On the dealloying mechanisms of a rapidly solidified Al80Ag20 alloy using in-situ X-ray diffraction
Introduction
Nanoporous (np) metals have received great interests due to their unique three-dimensional bicontinuous nanoscale ligament-channel structure and potential applications in catalysis, electro-catalysis, actuation, as well as energy conversion and storage systems [[1], [2], [3], [4], [5], [6]]. Dealloying, during which a less noble element is selectively etched away from an alloy and a more noble element diffuses/re-organizes into a porous structure, is an important method to fabricate nanoporous metals [7,8]. Except for chemical/electrochemical dealloying in conventional aqueous solutions, some novel routes have recently been developed such as dealloying in lithium ion batteries with organic electrolytes [9], liquid metal dealloying [10], vapor phase dealloying [11], and so forth.
Understanding of dealloying mechanisms by theories and/or experiments is of significance to probe the formation of nanoporous structures and to modulate the microstructures/properties of nanoporous metals. Erlebacher provided an atomic description of dealloying using a kinetic Monte Carlo model, addressing the scenarios of nanoporosity evolution, the critical potential and rate-limiting behavior of solid solution alloys like Ag–Au [12]. Specially, in-situ (operando) experiments including X-ray diffraction (XRD), Raman, scanning tunneling microscopy (STM), transmission electron microscopy (TEM) could give deeper insight into corrosion mechanisms and porosity evolution during dealloying, as compared with pre-/post-dealloying characterizations [[13], [14], [15], [16], [17]]. Oppenheim et al. [13] investigated corrosion of low Ag content Ag–Au alloys using in-situ STM and found that dealloying leads to surface roughening by dissolution of Ag atoms from terrace sites. Renner et al. [14] reported atomic-scale observations of the initial corrosion stages of a Cu3Au(111) single crystal alloy by in-situ XRD with picometre-scale resolution. Dotzler et al. [15] investigated strain development and porosity evolution in np-Au foils during dealloying of Ag65Au35 alloy by combining synchrotron small angle X-ray scattering (SAXS) and XRD. Most recently, Liu et al. [17] reported dealloying kinetics of AgAu nanoparticles by in-situ liquid-cell scanning transmission electron microscopy. Noticeably, in-situ XRD experiments were carried out on synchrotron beamlines (for example, the Australian Synchrotron [15,16]) due to the strong interference of aqueous solutions on X-ray signals. Thus, it is a great challenge to realize in-situ XRD measurements of dealloying processes on normal X-ray apparatuses rather than synchrotron beamlines.
In this work, we built a special sample holder for in-situ XRD experiments of dealloying, and a rapidly solidified Al80Ag20 alloy was selected as a model precursor for in-situ XRD studies. Although it has been reported that the dealloying of Al–Ag alloys results in the formation of np-Ag [18,19], the underlying mechanisms are still unclear, especially lack of in-situ information. Herein, the influence of the solution type and corrosion duration on the dealloying processes and involved phase evolutions was detailedly investigated. Some new dealloying scenarios were identified, including the inert nature of second phase, the lattice evolution of formed phase, the size effect, and so forth.
Section snippets
Experimental
The Al80Ag20 (nominal composition, at.%) alloy was prepared from Al and Ag metals (99.9 wt% purity) by induction melting, and the rapidly solidified ribbons were further fabricated by melt-spinning. The detailed procedure was available in the literature [18]. The in-situ dealloying was performed on an XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd, China) with a home-made sample holder (Fig. S1). A continuous scanning mode was used with a scan speed of 4 deg. min−1, and a
Results and discussion
Fig. 1a shows the XRD pattern of the rapidly solidified Al80Ag20 ribbons. Clearly, the precursor alloy is mainly composed of α-Al(Ag) solid solution. Although the maximum solid solution of Ag in α-Al exceeds 20 at.% [20], a minor Ag2Al phase can still be detected in the Al80Ag20 precursor. The back-scattered electron (BSE)-SEM images verify the two-phase microstructure of the Al80Ag20 precursor (Fig. 1b and c). The Ag2Al phase distributes along the grain boundaries of α-Al(Ag), forming a
Conclusions
In summary, in-situ XRD is a powerful tool to give insights into the dealloying mechanisms and to probe the evolution of involved phases during dealloying. In this work, the dealloying of the two-phase Al80Ag20 precursor was investigated using in-situ XRD and ex-situ SEM, and the following conclusions can be drawn.
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No intermediate or metastable phase can be detected during the dealloying of Al80Ag20 in both the NaOH and HCl solutions. The peak positions of the formed f.c.c. Ag show continuous
CRediT authorship contribution statement
Fakui Luo: Conceptualization, Methodology, Writing - original draft. Ying Zhang: Conceptualization, Methodology, Visualization. Congcong Wei: Visualization, Investigation, Formal analysis. Chi Zhang: Validation, Investigation. Jianfeng Wang: Data curation, Resources. Zhonghua Zhang: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51871133 and 51671115), and the support of Taishan Scholar Foundation of Shandong Province, Department of Science and Technology of Shandong Province, the program of Jinan Science and Technology Bureau, and Innovation Projects of Department of Education of Guangdong Province, China.
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These authors equally contributed to this work.