Materials Science Forum Vols. 495-497

Abstract: The aim of this work is to discuss a new non linear intermediate model for large viscoplastic deformations that could predict the texture transition and stress-strain behavior in a range that spans from the upper bound (Taylor) to the lower bound (Sachs or static) estimates. In this model, we introduced a single parameter, f , as a weight function to formulate the proposed intermediate approach which combines the Taylor and Sachs estimates. This formulation leads to an interaction law by the minimization of a tensorial function which depends on the parameter f . For the applications, we focus on the uniaxial tension test. The results for texture evolution in an FCC polycrystal show that a transition between the copper type and brass type textures can be obtained by the proposed non-linear intermediate model.

Abstract: A reaction stress model is introduced in this paper and its applications for the crystallographic texture simulation are also discussed with the comparison to the classic Taylor type model and the self consistent model. This model took the external deformation stress tensor as an initial point, and the activation process of slip systems as well as the orientation evolutions was simulated step by step. Certain relaxation of reaction stress were introduced during tensile or drawing deformation, which predicts the tensile direction distribution along the orientation line between <111> and <100> in the inverse pole figure besides the <111> and <100> fiber texture. The simulation agrees with the common experimental observations. The model supplies a simple way to follow the deformation process in the main part of polycrystals, in which the effect of grain orientation and its interaction with the surrounding matrix are considered.

Abstract: Hexagonal materials deform plastically by activating diverse slip and twinning modes. The activation of such modes depends on their relative critical stresses, function of temperature and strain rate, and the orientation of the crystals with respect to the loading direction. For a constitutive description of these materials to be reliable, it has to account for texture evolution associated with twin reorientation, and for the effect of the twin barriers on dislocation propagation and on the stress-strain response. In this work we introduce a model for twinning which accounts explicitly for the composite character of the grain, formed by a matrix with embedded twin lamellae which evolve with deformation. Texture evolution takes place through reorientation due to slip and twinning. The role of the twins as barriers to dislocations is explicitly incorporated into the hardening description via a directional Hall-Petch mechanism. We apply this model to the interpretation of compression experiments both, monotonic and changing the loading direction, done in rolled Zr at 76K.

Abstract: Crystallographic slip, i.e. movement of dislocations on distinct slip planes, is the main source of plastic deformation of most metals. Therefore, it was an obvious idea to build a constitutive model based on dislocation densities as internal state variables in the crystal plasticity. In this paper the dislocation model recently proposed by Ma and Roters (Ma A. and Roters F., Acta Materialia, 52, 3603-3612, 2004) has been extended to a nonlocal model through separating the statistically stored dislocation and geometrically necessary dislocation densities. A nonlocal integration algorithm is proposed, which can be more easily used in conjunction with commercial software such as MARC and ABAQUS than the model proposed in the work of Evers(Evers L.P., Brekelmans W.A.M., Geers M.G.D., Journal of the Mechanics and Physics of Solids, 52, 2379-2401, 2004).

Abstract: This paper combines experimental evidence at the mesoscopic and microscopic levels of the physical slip underlying plastic deformation in an aluminium polycrystal in the evaluation of crystal plasticity models. At the mesoscopic level, lattice rotations of individual bulk grains during deformation have been measured using high-energy synchrotron radation (3DXRD). At the microscopic level, deformation induced dislocation structures have been investigated by transmission electron microscopy. The performance of two different versions of the Taylor model taking different approaches to the ambiguity problem is evaluated.

Abstract: The substructure of a single grain in an electron backscatter diffraction (EBSD) data map is studied, focusing on the influence of the grain boundary configuration on the misorientation to the average grain orientation of data points close to the grain boundary. For most grain boundary segments a certain degree of linking between the misorientations to the average orientation of the grain exists and large deviations from the average orientation of the grain are observed close to the triple junctions of the boundary segments. Changes of the misorientation over one boundary segment are analysed and possible explanations for these variations are discussed. It is suggested that the variations of the misorientation over the boundary segment can be attributed to the requirements of stress equilibrium and strain compatibility. Also the tendency of the grain boundary to lower its surface energy might have a significant influence on the misorientation profile and therefore on the subdivision behaviour of the grains.

Abstract: A combination of monotonic and reverse tests has been carried out in order to assess the strain path effects on an austenitic stainless steel hot deformed by torsion. Microstructural results have been obtained by EBSD. The misorientation average parameter measured at different step size scans, the Kernel parameter and the orientation spread average parameter, provide a picture of the in-grain curvature developed during the different strain paths. The results show that these parameters are sensitive to the strain path.

Abstract: Plastic deformation in cubic metals is relatively simple due to the high crystallographic symmetry of the underlying structure. Typically, one unique slip mode can provide arbitrary deformation. This is not true in lower symmetry hexagonal metals, where prismatic and basal slip (the usual favored modes) are insufficient to provide arbitrary deformation. Often, either pyramidal slip and/or deformation twinning must be activated to accommodate imposed plastic deformation. The varied difficulty of activating each of these deformation mechanisms results in a highly anisotropic yield surface and subsequent mechanical properties. Further, the relative activity of each deformation mode may be manipulated through control of the initial crystallographic texture, opening new opportunities for the optimization of mechanical properties for a given application.

Abstract: A new phenomenological latent hardening model is developed for rate-dependent single crystal plasticity. The model quantitatively predicts the latent hardening evolution and latent hardening material dependence for f.c.c. single crystals. Increased overshoot, typically observed in copper alloys as opposed to copper, is rationalized based on the history dependence of latent hardening.