Stabilization of Al 3 Zr allotropes in dilute aluminum alloys via the addition of ternary elements
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The formation of Al3Zr particles within dilute aluminum alloys can contribute effectively to controlling microstructure evolution and enhancing material properties. However, the possible transformation of Al3Zr from its initial metastable crystal structure L12 into its stable, tetragonal structure D023 is associated with faster coherency loss and the coarsening of Al3Zr particles. In this regard, our study aims at identifying ternary elements that can disrupt this mechanism. For this purpose, nine ternary Al-Zr-X alloys (Er, Sc, Hf, Y, Nb, Mn, Cu, Zn and Si) plus a base alloy (Al-Zr) were produced. Isochronal aging was performed at 475 °C and 550 °C, and an investigation of the particle landscape was carried out by STEM and HR-TEM. In parallel, we conducted ab initio calculations to investigate fundamental properties of ternary AlZrX-particles such as substitution likeliness, heat of formation and transformation mechanisms. The elements investigated show various behaviors. Fewer than half of the elements (Er, Sc, Hf and Si) are found to be incorporated into Zr-rich particles to any large extent; Er and Sc exhibit the well-known core-shell structure. Y and Zn do not interfere at all with the precipitation process. Nb, Mn and Cu form particles on their own, with Zr particles often attached to them. Concerning crystal structures, all element additions except for Y and Si widen the stability regime of L12.