Soroosh Naghdy

Microstructural evolution study of severely deformed commercial aluminium by transmission Kikuchi diffraction

Commercially pure aluminium was selected as a model fcc material for a detailed investigation of the microstructural and textural evolution during processing by high pressure torsion (HPT). Vickers hardness measurements reveal that: the hardness is the lowest where the lowest strain is imposed and increases with increasing strain until saturation hardness is obtained. The microstructural evolution is investigated by means of electron backscattered diffraction and transmission Kikuchi diffraction. During HPT a grain fragmentation process takes place: within the original grains dislocations aggregate, resulting in the creation of very fine grains separated first by low and later by high angle grain boundaries. At a strain level of a simple shear texture of fcc structure was observed. This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.

Morphological and crystallographic anisotropy of severely deformed commercially pure aluminium by three-dimensional electron backscatter diffraction

International Union of Crystallography. The aim of this paper is to examine the morphological and crystallographic anisotropy that develops during high-pressure torsion (HPT) processing. Commercially pure aluminium was subjected to monotonic HPT deformation at room temperature. The microstructure and texture were studied by large-area electron backscatter diffraction (EBSD) scans. Three-dimensional EBSD scans served to s crutinize the morphological anisotropy and local texture. It was observed that two distinct stages of grain fragmentation and saturation occur during processing. Grains exhibited an ellipsoidal shape rather than an equi-axed one. The major axes of the ellipsoids showed a favorable orientation at the steady-state stage: an almost 20° inclination towards the shear direction. The global texture was characterized by typical shear components of face-centered cubic metals at both stages. However, the local texture revealed a preferential fragmentation pattern in the first stage: orientations in the vicinity of ideal fibers became less heavily fragmented while non-ideal orientations broke up more severely. This phenomenon was linked with the lattice rotation required to bring an initial orientation close to a stable one. Although the texture weakened considerably in the fragmentation stage, the texture index did not further decrease in the saturation stage. Saturation of texture, grain refinement and formation of microstructure are discussed in the light of different microstructural coarsening mechanisms.Morphological and crystallographic anisotropy of severely deformed commercially pure aluminium is studied by conventional and three-dimensional electron backscatter diffraction. Saturation of texture, grain refinement and the formation of microstructure are discussed in the light of different microstructural coarsening mechanisms.

Mechanical behavior and texture prediction of Ti-6Al-4V based on elastic viscoplastic self-consistent modelling

The goal of this study is to apply an elastic viscoplastic self-consistent crystal plasticity model to predict the texture evolution in a Ti-6Al-4V alloy which has a (mainly) hexagonal crystal structure. The model under consideration is an extension of the viscoplastic self-consistent model proposed by Lebensohn and Tome [1993] which has been adapted to account for elasticity and has been integrated with a new algorithm, making it more computationally efficient within an implicit FE scheme. The flow behavior of Ti-6Al-4V is strongly dependent on strain rate and temperature. To estimate the model parameters, the flow behavior of quasi-static experiments is used. A temperature sensitivity term has been introduced to correct the effects of temperature increase during the dynamic experiments. In order to have a meaningful rate sensitivity exponent, a value is calculated based on valid experimental data, rather than choosing an arbitrary large numerical value. In this way the behavior of Ti-6Al-4V is captured at different strain rates. Predictions of the model are compared to experimental data.

Comparison of the Johnson-Cook and VPSC models to describe the constitutive behaviour of Ti-6Al-4V in an implicit finite element scheme

The use of finite element simulations has become one of the main tools of the mechanical engineer. The method is applied to the analysis and design of engineering structures, the study of manufacturing processes and even to perform virtual experiments. Traditionally, the constitutive laws chosen for finite element analysis have been as simple as possible, mainly due to the limitation imposed by the available computing power. However, the development of more powerful computers and more efficient methods is opening the possibility of using more elaborated (and, most often, more accurate) material models. In particular, polycrystal models capable of predicting not only the mechanical behaviour of the material, but also of the evolution of properties with increasing strain, are particularly well suited for the simulation of forming processes, for which a precise knowledge of the properties of the resulting product is of paramount importance.The present work studies how the Visco Plastic Self-Consistent model (VPSC) can be used in combination with the implicit finite element package Abaqus/Standard to simulate the behaviour of Ti-6Al-4V sheet, and compares it with the more common (and much simpler) Johnson-Cook model. More specifically, the goal of this study is to determine whether or not, with using similar experimental calibration data, the use of the much more complex polycrystal model, justifies the increased complexity and execution time. Using standard tensile experiments at different strain rates, the parameters of the VPSC and Johnson-Cook models are fitted using a minimization method. Then, both models are used in finite element simulations and the results given by both models are compared.