Denis Solas

Dynamic Recrystallization Modeling during Hot Forging of a Nickel Based Superalloy

A crystalline modeling of deformation implemented in the Finite Element code Abaqus® coupled to a recrystallization Cellular Automaton code is proposed and applied to the hot forging process. A sequential modeling is used in order to obtain a better understanding of the experimental observations and to improve our knowledge of the dynamic recrystallization process. Modeling is performed on aggregates built up from Electron Back Scattered Diffraction measurements. At the deformation temperature, the material presents two phases with a γ matrix of FCC structure and a γ’ hardening phase under a precipitate shape (Ni3(Ti,Al)) of SC structure. The crystalline approach can describe the interactions between the two phases and can compute the evolution of the local strain and stress fields as well as the dislocation density and the lattice rotation in the different grains. A Cellular Automaton algorithm is used for simulating the microstructure evolution during dynamic recrystallization. Nucleation and grain boundary mobility depend on the misorientation and on the local variation in stored energy. This presentation mainly details the different assumptions introduced in the recrystallization code and their influences on the microstructure evolution.

Texture Evolution in Invar® Deformed by Asymmetrical Rolling

Asymmetrical rolling, in which the circumferential velocities of the working rolls are different, is a method to impose shear deformation and in turn shear deformation texture to sheet through the thickness. The Invar® alloy has been deformed by asymmetrical rolling with a 84% thickness reduction. The texture of the deformed and annealed alloy was measured by X-ray diffraction at different levels through the thickness: upper side- middle- down side, with unidirectional rolling. The deformed texture is a copper type texture but the components were rotated about 5-7° around the Transverse Direction (TD) axis as compared to the ideal position of these components in the pole figure representation. During recrystallization, the rolling components (brass {011}<112>,copper {112}<111>, aluminum {123}<634>) decrease quickly whereas the cube component {001}<100> is preferentially developed after a short annealing time. However, the rolling components do not disappear completely after complete recrystallization (120 minutes annealing). As a consequence the final texture contains a high cube component and rolling components.

INCONEL 718 Recrystallization in the Delta Supersolvus Domain

An experimental study of compression tests at high temperature and different engineering strains was carried out on INCONEL 718 above the delta phase solvus. The objective is to investigate the mechanical behaviour in relation with the microstructure evolution. After deformation the samples were quenched with helium gas to avoid metadynamic recrystallization (MDRX). The quench efficiency is discussed by microstructural and hardness comparison. During forging process and without MDRX, there is generally a competition between deformation and dynamic recrystallization state (DRX) i.e. a dependence on dislocation density increase and dislocation annihilation, respectively. To investigate this competition, samples are characterized at different scales by EBSD method to determine local texture and grain size and by TEM to understand the dislocation evolution and determine the nucleation mechanism. In parallel, a numerical model using a three-dimensional finite element model of crystalline plasticity (CristalECP) has been developed in ABAQUS™ finite element code and coupled with a Recrystallization Cellular Automaton (CA_ReX). Results of forging process simulations are compared to those of experimental studies presented before and then discussed in terms of evolution.

Monte Carlo Method for Simulating Grain Growth in 3D Influence of Lattice Site Arrangements

A three-dimensional Monte Carlo computer simulation technique has been applied to the problem of normal grain growth. A continuum system is modelled employing a discrete lattice. In this paper we investigate the connectivity of the points that represent the discretized microstructure. The lattice can have a strong influence on the result of the simulation. Only the BCC lattice with 14 neighbours gives similar results than the traditional simple cubic model with 26 neighbours. If we consider the computing time and the required computer memory, the BCC-14 model is a good alternative to the SC-26 model for simulating normal grain growth.

Temperature and Deformation Effects on the Recrystallization Microstructure and Texture of Wire Draw Steel

In this work, the effect of the deformation, caused by cold wire drawing, on the microstructure and the texture of low carbon steel wire (0.06 wt % C) is examined. The combined influence of the deformation level and the recrystallization temperature on the development of new grains is studied for all wires. Isothermal tests of annealing allow the determination of the critical temperature of recrystallization estimated above 450°C. The temperature effect is studied below the eutectoid level, at 500°C, 600°C and 680°C. The appearance of a homogeneous recrystallization is noted over the section of the wire. The recrystallized grains keep the same orientation as the deformed grains. The expansion of time of annealing lead to recrystallization in the ferritic grains accompanied by a spheroidization of the lamellar pearlite. The kinetics of recrystallization and spheroidization are accelerated by increasing of annealing temperature and the deformation level. The experimental techniques used in this study are: the Scanning Electron Microscope (SEM), the Electron Back Scattered Diffraction (EBSD), the X-ray diffraction and Vickers microhardness.

Evolution of Microstructure and Texture during Annealing of a Copper Processed by ECAE

A submicron-grained (SMG) microstructure, with an average grain size of ~0.4 µm was produced by equal channel angular extrusion (ECAE). The SMG microstructure was composed of large dynamic recrystallized grains within a matrix of deformed elongated cells. Samples were annealed for various times at 473 K and then examined using transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). The results specify that a large recovery takes place during the first annealing times. Moreover, MET investigations show nucleation of grains which orientations are found in the recrystallized texture. The EBSD measurements established that, after 7min30s at 473 K, the microstructure is equiaxed and stable with an average grain size of about 2 µm.

Texture and Microstructure of Invar® Deformed by Asymmetrical Rolling

Asymmetrical rolling, in which the circumferential velocities of the working rolls are different, is a method to impose shear deformation in addition to the thickness reduction. As a consequence, the deformation texture can be modified as compared to the classical rolling. In this work, the asymmetrical rolling of invar (Fe-36%Ni) and the influence of the deformation route are studied. The Invar® alloy has been deformed by asymmetrical rolling with a 83% thickness reduction. The texture of the deformed alloy was measured by X-ray diffraction at different levels through the thickness: upper side- middle- down side. With asymmetrical rolling, the deformed texture is a copper type texture but the components were rotated about 5-7° around the Transverse Direction (TD) axis as compared to the ideal position of these components in the pole figure representation. The rotation of the pole figure is an indicator of the amount of shear really introduced in the material during asymmetrical rolling. Finally, a simple model was developed in order to establish the condition to obtain either shear texture or grain refinement.

Microstructural Evolution Modelling of Nickel Base Superalloy during Forging

In order to obtain a better understanding of mechanisms governing the microstructural evolution of nickel base superalloys during forging, experimental and numerical studies have been undertaken. For the experimental part, isothermal compression tests were performed at 1100°C for several ratio and strain rates to reproduce microstructural evolution during industrial forging. The resulting microstructures were analysed by Electron Back Scattered Diffraction method and Transmission Electron Microscopy to identify the dynamic recrystallization mechanisms. In parallel, numerical studies has been carried out in which a crystalline plasticity modelling implemented in the finite element code Abaqus® coupled to a recrystallization Cellular Automaton code was used to simulate forging. The first model allows us to obtain the local mechanical fields (strain, stress, crystallographic orientation, dislocation density …) and the second one predicts the dynamic recrystallization.

Monte Carlo Modelling of Recrystallization Mechanisms in Wire-Drawn Copper from EBSD Data

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.

Jan T. Bonarski (1957–2016)

CRISMAT-ENSICAEN and IUT-Caen, Université de Caen Normandie, Campus 2 6, Boulevard M. Juin, 14050 Caen, France, ICMMO – UMR 8182, Université Paris-Sud, Bâtiment 410, Rue du Doyen Georges Poitou, 91405 Orsay Cedex, France, Laboratoire d’Étude des Microstructures et de Mécanique des Matériaux (LEM3), CNRS UMR 7239, Université de Lorraine-Metz, 57045 Metz, France, Laboratoire d’Excellence ‘DAMAS’: Design of Metal Alloys for Low-Mass Structures, Université de Lorraine-Metz, 57045 Metz, France, and Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Reymonta 25, 30-059 Kraków, Poland. *Correspondence e-mail: daniel.chateigner@ensicaen.fr ISSN 1600-5767