Autor: RODRÍGUEZ GALÁN, Daniel

Título: Standard and strain gradient crystal plasticity models: application to Titanium

Fecha: 2017

Materia: Sin materia definida

Escuela: E.T.S. DE INGENIEROS DE CAMINOS, CANALES Y PUERTOS

Departamento: CIENCIA DE LOS MATERIALES

Acceso electrónico: http://oa.upm.es/45404/

Director/a(s):

• Director/a: ROMERO OLLEROS, Ignacio
• Director/a: SEGURADO ESCUDERO, Javier

Resumen: The necessity of improvements in metallic materials requires to understand the mechanisms that give the evolution of its mechanical properties at the micrometer scale. These mechanisms whose dependence and effects span several scales over length and time derive from plasticity in metals. In this realm, the current work is focused on the simulation at the continuum scale of the mechanical behavior of metals with a micromechanical description based on crystal plasticity because of its proven suitability to characterize the microstructural behaviour in crystalline materials. However, local crystal plasticity models are not able to capture the size effects. For this reason, strain gradient plasticity theories are introduced. This work focuses on lower-order theories because they employ conventional stresses, equilibrium equations and boundary conditions. Here, the model is presented within a continuum thermodynamic framework. A bending benchmark is proposed to assess strain gradient crystal plasticity formulations from a numerical standpoint, with the aim to determine the robustness and accuracy. This benchmark is specially suitable for lower-order strain gradient crystal plasticity theories because it does not involve higherorder boundary conditions, and was used to assess the non-local element proposed by Busso et al. (2000) and the recovery technique proposed by Han et al. (2007). The conclusions of this study are that the non-local element presents convergence problems that discourage its use. On the other hand, the recovery technique shows satisfactory results and proves to be a feasible technique. On the other hand, the behavior of nanostructured pure Ti was studied experimentally and theoretically using a crystal plasticity finite element (CPFE) polycrystalline model. The actual polycrystalline microstructure (grain shape and orientation distributions) was accounted in voxel-based representative volume elements. The crystal behavior was described by a standard crystal plasticity model with a physically-based description of the plastic slip rate based on the theory of thermally activated dislocation motion. Prismatic, basal and pyramidal slip systems were considered. The parameters of the crystal plasticity model were obtained by combining experimental measurements (i.e. dislocation densities) and an inverse analysis of the macroscopic experimental results. The resulting polycrystalline model was validated by an accurate reproduction of independent experimental tests performed at different temperatures and strain rates. The critical resolved shear stresses (CRSS), predicted by the model for the different slip systems, show the expected increase with respect to those for coarse grained pure Ti. The nanostructured Ti shows lower strain rate sensitivity and activation volumes than coarse grained pure Ti. The ratios between the CRSSs of the different slip systems at room temperature were almost independent of grain size. The model was used to predict the evolution of the CRSSs as a function of temperature and a faster decay of pyramidal CRSS was found compared to prismatic and basal ones.