Papers by Author: C. Pinna

Abstract: A three dimensional phase field model has been developed to simulate the texture formed during the static recrystallisation of FCC metals with medium or high stacking fault energy, such as aluminium, copper and nickel. Before recrystallisation the deformation texture as well as the stored energy was simulated using a three dimensional crystal plasticity finite element model. This output calculated on the distorted finite element mesh was first mapped onto the regular grid of the phase field model using a linear interpolation method and then used as initial condition for the subsequent recrystallisation texture modelling. This model has successfully predicted the typical recrystallisation texture components: cube {001}<100>, R {124}<211> in the aluminium alloy. In addition, the softening fraction and three dimensional microstructure produced during static recrystallisation have also been simulated by this model.

Abstract: The Plane Strain Compression and Static Recrystallisation Textures of BCC Metals Have Been Simulated Using a Coupled 3D Crystal Plasticity Finite Element (CPFE)-Single Phase Field (PF) Model Using an Interstitial-Free (IF) Steel as an Example. the Recrystallisation Nucleation Is Modelled Based on the Orientation Dependent Recovery (ODR) Theory which Assumes that Deformation Texture Components with a Relatively High Number of Slip Systems Activated during the Plane Strain Compression Undergo a Faster Recovery Process during the Subsequent Annealing due to the Cross Slip of Dislocations and as a Result Will Nucleate Earlier than Others. the Growth of Strain-Free Grains Is Simulated Using the Mis-Orientation Angle Dependent Grain Boundary Energy and Interface Mobility. A Linear Interpolation Method Is Adopted to Map the Data between the CPFE Model of Deformation and the Single PF Model of Recrystallisation. Simulated Results Show a Qualitative Agreement with the Typical Rolling and Annealing Textures Measured Experimentally for BCC Metals. Apart from the Texture and Grain Structure Formed during Deformation and Annealing, the Softening Fraction Can Also Be Simulated Using the Developed Model.

Abstract: An integrated crystal plasticity-phase field model has been developed to simulate the static recrystallisation textures of both Face-Centred Cubic (FCC) and Body-Centred Cubic (BCC) metals. Nucleation sites are determined using the Orientation Dependent Recovery (ODR) theory. Both the interface mobility and the grain boundary energy are set to be dependent on mis-orientation angles in the simulations. A pre-deformed microstructure without a particular texture is generated using a Monte Carlo simulation. Plane strain compression textures before recrystallisation are predicted by a Crystal Plasticity Finite Element (CPFE) model showing a good agreement with the typical experimental rolling textures. It is shown that the typical recrystallisation textures for FCC and BCC metals can be simulated correctly using a Phase Field (PF) method by choosing appropriate critical values for the nucleation criterion. A comparison between the two different nucleation criteria based on the ODR theory or the stored energy is also presented.

Abstract: Digital Image Correlation (DIC) together with in-situ tensile testing has been used to measure in DP1000 steel the evolution of plastic strains at the microstructure scale. Interrupted tensile tests were performed on specially designed samples and scanning-electron micrographs were taken at regular applied strain intervals. Patterns defined by the microstructural features of the material have been used for the correlation carried out using LAVision software. The full field strain maps produced by DIC show a progressive localisation of deformation into bands at about 45o with respect to the loading direction. Plastic strains as high as 130% have been measured within the ferrite phase.

Abstract: The deformation of metals at high homologous temperatures typical of industrial forming processes such as hot rolling is investigated using an electron micro-lithography technique enabling strain measurements at the scale of the microstructure of these metals. Grids with a typical 5 μm pitch have been laid on the surface of an aluminium alloy and a stainless steel deformed by plane strain compression at 400oC and 850oC respectively. In-plane strain values can be computed from the displacements of the intersections of the grids. Strain maps can then be plotted over representative areas of the microstructures together with strain distributions within each phase of the microstructure. Results obtained for a commercial high-purity aluminium alloy with 5% magnesium show a strong localisation of deformation at triple points with a local equivalent strain value of 1.7 for a 0.22 applied strain at 400oC. As for the two-phase stainless steel deformed to a strain of 0.3, results show a high heterogeneity of deformation within each phase with a localisation of deformation into bands in the ferrite phase and local values reaching more than two times the applied compression.

Abstract: The grid technique is an experimental method for measuring the deformation in hot rolling. An AA3004 sample -fitted with an insert - was rolled in a single hot rolling pass at 400 oC. The insert was hand engraved with a 1x1 mm grid and the analysis of the image of the deformed grid enabled the calculation of the components of the deformation gradient tensor. In order to prevent relative motion between the insert and the work-piece, four steel pins were used; after the test no detachment was observed between insert and sample. The temperature was monitored during rolling using two embedded thermocouples, one close to the surface and the other on the centre-line of the slab. The commercial finite element (FE) code ABAQUS was used to build a threedimensional model of the rolling process. The recorded temperature was compared with the FE values evaluated after tuning the heat transfer coefficient. The FE model was run several times with different friction coefficients and the deformation gradient checked against the experimental measurement of the deformed grid in order to obtain the optimum friction coefficient. The experimentally determined deformation gradient and the measured temperature agreed well with the numerical values.