In-situ and ex-situ synchrotron X-ray diffraction studies of microstructural length scale controlled dealloying

This paper reports the critical role of microstructural length scale in dealloying for the fabrication of bimodal or monolithic functional nanoporous metal structures and the underlying mechanisms and kinetics. Two dual-phased (Al2CuAlCu) precursor alloys Al65Cu35 and Al55Cu45 (at.%) were selected to demonstrate the concept. Microstructural observations revealed that the two constituent phases Al2Cu and AlCu in each alloy can undergo either sequential dealloying, which leads to bimodal nanoporous Cu, or simultaneous dealloying, which results in monolithic nanoporous Cu. In-situ and ex-situ synchrotron X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and potentiodynamic polarization scans were used to identify the detailed phase evolution processes and kinetics. It is concluded that microstructural length scale plays a decisive role in regulating the dealloying pathways. Sequential dealloying of Al2Cu and AlCu occurs when both phases are micrometer-scaled, while simultaneous dealloying takes over when both phases are nanoscaled. The nanosize effect of Al2Cu and AlCu can override their intrinsic difference in electrochemical potential at the micro- or macro-scale, and the advantage of tetragonal Al2Cu over monoclinic AlCu in crystallographic transition to face-centered-cubic (FCC) Cu by dealloying. The high-resolution in-situ synchrotron XRD data revealed a two-stage kinetic process for dealloying of Al2Cu to Cu. The Avrami-Erofe′ev kinetic model provides an excellent description of each stage. The underlying rationales and implications are discussed.