Thermo-micro-mechanical simulation of bulk metal forming processes

The newly proposed microstructural constitutive model for polycrystal viscoplasticity in cold and warm regimes (Motaman and Prahl, 2019), is implemented as a microstructural solver via user-defined material subroutine in a finite element (FE) software. Addition of the microstructural solver to the default thermal and mechanical solvers of a standard FE package enabled coupled thermo-micro-mechanical or thermal-microstructural-mechanical (TMM) simulation of cold and warm bulk metal forming processes. The microstructural solver, which incrementally calculates the evolution of microstructural state variables (MSVs) and their correlation to the thermal and mechanical variables, is implemented based on the constitutive theory of isotropic hypoelasto-viscoplastic (HEVP) finite (large) strain/deformation. The numerical integration and algorithmic procedure of the FE implementation are explained in detail. Then, the viability of this approach is shown for (TMM-) FE simulation of an industrial multistep warm forging

Microstructural evolution in materials during thermal processing

Copyright © 2012 Joseph K. L. Lai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article has been made available through the Brunel Open Access Publishing Fund.This article is made available through the Brunel Open Access Publishing Fund

Effect of Cycle Frequency on High Temperature Oxidation Behavior of Alumina-forming Coatings Used for Industrial Gas Turbine Blades

Oxidation kinetics of platinum modified aluminide and overlay coatings on nickel base superalloys were investigated. Isothermal oxidation tests were carried out at 1050°C in synthetic air. Cyclic oxidation tests were performed with 2 cycle frequencies : - Short term cycles : 1h dwells at 1050°C in synthetic air ×1800 cycles - Long term cycles : 300h dwells at 1050°C in laboratory air × 6 cycles (experiment planned to totalize at least 10 000 hours at high temperature) The mass gain curves point out a large effect of the cycle frequency at 1050°C for overlay NiCoCrAlYTa coating whereas the effect is less significant for Pt-modified nickel aluminide coating. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy was used to evaluate the effect of cycle frequency on microstructural evolution. A simple statistical spalling model [1,2], assuming that the parabolic rate constant kp and the spalling probability p are constant, is tentatively applied and discussed in view of the microstructural evolution complexity

Effect of a heterogeneous distribution of particles on the formation of banded grain structure in wrought Alloy 718

Alloy 718 is known to be sensitive to interdendritic segregation formed during ingot solidification. The occurrence of banded grain structures under heat treating conditions close to 1000 ° C related to interdendritic segregation is often reported. In order to have a better understanding of this microstructural evolution, an extensive experimental program has been carried out. Consequently, a model taking into account the selective dissolution of δ-phase (Ni3Nb) is proposed. A grain growth simulation by Monte-Carlo method is then used to illustrate the grain structure evolution in a banded particle distribution. By comparing experimental data and computer simulation, the relationship between the Monte-Carlo step and the real time is assessed and the range of parameters when heterogeneous microstructures appear is specified

Parallelized Hybrid Monte Carlo Simulation of Stress-Induced Texture Evolution

A parallelized hybrid Monte Carlo (HMC) methodology is devised to quantify the microstructural evolution of polycrystalline material under elastic loading. The approach combines a time explicit material point method (MPM) for the mechanical stresses with a calibrated Monte Carlo (cMC) model for grain boundary kinetics. The computed elastic stress generates an additional driving force for grain boundary migration. The paradigm is developed, tested, and subsequently used to quantify the effect of elastic stress on the evolution of texture in nickel polycrystals. As expected, elastic loading favors grains which appear softer with respect to the loading direction. The rate of texture evolution is also quantified, and an internal variable rate equation is constructed which predicts the time evolution of the distribution of orientations.Comment: 20 pages, 8 figure

Biomass-derived carbon materials for energy storage applications

Energy storage systems are an essential link in the implementation of renewable energies and in the development of electric vehicles, which are needed to reduce our dependence on fossil fuels and the emission of greenhouse gases. Various technologies have been proposed for energy storage based on different working principles, including lithium-ion batteries, emerging sodium-ion batteries and electric-double layer capacitors. Besides the quest for improving key aspects such as energy and power densities, current research efforts are devoted to foster the manufacturing of more environmentally friendly devices using sustainable materials. Carbon-based electrodes hold considerable promise in such terms due to their low cost, tailorable morphology and microstructure, and the possibility of processing them by direct carbonization of eco-friendly and naturally-available biomass resources. The main goal of this thesis is to develop carbon materials from biomass resources and study their applications as electrode for lithium-ion batteries, sodium-ion batteries and electric-double layer capacitors. En route towards that goal, it also aims at expanding our understanding of the microstructural changes of biomass-derived carbons with varying processing conditions and their effect on the electrochemical performance for each of these technologies. The first part of this work reports on the synthesis of graphitized carbon materials from biomass resources by means of an Fe catalyst, and the study of their electrochemical performance as anode materials for lithium-ion batteries (LIBs). Peak carbonization temperatures between 850 °C and 2000 ºC were covered to study the effect of crystallinity, surface and microstructural parameters on the anodic behavior, focusing on the first-cycle Coulombic efficiency, reversible specific capacity and rate performance. Reversible capacities of Fe-catalyzed biomass-derived carbons were compared to non-catalyzed hard carbon and soft carbons materials heated up to 2800 ºC. Moreover, in-situ characterization experiments were carried out to advance our understanding of the mechanisms responsible for catalytic graphitization. The second part of this work reports a comprehensive study on the structural evolution of hard carbons from biomass resources as a function of carbonization temperature (800 - 2000 ºC), and its correlation with electrochemical properties as anode materials for sodium-ion batteries (SIBs). Synchrotron X-ray total scattering experiments were performed and the associated atomic pair distribution function (PDF) extracted from the data to access quantitative information on local atomic arrangement in these amorphous materials at the nanoscale, as well as its evolution with increasing processing temperature. Then, electrochemical properties and the storage mechanisms involved on Na ions insertion into hard carbon structures at each characteristic potential regions were elucidated and correlated with microstructural properties. Finally, the third part of this work reports on the synthesis of nanostructured porous graphene-like materials from biomass resources using an explosion-assisted activation strategy by nitrate compounds and Ni as a graphitization catalyst. The thermal behavior during carbonization as well as the resulting microstructural and surface properties were evaluated at two different processing temperatures, 300 and 1000 ºC. Finally, their application as electrode materials for electric-double layer capacitors (EDLCs) and LIBs is investigated, with a view to their performance under high charge/discharge specific current densities experiments.Premio Extraordinario de Doctorado U

Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclicoxidation tests at 1,050 C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 C for 100 h and cyclicoxidation tests at 900 C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the b to c0 transformation leads to the formation of a two-phase b/c0 layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this b to c0 transformation occurs gradually by an inward transformation of b leading to the formation of a continuous layer of c0 phase, parallel to the metal/scale interface

X-ray reflectivity, diffraction and grazing incidence small angle X-ray scattering as complementary methods in the microstructural study of sol–gel zirconia thin films

X-ray reflectometry, X-ray diffraction and grazing incidence small angle X-ray scattering have been complementary used to fully characterize zirconia (ZrO2) thin films obtained by the sol–gel route. The films were synthesized on various sapphire (Al2O3), silicon (Si) and glass mirrorpolished wafers by a dip-coating process in a zirconia precursor sol. Versus the synthesis parameters as alkoxide sol concentration, withdrawal speed and annealing temperature, the microstructure of the layer is managed and its different microstructural parameters such as thickness, mass density, crystalline phase, grain size and spatial arrangement have been determined. The as prepared layers are amorphous. During a thermal treatment at low temperature (<1000 -C), the layers thickness decreases while their mass density increases. Simultaneously the zirconia precursor crystallises in the zirconia tetragonal form and the coating is made of randomly oriented nanocrystals which self organise in a dense close-packed microstructure. At low temperature, this microstructural evolution is similar whatever the substrate. Moreover, the layer evolves as the corresponding bulk xerogel showing that the presence of the interface does not modify the thermal microstructure evolution of the layer which is controlled by a normal grain growth leading to relatively dense nanocrystalline thin films