Staff

Abstract:
The objective of this work is to deconvolute the interaction of slip, twinning, and notch on the deformation response of an austenitic manganese (Hadfield) steel using detailed finite element simulations. The simulations employ a rate-dependent crystal plasticity constitutive model that incorporates both slip and twinning deformation mechanisms. The model accounts for the spatially non-uniform appearance of new twin-related orientations, hardening due to slip–twin interactions, and modified properties of the twinned crystal. Limited experiments on single-crystal dog-bone and single-edge notch specimens, with two crystal orientations, are also conducted to aid the simulation. Several features of the experimental observations are accurately captured in the simulations. For example, simulations accurately capture distinct stress–strain responses associated with different crystallographic orientations, including variations in initial hardening behavior followed by either decreasing or increasing hardening depending on the dominant deformation mechanisms. The simulation also captures the observed orientation-dependent asymmetric deformation of the notch in single-edge notch specimens. Additionally, by selectively activating deformation mechanisms, the role of twinning is isolated and its influence on both global and local response is clearly demonstrated. These results provide a mechanistic understanding of how deformation mode interactions and local geometry (i.e., notch) influence the response of these materials.

Keywords:
Austenitic steel, Single crystals, Anisotropy, Deformation twinning, Crystal plasticity, Fracture, Finite element method

Abstract:
The crystal plasticity finite element method (CPFEM) is used to investigate the coupling between the cylindrical void growth or collapse and grain refinement in face-centred cubic (FCC) single crystals. A 2D plane strain model with one void is used. The effect of the initial lattice orientation, similarities, and differences between stress- and strain-driven loading scenarios are explored. To this end, boundary conditions are enforced in two different ways. The first one is based on maintaining constant in-plane stress biaxiality via a dedicated truss element, while the second one is imposing a constant displacement biaxiality factor. Uniaxial and biaxial loading cases are studied. For the uniaxial loading case a special configuration, which enforces an equivalent pattern of plastic deformation in the pristine crystal, is selected in order to investigate the mutual interactions between the evolving void and the developed lattice rotation heterogeneity. Next, biaxial loading cases are considered for three crystal orientations, one of which is not symmetric with respect to loading directions. It is analysed how stress or strain biaxility factors and initial lattice orientation influence the void evolution in terms of its size and shape. Moreover, the consequences of variations in the resulting heterogeneity of lattice rotation are studied in the context of the grain refinement phenomenon accompanying the void evolution. Scenarios that may lead to more advanced grain fragmentation are identified.

Keywords:
Crystal plasticity , Finite element method, Void evolution, Grain refinement

Abstract:
Evolutionary algorithms have become an extensively used tool for identification of crystal plasticity parameters of hexagonal close packed metals and alloys. However, the fitness functions were usually built using the experimentally measured stress–strain curves. Here, the fitness function is built by means of numerical comparison of the simulated and experimental textures. Namely, the normalized texture difference index is minimized. The evolutionary algorithm with the newly developed fitness function is tested by performing crystal plasticity parameter optimization for both pure zinc and zinc-magnesium alloy. These materials are promising candidates for bioabsorbable implants due to good biocompatibility and optimal corrosion rate. Although their mechanical properties in the as-cast state do not fulfill the requirements, they can be increased by means of hydrostatic extrusion. The developed modeling approach enabled acquisition of the crystal plasticity parameters and analysis of the active deformation mechanisms in zinc and zinc-magnesium alloy subjected to hydrostatic extrusion. It was shown that although slip systems are the main deformation carrier, compressive twinning plays an important role in texture evolution. However, the texture is also partially affected by dynamic recrystallization which is not considered within the developed framework.