Dealloying of Binary Alloys: Evolution of Nanoporosity
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Nanoporous metals (NPMs) are materials formed during selective chemical or electrochemical dissolution of one or more components out of a homogenous alloy. This process is known as dealloying or selective dissolution. Dealloying transforms initially dense alloys into materials with a uniform, open porosity, with ligament widths and spacings tunable between just a few nanometers to many hundreds of nanometers, and usually employing only the simplest electrochemical processing. This article reviews the physical characteristics of NPMs, the experimental variables that can be fruitfully varied in their fabrication, and the theoretical models that attempt to explain nanoporosity evolution during dealloying.
Historically, selective dissolution has been primarily studied in those technologically important systems for which the formation of nanoporosity is undesirable. For instance, Zn can be selectively dissolved from brass (Cu/Zn alloys) and can lead to stress corrosion cracking. The dealloying of strength-hardening S-phase precipitates in the important aviation alloy Al-2024 can lead to pitting corrosion and, ultimately, materials failure; even stainless steels may be dealloyed of iron if the chromium content is too small. Within the context of corrosion, dealloying is obviously problematical, and a significant amount of effort has been made in its avoidance.