Papers by Author: Nathalie Bozzolo

Abstract: The mechanical and thermal properties of metallic materials are strongly related to theirmicrostructure. An accurate and quantitative prediction of microstructural evolutions is then crucialwhen it comes to optimize the forming process. Recently a new full field approach, based on a Level-Set (LS) description of interfaces in a finite element (FE) context has been introduced to model 2D and3D primary recrystallization (ReX), including the nucleation stage [1, 2], and has been extended to takeinto account the grain growth (GG) stage [3, 4]. The ability of this approach to model also the Zenerpinning (ZP) phenomenon without any assumption concerning the shape of second phase particleswas also demonstrated [5]. Moreover, recent developments have also illustrated the capability of thisapproach to take into account the characteristics of twin interfaces during grain boundary motion [6,7]. Current work concerns also the improvement of the numerical cost of this new approach [8]. Allthese developments are necessary to account for the microstructural complexity of ReX phenomenon.

Abstract: Under certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate.

Abstract: Grain boundary engineered (GBE) materials have improved properties that are associated with the high fraction special Σ3n boundaries in the microstructure, where n = 1,2,3. Previous experimental studies with high purity nickel before and after thermomechanical processing have shown that the fraction of Σ3 boundaries increased by at least factor of two [1]. Electron backscatter diffraction (EBSD) is used to characterize the evolution of these special boundaries throughout the recrystallization process of a 25% cold rolled sample annealed at 490°C. The fractions of the Σ3 boundaries and coherent twins have been measured over time revealing a steadily increasing behavior over the entire microstructure. However partitioning to only include recrystallized regions reveals a different behavior in the Σ3 boundaries as fractions, which increase rapidly at first and then stagnate over time. Additional triple junction characterization was performed to monitor the evolution of triple junctions containing special boundaries.

Abstract: Understanding of the mechanisms of annealing twin formation is fundamental for grain boundary engineering. In this work, the formation of annealing twins in a 304L austenitic stainless steel is examined in relation to the thermo-mechanical history. The behaviour of annealing twins of various morphologies is analysed using an in-situ annealing device and EBSD. The results confirm that there is a synergistic effect of prior strain level on annealing twin density generated during recrystallization. The higher the prior strain level, the higher the velocity of grain boundary migration and the higher the annealing twin density in the recrystallized grains. This effect decreases as the recrystallization fraction increases. The existing mathematical models (Pande's model and Gleiter's model), which were established to predict annealing twin density in the grain growth regime, can not predict this phenomenon.

Abstract: A heating stage as been developed to perform in-situ annealing in a SEM equipped with an EBSD system in order to study recrystallization mechanisms. High temperature treatments could then be performed inside the SEM, up to 1180°C and with high heating-and cooling-rates (~100°C.s^-1). Samples were cooled down to room temperature to perform EBSD orientation mapping in between successive short-duration heat-treatments. Microstructure evolution snapshots obtained this way are presented in this paper to show recrystallization in Zircaloy4 and in pure tantalum.

Abstract: The effect of a magnetic field on texture and microstructure development in cold rolled (80%) commercially pure zirconium (Zr701) was investigated. X-ray diffraction and EBSD measurements were utilized for the texture and microstructure characterization. The results revealed that a magnetic field promotes grain growth in the investigated material. During annealings at 550°C this is particularly apparent from the faster development of specific (0/180, 35, 30) texture components and the bigger mean grain size after magnetic annealing. The magnetic annealing at 700°C resulted in an asymmetry of the two major texture components. This is due to a magnetic driving force for grain growth arising from the anisotropic magnetic susceptibility of zirconium. During annealing at 700°C the abnormal grain growth occurred. This behavior is attributed to the higher mobility of grain boundaries between grains misoriented by 30° around [000. The magnetic field essentially enhanced the observed abnormal grain growth.

Abstract: According to various studies, Grain Boundary Engineering (GBE) is likely to enhance mechanical properties of polycrystalline materials. The present investigation highlights some relationships between thermomechanical process (TMP) parameters of a commercial nickel-base superalloy PER72, supplied by Aubert & Duval (equivalent to Udimet®720™) and the resulting microstructure. The long-term goal is to develop TMPs that modify the Grain Boundary Character Distributions (GBCD) in order to improve high temperature properties. In this context, Grain Boundary Engineering (GBE) techniques are considered, thinking of replacing standard forming processes by optimised thermomechanical treatments. Mechanical testing at high temperature (compression and torsion tests) has been carried out and it is shown that multi-step treatments promote twinning. Some clues are then presented in an attempt to explain when and how twins are created.

Abstract: A 2D cellular automaton model developed for the simulation of grain growth in hexagonal metals is presented here. It allows the direct use of experimental measurement as input data. Texture evolution of a titanium alloy and a zirconium alloy are simulated on the basis of simple hypothesis and compared with experimental evolution as well as the results from a 3D Monte Carlo model. Results from both models are discussed with regards to their characteristics.

Abstract: Grain boundary character in samples of Zr701 annealed at two different temperatures has been investigated in terms of lattice misorientation. The main difference between the two samples was the extent of grain growth post-recrystallization. The textures were typical for the material. Differences between the texture-based misorientation distribution function (T-MDF) and the microstructure-based MDF (M-MDF) revealed significant preferences for certain grain boundary types, notably those with <11-20> rotation axes.

Abstract: Zirconium alloys are widely used for different applications in nuclear industry. Precise knowledge of their texture is of great relevance since this hcp metal exhibits a strong crystal anisotropy. Despite that, the mechanisms of texture change during its deformation and subsequent annealing are still not precisely known. Thus, there is a need for a better understanding of the fundamental mechanisms of recrystallisation. Earlier works on Zr702 [1-3] suggested that the kinetics and local mechanisms of recrystallisation after cold-rolling was controlled by the heterogeneity of the deformed microstructure and that, at the end of recrystallisation (corresponding to the disappearance of the deformed matrix), the position of the major texture components remained almost unaffected. The aim of the present work is to confirm whether these statements can be generalized for various deformation conditions or not.