Rationalising the Role of Microalloying Additions in Blended Precipitation

The precipitation of ${\theta }^{\prime }$ (Al₂Cu) in Al-Cu alloys is greatly influenced by microalloying elements. Combining high-resolution scanning transmission electron microscopy (STEM) with density functional theory (DFT) and classical nucleation theory (CNT) calculations, we have investigated the generality of a recently discovered mechanism that enhances the precipitation of the ${\theta }^{\prime }$ precipitate phase through the dissolution of trace Au additions within ${\theta }^{\prime }$. We have designed a workflow to systematically screen chemical elements and found that, Pd and Pt can also enhance the precipitation of ${\theta }^{\prime }$ by the same mechanism as Au. All these three elements are found to substitute Cu atoms within ${\theta }^{\prime }$, forming what we call “blended precipitates,” namely, precipitates containing regions of both ${\theta }^{\prime }$ and another phase of nearly identical crystal structure (i.e., $\eta$ (Al₂Au), Al₂Pt or Al₂Pd). According to our calculations, enhanced precipitation originates from the lowering of different energy contributions to the substitution of Cu atoms. Among these elements, Pt is the most promising choice for microalloying in the Al-Cu alloy system as it decreases both the formation energy of the ${\theta }^{\prime }$ blended precipitate phase as well as the energy of its interface with the Al matrix. This work illustrates the effectiveness of the workflow developed here and should stimulate the exploration of other alloy systems displaying blended precipitate phases, with potentially improved mechanical properties.