Revealing Multistep Phase Separation in Metal Alloy Nanoparticles with In Situ Transmission Electron Microscopy

Phase separation plays a crucial role in many natural and industrial processes, such as the formation of clouds and minerals and the distillation of crude oil. In metals and alloys, phase separation is an important approach often utilized to improve their mechanical strength for use in construction, automobile, and aerospace manufacturing. Despite its importance in many processes, the atomic details of phase separation are largely unknown. In particular, it is unclear how a different crystal phase emerges from the parent alloy. Here, using real-time in situ transmission electron microscopy, we describe the stages of the phase separation in face-centered cubic (fcc) AuRu alloy nanoparticles, resulting in a Ru phase with a hexagonal close-packed (hcp) crystal structure. Our observation reveals that the hcp Ru phase forms in two steps: the spinodal decomposition of the alloy produces metastable fcc Ru clusters, and as they grow larger, these clusters transform into hcp Ru domains. Our calculations indicate that the primary reason for the fcc-to-hcp transformation is the size-dependent competition between the interfacial and bulk energies of Ru domains. These insights into elusive, transient steps in the phase separation of alloys can aid in engineering nanomaterials with unconventional phases.