High Resolution Studies of Dealloyed Layers
Authors: El-Zoka Ayman A. , Perovic Doug D. , Newman Roger C. , Langelier Brian 
Published: February 2020

The nanoscale morphology of dealloyed materials has been studied for many years, from Pickering and Swann [1], through A.J. Forty [2], to recent TEM and atom-probe studies [3, 4]. Modern instruments have the ability to reveal key features of dealloying at unprecedented resolution, as well as having facilities for heating and environment control. Our particular interest is the dealloying behaviour of ternary alloys, such as AgAuPt, following the recent work of Vega [5]. Another area of interest is stress corrosion cracking of such materials, which features film-induced brittle events [6, 7].

Progress will be reported in several areas, including –

High-resolution ATEM studies of dealloyed binary and ternary alloys
, to observe and account for the distributions of Ag, Au and Pt for various dealloying conditions (binary AgPt is included in this part of the study).

In situ heating studies of dealloyed binary and ternary alloys
, to gain more insight into the observations of Vega [8] regarding oxygen-induced surface segregation of Pt, and other ways to manipulate the surface composition of the ligaments within the nanoporous material.

Conventional underpotential deposition and novel “sub”-potential deposition of bulk Cu
, as shown by Lee et al. for deposition of Cu into dealloyed CuPt [9].

Pore-filling of dealloyed materials by electrodeposition
, often believed to be impossible, but actually a relatively easy method, provided the conditions are controlled very precisely and one is only dealing with a surface layer. Naturally this method is more challenging for ternary than binary alloys, owing to the smaller pore size. Copper is the initial metal of choice for pore filling. Atom-probe tomography studies are in progress, as favourable sites within the sample can be chosen for tip fabrication, even when pore filling is uneven. Initial results show good promise as a way to determine definitively the elemental distributions.

References

[1] H. W. Pickering and P. R. Swann. Electron metallography of chemical attack upon some alloys susceptible to stress corrosion cracking, Corrosion, 1963, 19, 373t.

[2] A. J. Forty. Corrosion micro-morphology of noble-metal alloys and depletion gilding, Nature, 1979, 282, 597.

[3] B. Pfeiffer, T. Erichsen, E. Epler, C. A. Volkert, P. Trompenaars, and C. Nowak. Characterization of nanoporous materials with atom probe tomography, Microscopy and Microanalysis, 2015, 21, 557-563.

[4] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher and M. Chen. Atomic origins of the high catalytic activity of nanoporous gold, Nature Materials, 2012, 11, 775–780.

[5] A. A. Vega and R. C. Newman. Nanoporous metals fabricated through electrochemical dealloying of Ag-Au-Pt with systematic variation of Au:Pt ratio. Journal of the Electrochemical Society, 2014, 161, C1-C10.

[6] Andrew Barnes, N.A. Senior and R. C. Newman. Film-induced cleavage of Ag-Au alloys. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 2009, 40, 58-68.

[7] S. Sun, X. Chen, N. Badwe and K. Sieradzki. Potential-dependent dynamic fracture of nanoporous gold. Nature Materials, 2015, 14, 894-898.

[8] A. A. Vega and R. C. Newman. Beneficial effects of adsorbate-induced surface segregation of Pt in nanoporous metals fabricated by dealloying of Ag-Au-Pt alloys. Journal of the Electrochemical Society, 2014, 161, C11-C19.

[9] L. Lee, D. He, A.G. Carcea and R.C. Newman. Exploring the reactivity and nanoscale morphology of de-alloyed layers. Corrosion Science, 2007,
49, 72–80.