Seminar of Prof. Dan Mordehai from Technion (Faculty of Mechanical Engineering) in Israel, entitle “Microstructure-based Plasticity of Metals – Going from the Atomic to the Continuum Scale” – 22nd of April, at 12:00 pm in the Auditorium

Abstract:

Microstructural defects on the atomic level are known to be responsible to the macroscopic deformation of metals. Despite, the study of microstructure-based plasticity of metals is still a great challenge owing to the multiphysics and multiscale nature of plasticity. In this talk, I will present the activity of our group at the Technion in recent years, and will detail on two examples on how we use information gained from the atomic scale to study plasticity. First, I will introduce a dislocation-based constitutive rule for the dynamic strength of metals. The constitutive rule is informed with mobility rules obtained from molecular dynamics (MD) simulations. In addition, thermally-activated dislocation nucleation is implemented in the model. The continuum simulations are shown to predict accurately the dynamic strength in a large range of strain rates and temperatures, as well as the Richtmyer-Meshkov instability. With the help of the model, we revisit the interpretation of the dynamic strength in metals. In the second example, I will demonstrate analysis of the deformation of nanoporous Au structures under compression. Using MD simulations, we show that the stress-strain curves demonstrate an initial non-linear regime up to compressive true strains of about 5%, followed by a stress plateau and a strong hardening stage. At low strains, the ligament size-dependent elastic modulus of nanoporous nanopillars at a given solid fraction was found to depend on both the geometrical characteristics of the nanopillars and topology of the ligament structure. To rationalize the topology-dependent elastic behavior, we skeletonized the nanoporous structure and we show that load-bearing ligaments are contributing to the elastic response. The skeletonization is then combined with dislocation analysis, in order to identify yielded ligaments. We found that the elastic-plastic transition starts at a strain substantially lower than that of the plateau and the load bearing capacity is further retained by the unyielded ligaments. At the onset of the plateau stage, the unyielded ligaments cannot bear the load, which manifests itself in nullification of the genus of the subnetwork of unyielded ligaments. The stress starts increasing again as the ligaments coalesce and the nanoporous structure densifies. We propose a correlation between the hardening and the topology of the structure. During coalescence, grain boundaries were formed between coalesced ligaments, some are either removed or retained. As a result, at very large compressive strains the single crystal nanoporous nanopillar turns into a polycrystalline specimen with some disconnected pores.