Abstract: During transport of spent Zircaloy-4 fuel rods, cladding temperatures can be
expected to rise well over 400°C for transportation periods longer than 10 days. The kinetics of creep under these conditions will be controlled by both strain hardening and the softening effect of static annealing of cold work and irradiation defects. This paper will focus on the development of a coupled recovery/recrystallisation model for Zircaloy-4 from 400 – 520°C.
Abstract: In order to simulate the recrystallization process, Monte Carlo modelling has been
applied to the case of wire-drawn copper deformed to a moderate strain. The complete experimental set of data was taken mainly from Electron Back Scattered Diffraction measurements in a Scanning Electron Microscope. Several nucleation hypothesis have been introduced and tested into the model. It has been shown that nucleation taking into account the sites associated with the highest stored energy and highest local misorientation leads to the best results in terms of recrystallization
microstructure and texture. An important number of new orientations - that come only from annealing twinning - are not reproduced with the model, indicating the major role of this particular mechanism during the recrystallization process.
Abstract: Because both nucleation and growth are local phenomena, recrystallization depends on the spatial distribution of strains in a plastically deformed polycrystal. Using a polycrystal plasticity finite element model, we calculated these distributions for equiaxed polycrystals of copper with a random texture. We incorporated the results into a mesoscale recrystallization simulation with a nucleation model based on subgrain evolution. The coupled simulation results indicate that differences in local structure cause significant differences in recrystallization kinetics and grain size distribution. Furthermore, recrystallization in non-uniformly deformed
polycrystals, even those with a random texture, is quantitatively and qualitatively different than predicted by continuum models that assume a uniform distribution of strains. This work highlights the need to examine all length scales relevant to the recrystallization process.
Abstract: The localization of deformation in recrystallizing materials is investigated via a series of two-dimensional grain-scale numerical simulations. These simulations couple a grain size and strain dependant viscous rheology with grain size reduction and grain growth processes. The simulations are able to predict the mechanical, microstructural and strain evolution of the polycrystals to high strain, and allow us to examine the nature of the time dependent feedback between mechanical and
microstructural behavior. It was found that significant strain localization occurred only when the grain size dependence of the viscosity was non-linear, and was greatly enhanced by the activity of the grain size modifying processes. The intensity and location of the zone of strain localization varied spatially and temporally, with the result that the finite strain state showed a much broader, and hence less intense, zone of localized deformation than the instantaneous state.
Abstract: The recrystallization process in polycrystalline material was studied using the newly
developed two–dimensional model based on the vertex concept. In the model presented below the microstructure of polycrystalline material is represented by two-dimensional network of grains. The initial microstructure is characterized by topology, crystal orientations and stored energy values of the grains. The boundary energies and mobilities are anisotropic in general. Additional driving forces in recrystallization, are exerted on vertices and are derived from the stored energy gradients between adjacent grains. The nucleation mechanism of a given type is selected at the start of the calculations. Two different nucleation types were tested. Deformation texture, stored energy distribution and initial microstructure are input parameters of the model. The goal of the calculations is the prediction of texture and microstructure modification during recrystallization. A comparison of predicted and experimental characteristics enables the verification of the model assumptions.
Abstract: Analytical modeling of recrystallization considers two arrangements of nuclei: a) random, b) “periodic”. If nucleation departs from these extremes, no exact analytical treatment is available. When classical Johnson-Mehl, Avrami, Kolmogorov approach is used there is often the doubt whether the nucleation is truly random. Therefore, it would be of some interest to assess to what extent the exact analytical theory is valid if the nucleation departs from randomness. In this work, recrystallization is simulated using cellular automata in two dimensions in order to investigate the effect of nuclei distribution on the kinetics. Simulations are carried out for nuclei distribution ranging from periodic to random. The effect of departure from the exact analytical solutions is assessed by means of the overall kinetics and the microstructural path.
Abstract: Recently, some authors have used the Monte Carlo modelling using complete set of
experimental data to get a better correlation between experimental observations and calculations concerning recrystallization process [1, 2]. Simulations using Monte Carlo technique have been performed these last years for IF-Ti steels in order to predict the microstructure and the texture evolution after high reduction amounts by cold rolling [3, 4]. On the contrary, in the present work, this evolution is simulated in an IF-Ti steel cold rolled after low deformation amount (reduction amount R = 40 %). Microstructure is characterized by Electron Back-Scattered Diffraction and introduced in the model. The quality index of the Kikuchi patterns (EBSD data) is used to qualitatively evaluate the stored energy for each grain. Different hypothesis of nucleation mechanisms have been introduced into the model. It has been shown that the better recrystallization texture correlation between experiment and simulation is obtained by taking into account the nucleation in the low stored energy sites and highly misorientation regions. Finally a simulation issue was compared with EBSD and TEM experimental results: microstructure, recrystallization kinetics and Avrami coefficients values.
Abstract: The observation of inhomogeneous, ‘sluggish’ recrystallization in ferritic steels has been extensively documented and discussed, particularly with reference to low carbon steels. Stabilized ferritic stainless steels are also prone to this phenomenon and, in many cases, exhibit the effect more strongly than their carbon counterparts. The situation for stainless steels is exacerbated in part by the topology of the cold rolled microstructure, which is composed of highly elongated and layered grains. In this work an attempt has been made to probe the key features of this process by means of a two-dimensional vertex simulation. Key microstructural characteristics such as subgrain and grain size, topology, misorientation and energy are varied in these simulations in an attempt to elucidate the mechanisms responsible for the final recrystallization. These simulations are compared and contrasted with experimental observations from the recrystallization of an AISI409 stainless steel.
Abstract: In the present work a detailed characterisation of a cold rolled and annealed AA3103- alloy has been carried out. The effect of different concentrations of Mn in supersaturated solid solution on the deformation and subsequent annealing behaviour has been studied in detail. A physically based microstructure model has been used to predict the evolution in microstructure and flow stress during annealing of the given alloy, with particular focus on the effect of concurrent precipitation on the softening behaviour.
Abstract: Forging of high strength nickel base superalloys 720 and 718 for aircraft parts requires the usage of finite element simulations to ensure a proper thermo-mechanical treatment. Because of the strong mechanical requirements and narrow specifications of such parts not only a correct, defect free final geometry is necessary, but also a defined microstructure. The crucial point is therefore, to control all process parameters in a way to achieve the demanded properties. The typical forging processes like hydraulic, screw press and hammer forging imply a broad spectrum of strain rates. The influence of this different strain rates as well as forging temperature and strain on dynamic and post-dynamic recrystallization have been examined experimentally. Annealing tests at various temperatures and
time periods have been performed, to investigate the grain growth behavior and dissolution processes in this before mentioned materials during heating periods. The obtained data was used to build phenomenological models, which were implemented into finite element code of a commercial special purpose finite element program. 2D and 3D Simulations of multiple step thermo-mechanical processes are compared with microstructure examinations of forged parts to show the usability and accuracy of such models as a tool to optimize complex forging processes of critical aircraft parts. In combination with systematic process data collection during production a stable processes and satisfactory mechanical product properties are guaranteed.