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

Understanding phase transformations in binary alloy nanoparticles is an important scientific and technological problem. We tackled the challenge by experimentally studying the solidification pathways of single AuGe nanoparticles on Si3N4 substrate using in situ TEM techniques. The nanoparticles with sizes of 100–160 nm were slowly cooled from temperatures below and above the liquidus, enabling visualization of solidification pathways of liquid and partly molten alloy on the same AuGe nanoparticle in a TEM. Consequently, the crystalline structure, size, and composition of equilibrium and metastable phases formed in solidifying nanoparticles of near-eutectic (28 at. % Ge), hypo-eutectic (25 at. % Ge), and hyper-eutectic (38 at. % Ge) compositions were determined. We found a nonclassical solidification pathway of near-eutectic and hypo-eutectic composition AuGe nanoparticles. Solidification of highly undercooled nanoparticles occurred over a temperature range rather than at a single temperature. The heterogeneous nucleation process governed the onset solidification temperature, while the thermodynamic stability of the undercooled liquid defined the terminal temperature. It was shown that the number of phases coexisting in a single nanoparticle during the solidification depended on its composition and initial phase state, reaching a maximum of four phases. Thus, the liquid phase, fcc Au, hcp Au, and Ge coexisted in nanoparticles of hypo-eutectic composition in the temperature range of 260–210 °C. For the first time, a single-crystalline hexagonal Au phase was observed in liquid AuGe nanoparticles. Thermodynamic analysis showed that hcp Au is the equilibrium phase of the nucleus during the solidification of undercooled AuGe nanoparticles.