Crystal Plane-Orientation Dependent Phase Evolution from Precursor to Porous Intermediate Phase in the Vapor Phase Dealloying of a Co-Zn Alloy

Abstract

Understanding phase transitions and pore formation during vapor phase dealloying (VPD) is essential to optimize the microstructure and composition of nanoporous metals for versatile applications. Nevertheless, the underlying atomic mechanisms of phase formation and pore evolution during VPD are unknown. Using a binary γ-CoZn precursor alloy as a prototype system, we found a two-step dealloying process. The microsized porous β-CoZn intermediate phase that formed at the dealloying front facilitated the subsequent growth of completely dealloyed hierarchical nanoporous α-Co with the micropore structure of the intermediate phase. Combining aberration-corrected scanning transmission electron microscopy with energy dispersion X-ray spectrometry, we found that the intermediate phase preferentially formed on specific crystal planes of the precursor, and the vacancies generated by sublimation of Zn atoms diffused dominantly on the {110} planes of the precursor close to the intermediate phase. Theoretical calculations indicated that the energy barrier to the diffusion of Zn vacancies on the low-index {110} plane was lower than on other planes. The atomic-scale phase evolution played a key role in the subsequent evolution of the porous structure and provided deep insight into the phase transition during VPD. This insight may provide a new approach to tune the pore structure and composition of nanoporous metals by designing and regulating their intermediate phases.

Keywords: Vapor phase dealloying, porous structure, intermediate phase, Phase evolution