Using High Sensitivity – Low Energy Ion Scattering Spectroscopy (HS-LEIS) to unravel the complex nature of multi-principal element alloys

Abstract

Complex metallic materials such as Multi-Principal Alloys (MPEAs) and High Entropy Alloys (HEAs) have emerged as a promising class of materials given their unique inherent characteristics. First mentioned by Cantor [1] and Yeh [2], the Cantor alloy (CrMnFeCoNi) and associated subsystems are among the best-known representatives. Excellent mechanical, thermal, and corrosion properties allow for a broad spectrum of applications. However, due to the multi-element nature of these alloys, characterization of the composition and microstructure, and subsequent linking to material behavior, proves to be a challenging task. Especially with regard to corrosion-protective passivation films, the complex correlations with the corrosion behaviour are fully unclear to date, and require an in-depth atomic level characterisation and rationalisation. Since the thickness of such oxides lies in the region of 1-5 nm, the precise layer by layer structure of these passive films is particularly demanding to assess. Conventional advanced surface techniques, such as XPS (X-ray photoelectron spectroscopy) or AES (Auger electron spectroscopy), are employed to address this analytical question, providing valuable information about the outer few nanometres of the material. However, due to analysis penetration depths of several nanometres, they cannot achieve atomic layer resolution. To fully understand and quantify the passivation layer structure, such an atomic layer resolution of the surface region is necessary, due to the complexity of the alloys. In order to obtain an exact understanding of the atomistic mechanism at the monoatomic layer level, High-Sensitivity - Low Energy Ion Scattering Spectroscopy (HS-LEIS), was applied. This technique is based on the scattering of low-energy noble gas ions on the material surface. Due to the low energies of the primary ions – typically 1-8 keV, there is a high probability of neutralization upon interaction with the surface. Since only charged particles are registered by the detector, only those ions scattered at the outermost atomic layer have sufficiently short interaction times to avoid neutralization. [3] By exploiting this principle, the required monolayer resolution to study the passivation layers of such complex multi-component alloys can be provided. In this work, the quinary Cantor alloy and different subsystems were investigated with LEIS. The unique surface sensitivity combined with the implementation of novel in-situ treatment methods enabled the real-time study of oxide layer growth, as well as the analysis of temperature-dependent changes in the elemental surface composition. Due to the high resolution achieved by static and dynamic sputter depth profile modes, we could determine the exact composition of the HEA passivation layer with resolution on atomic monolayer scale. The findings provide the potential to significantly advance the current understanding of the passivation behaviour of MPEAs and HEAs, and the development of novel metallic materials with superior properties. Valuable insights for understanding the material characteristics for those highly advanced materials could thereby be generated. For the LEIS instrument, the funding from the state of Lower Austria and the European Regional Development Fund under grant number WST3‐F‐542638/004‐2021 is gratefully acknowledged. Financial support of “Gesellschaft fuer Forschungsfoerderung NOE” (FTI21 - Dissertationen) and the FFG COMET funding scheme (Competence Centers for Excellent Technologies by BMVIT, BMDW as well as the Province of Lower Austria and Upper Austria) is gratefully acknowledged. [1] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Materials Science and Engineering: A, 375-377 (2004) 213-218. [2] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Advanced Engineering Materials, 6 (2004) 299-303. [3] C.V. Cushman, P. Brüner, J. Zakel, G.H. Major, B.M. Lunt, N.J. Smith, T. Grehl, M.R. Linford, Anal. Methods, 8 (2016) 3419-3439.