Sub-micron metallurgy: How intermittent plasticity can be mitigated? – SUMMIT

The extreme miniaturization in modern technology calls for deeper insights into the non-conventional, fluctuation dominated mechanics of materials at micro- to nano-scales. At these scales, the nature of dislocation-mediated plasticity changes radically: sub-micron metallic samples exhibit high, but strongly scattered, yield strength, however corrupted by intermittent strain fluctuations compromising forming processes and endangering structural stability. Despite many observations showing that quenched disorder can suppress plastic fluctuations, this effect has not been quantified in terms of avalanche statistics. Consequently, the theoretical framework based on the quantitative knowledge of the relationship between the material characteristics and mechanical fluctuations has not been established yet. This imposes severe restrictions on utilizing such strategy in practice, particularly in the structural analysis and design of microelectromechanical systems (MEMS).
Combining the expertises of the Chinese team in material elaboration and micro-mechanical testing and of the French team in theoretical modeling and numerical simulations, we expect to build a quantitative approach to (1) systematically understand the origin of fluctuations, and its dependences on the material properties, (2) explore how the introduction of disorder, such as solutes or precipitates, used in classical metallurgy to harden materials, can be used at sub-micron scales to suppress intermittent fluctuations, and (3) propose metallurgical strategies/methodologies to mitigate intermittency and instabilities at small scales. This will be done from compression experiments on micro-pillars in various, pure or alloyed, metallic materials, a statistical analysis of strain fluctuations, and the development of a theoretical framework associated with a new finite-element simulation tool introducing stochasticity.
The French and Chinese groups launched collaboration in 2015. They made a first step towards the quantification of fluctuations in pure Aluminium and Al alloys, revealed a disorder-related effect on intermittent plasticity, and built a theoretical framework from a constitutive modelling introducing stochasticity that unifies the combined effects of external size and disorder (Zhang P, Salman O.U., Zhang J-Y, Liu G, Weiss J, Truskinovsky L, Sun J. Taming intermittent plasticity at small scales. Acta Mater. 2017). However, this analysis was restricted to face-centered cubic (FCC) Al alloys. In order to get a comprehensive perspective on intermittent plasticity at nano- to micro-scales, and on the strategies to mitigate the associated deleterious effects, we are applying for an extension of this seminal work to body-centered cubic (BCC) and hexagonal close-packed (HCP) metals, for which different mechanisms may take place.