Predicting the Early Stages of Solid-State Precipitation in Al-rich Al-Pt Alloys

Abstract: The high strength of structural aluminium alloys depends strongly on the controlled precipitation of specific intermetallic phases, whose identity and crystal structure can be difficult to predict. Here, we investigate the Al-Pt system, which shows some similarity to the Al-Cu system as one of their main intermetallic phases, $Al_2Pt$, is nearly isostructural with $\theta^{\prime} (Al_2Cu)$, the metastable phase responsible for the high-strength of Al-Cu alloys. However, phases in Al-Pt alloys are complex and have not been studied in detail. Using a combination of density-functional theory (DFT) calculations and classical nucleation theory (CNT) applied to the Al-Pt system, we design a workflow to predict the thermodynamics of solid solution, intermediate phases such as GP zones, stable and metastable precipitates, and their precipitation sequence. This workflow can be applied to an arbitrary binary alloying system. We confirm the known stable phases $Al_4Pt$, $Al_{21}Pt_8$, $Al_2Pt$, $Al_3Pt_2$, $AlPt (\alpha & \beta)$, $Al_3Pt_5$, $AlPt_2 (\alpha & \beta)$ and $AlPt_3 (\alpha & \beta)$. We also reveal the possible existence of two phases of chemical formulae $Al_5Pt$ and $Al_3Pt$. This large number of intermetallic phases is due to the strong bonding between Al and Pt, which also leads to significant favourable Pt solute formation energy in the Al matrix. Our findings are compared with the known precipitation characteristics of the binary Al-Cu and Al-Au systems. We find that the $\theta^{\prime}$-like $Al_2Pt$ precipitate phase has a lower coherent interfacial energy than $\theta^{\prime}$. Our calculations strongly suggest that $Al_2Pt$ will precipitate first in Al-rich Al-Pt alloys and will form bulk-like interfaces similar to $\eta (Al_2Au)$ rather than like $\theta^{\prime}(Al_2Cu)$.