Lower Scale - Research Programme - Centre for Advanced Structural Analysis - CASA
This programme concentrates on the lower length scales of materials, from atomic up to micrometre scale, and will provide experimental and calculated input to the multi-scale framework and constitutive models from the lower scale. This will provide input for microstructure evolution, strength and work hardening for metallic materials, such as aluminium and steels, and a foundation for development of physically based models for crystal plasticity, continuum plasticity, damage and fracture. Advanced constitutive models, including plastic anisotropy, non-linear isotropic and kinematic hardening, strain-rate and temperature dependence, damage evolution and failure, tend to have a large number of model parameters, some of which need to be taken from a lower scale.
Objective and scope
In the lower scale programme we explore the nanoscale of the materials, using modelling and experiments as input to the constitutive modelling in a top-down/bottom-up methodology for large-scale numerical simulations of metal structures. Qualitative and quantitative descriptions at the lower length scales (i.e., nano- and microscales) will be closely accompanied by well-designed experiments at the length scales of relevance for the phenomena of interest. In this programme the microstructure – dislocations, grain boundaries, precipitates and precipitate free zones – will be studied and modelled in details. The results will provide a more fundamental understanding of mechanical properties and deformation of metal structures in a multi-scale framework (from the nano-scale to the complete structure). This will work as a basis for achieving improved models and used in both model developments and validations.
In the lower scale programme we have 4 PhD students working on the following tasks:
Nanoscale Characterization of Deformed Aluminium Alloys
The project deals with micro- and nanostructure characterisation of deformed aluminium alloys using transmission electron microscopy (TEM). We want to increase our understanding of the underlying physical mechanisms of strength, work-hardening and ductile fracture of aluminium alloys and thus to develop enhanced models of these phenomena. This project is concerned with the interaction of dislocations with the aluminium microstructure and, in particular, the role played by the precipitation free zones (PFZs) and grain boundary precipitates in ductile fracture. A systematic study of PFZs close to grain boundaries (different types of grain boundaries) and how they evolve with straining (either in compression or tension) will be performed. Dislocation densities will be quantified and type/density of hardening precipitates will be studied systematically. Another topic is nucleation and growth of voids around grain boundary precipitates under tension dominated straining.
Micromechanical modelling of steel
This project focuses on combining experimental work and modelling activities to describe the correlation between microstructure and mechanical properties in multiphase steel. In all structural materials the microstructure will be heterogeneous and the mechanical properties will strongly depend on the local variations and the thermomechanical history of the material. A systematic experimental study including, nanomechanical study of local properties and in-situ testing using scanning electron microscope (SEM), with the possibility of cooling the material to sub-zero degrees, will give input data for mathematical models for understanding and describing the performance of heterogeneous materials based on the microstructure information. The experimental tests will be combined with digital image correlation (DIC) to obtain detailed information about the local deformation.
Work hardening and Portevin-Le Chatelier (PLC) effect
This project aims at an improved understanding and quantitative description (characterization and modelling) of solute solution strengthening and the Portevin-Le Chatelier (PLC) effect (or dynamic strain aging). Dynamic strain aging arises from the interaction between solute atoms and matrix dislocations in strained metallic alloys. It initiates jerky dislocation motion and abrupt softening, causing negative steady-state strain rate sensitivity. This effect leads to instable flow phenomena at the macroscopic scale, appearing as a serrated stress–strain response and deformation banding. In some cases, the PLC effect has been found to promote shear failure and thus to reduce the ductility of the alloy. In particular, the PLC effect is a well-known problem in Al–Mg alloys (5xxx-series alloys) where Mg is the main alloying element, used e.g. for automotive sheet-forming applications. However, the phenomenon is also highly relevant for plastic forming of age-hardening aluminium alloys (6xxx- and 7xxx-series alloys) in the W temper.
Atomistic and Multiscale modelling of Aluminium alloys
This project focuses on multiscale modelling of aluminium alloys. The main focus will be on the coupling of atomistic simulations and discrete meso-scale methods such as dislocation dynamics, and how to use these methods to calibrate hardening models at a continuum scale. The goal is to study the effect of solute atoms and hardening precipitates, and how these affect initial yielding and hardening under different conditions. Experimental results from other tasks will be used as input and verification for the simulations. A potential second step is to study the interaction between dislocations and grain boundaries and how to model this on an atomistically informed meso-scale.