viable pathway to develop Al-Zr alloys strengthened by a high volume fraction of L1₂ Al₃Zr phase.