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Localized necking in nanocrystalline metallic thin films under static and relaxation conditions

By Guerric Lemoine (iMMC/ Université catholique de Louvain)
Co-authors: Laurent Delannay (iMMC/ Université catholique de Louvain)
Thomas Pardoen (iMMC/ Université catholique de Louvain)

Ductility of thin metallic films is a major issue in a variety of technologies involving flexible electronics, MEMS and deformable coatings. An enriched Marciniak-Kuczynski type imperfection based localization analysis is developed here in order to investigate localized necking in metallic films [1]. The model considers the localization process in a finite length specimen under any proportional loading. A strain gradient plasticity contribution to the hardening stabilizes the localization process, relying on a simple estimate of the deformation gradient inside the necking zone. The model, with gradient plasticity effects, is validated towards 2D finite element simulations. The response of the material involves both strain-hardening and rate sensitivity, as well as possible creep relaxation mechanisms. The plastic flow parameters are related to the grain size and film thickness. In agreement with experiments, the predicted ductility value depends significantly on several parameters including yield strength, imperfection, strain hardening capacity and strain rate sensitivity resulting from thermally activated mechanisms at small grain sizes. Stabilization can be reinforced by gradient plasticity effects. Qualitative comparisons with experimental data obtained by the on-chip test method are made and the interplay between deformation mechanisms is analysed based on observations by Jonnalagadda et al. on Au thin films [2]. References [1] Z. Marciniak, K. Kuczynski, Limit strains in the processes of stretch-forming sheet metal, International Journal of Mechanical Sciences 9 (1967) 609–620. [2] K. Jonnalagadda, N. Karanjgaokar, I. Chasiotis, J. Chee, D. Peroulis, Strain rate sensitivity of nanocrystalline Au films at room temperature, Acta Materialia 58 (2010) 4674–4684.

Ⓒ Photos:Toerisme Leuven