Research project P6/24 (Research action P6)
General Context
Strength and deformability of polycrystalline metals are determined by phenomena at various length scales. At a scale which we would label "nanoscopic", dislocations are created and pushed forward by the applied stress, achieving plastic deformation at the nano-scale. At the micro-scale, large numbers of moving dislocations interact and organize themselves in complex patterns. Still at higher scales, massive collective behaviour of these dislocations and patterns allow the plastic deformation of entire grains. On their turn they interact with each other finally leading to a certain mechanical response of the material at the macroscopic scale (i.e., the smallest scale at which it can be looked upon as a continuous medium). The response of the material on applied stresses depends on all these phenomena. Events on the various length scales may also cause important changes in the material, such as microstructure, internal damage and mechanical properties (strength and ductility). Note that the role of dislocations can partially be taken over by mechanical twinning or other stress-induced phase transformations.
All this has been extensively studied on all these length scales.
These studies certainly do have their merits, and have led to important experimental observations and theoretical understanding of the material behaviour at these length scales. However, in recent years it has become strikingly clear the events of each length scale do influence the events on other length scales, and that more significant progress in the understand (and modelling) of the material behaviour may be achieved by studying these relations than by further refining knowledge on each relevant length scale separately.
Finally, strong size effects occur when structural dimensions such as for instance film thickness or grain size starts interacting with the dislocation mean free path or the dislocation cell size, revealing a completely new and almost unexplored physics.
The engineering motivation for looking at these phenomena involves the development of higher performance materials, the optimisation of the manufacturing operations, and the improvement of the design and integrity assessment methods for both traditional (transport, energy) and emerging (MeMS, multifunctional active panels) structures.
Here ends the description of the field to which the present project wants to contribute. To achieve this, the following objectives have formally been defined:
Objectives
1. Understanding and modelling of the two-way relationship between the structure of metals at nano/ micro/ meso-scale and the mechanical behaviour at macro-scale.
"Structure" is a general term including dislocation patterns, microstructure, grain structure, crystallographic texture and damage. "Mechanical Behaviour" includes flow stress, work hardening and work softening, effects of strain path changes, ductile fracture and plastic anisotropy. Attention must be given to the coupling between the length scales. The latter is believed to influence or being influenced by a lot of phenomena, of which the following will be taken into account: dislocation, mechanical twinning, the competition between these two, heterogeneous distribution of stress and strain, size effects and finally also comparing behaviour at low and very high strain rates.
2. Development of models for the mechanical behaviour of the material at macro-scale to be used in finite element simulations at engineering scale.
An essential aspect is the development of appropriate schemes for the identification of the parameters of these macro-scale models on the basis of predictions made by multilevel-models. The latter results from the achievement of the previous objective. Special emphasis will be given on numerical stability and efficiency of the finite element codes for the engineering scale after implementation of the macro-scale models, accounting for the need of incorporating physical length scales.
Choice of Case Studies
To achieve this, several challenging case studies have been selected which are to be carried out under the Work Packages described below. They involve bcc, fcc and hcp metals, both single-phase and two-phase.
Work Packages
WP1: "Multilevel approach of dislocation dominated plasticity" (P1,P2,P3,P4,P6,EU1,EU2,EU3)
Experimental and theoretical study of plastic deformation of Al, Nb, Fe, Ta, dual-phase steel, thin films. Attention will be focused on (where relevant) dislocation interactions, grain fragmentation, texture, work hardening/softening, also at strain path changes, strain gradients, size effects. Experimental techniques include severe plastic deformation, macro- and micro-mechanical testing, nano-indentation, TEM, SEM and EBSD. The theoretical approaches involve dislocation theory, crystal plasticity and macroscopic plasticity theory, multilevel modelling including the use of crystal plasticity finite element models (CPFEM).
WP2: "Multilevel approach of twinning dominated plasticity" (P1,P2,P3,P4,P5,P6,EU2,EU3)
Experimental and theoretical study of plastic deformation of Ti, Zr, and TWIP-steel. The focus will be on the competition dislocation glide/mechanical twinning, texture, work hardening/softening, strain gradients, size effects. Experimental techniques include mechanical testing, nano-indentation, TEM, SEM and EBSD. The theoretical approaches involve dislocation theory, theory of twinning, crystal plasticity and macroscopic plasticity theory, multilevel modelling including the use of CPFEM.
WP3 -"Multilevel approach of nucleation and evolution of damage" (P1,P3,P4,P5,P6,EU1,EU2)
Experimental and theoretical study of plastic deformation of "generic metals", dual phase and TWIP steels, and Ti alloys. The focus will be on the interaction between the nucleation, and evolution of voids and cracks, on the anisotropy, work hardening, softening and viscous effects associated to the matrix, as well as on possible size effects occurring with very small scale damage phenomena. Experimental techniques include mechanical testing, also at very high strain rates, TEM, SEM and EBSD. The theoretical approaches involve crystal plasticity and macroscopic plasticity theory, fracture mechanics, damage theory, multilevel modelling including the use of CPFEM.
WP4: "Hierarchical multilevel modelling of mechanical behaviour" (P1,P3,P4,P6,EU2)
Development of a modelling strategy for the onset of plasticity under multi-axial loading conditions in which the parameters of a model at the macro-scale are identified using the predictions of multilevel models. Attention to the special requirements of HCP metals (stress differential effects), as well as to evolution of texture/microstructure/damage during deformation. Experimental techniques include forming and mechanical testing. Theoretical work will also include implementation of the macro-scale models in engineering finite element models, including development of new element types.
Interactions between Work Packages.
- WP1 and WP2 have many issues in common: dislocation glide, size effects, interactions between grains, various experimental techniques, multilevel modelling. Transverse collaboration between the work packages will be organised on these issues.
- WP3 will heavily rely on the progress in the description of the plastic flow behaviour gained in WP1 and WP2
- WP1, WP2 and WP3 have to supply the multilevel models which WP4 needs in order to identify its macroscopic models, both for the generation of the stress states at which the onset of plasticity or damage takes place, as for the evolution of texture/microstructure/damage at subsequent deformation. These two applications will require different multilevel models of different types.
