Formation and Coexistence\nof Metastable Phases during\nSolidification of Single AuGe Nanoparticles: An In Situ TEM Study

Understanding phase transformations in binary alloy nanoparticles\nis an important scientific and technological problem. We tackled the\nchallenge by experimentally studying the solidification pathways of\nsingle AuGe nanoparticles on Si<sub>3</sub>N<sub>4</sub> substrate\nusing in situ TEM techniques. The nanoparticles with sizes of 100–160\nnm were slowly cooled from temperatures below and above the liquidus,\nenabling visualization of solidification pathways of liquid and partly\nmolten alloy on the same AuGe nanoparticle in a TEM. Consequently,\nthe crystalline structure, size, and composition of equilibrium and\nmetastable phases formed in solidifying nanoparticles of near-eutectic\n(28 at. % Ge), hypo-eutectic (25 at. % Ge), and hyper-eutectic (38\nat. % Ge) compositions were determined. We found a nonclassical solidification\npathway of near-eutectic and hypo-eutectic composition AuGe nanoparticles.\nSolidification of highly undercooled nanoparticles occurred over a\ntemperature range rather than at a single temperature. The heterogeneous\nnucleation process governed the onset solidification temperature,\nwhile the thermodynamic stability of the undercooled liquid defined\nthe terminal temperature. It was shown that the number of phases coexisting\nin a single nanoparticle during the solidification depended on its\ncomposition and initial phase state, reaching a maximum of four phases.\nThus, the liquid phase, fcc Au, hcp Au, and Ge coexisted in nanoparticles\nof hypo-eutectic composition in the temperature range of 260–210\n°C. For the first time, a single-crystalline hexagonal Au phase\nwas observed in liquid AuGe nanoparticles. Thermodynamic analysis\nshowed that hcp Au is the equilibrium phase of the nucleus during\nthe solidification of undercooled AuGe nanoparticles.