Papers by Keyword: Microstructural Development

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Authors: Maria Giuseppina Mecozzi, C. Bos, Jilt Sietsma
Abstract: A three-dimensional cellular automata (CA) model is developed for the kinetic and microstructural modelling of the relevant metallurgical mechanisms occurring in the annealing stage of low–alloy steels: recrystallisation, pearlite–to–austenite transformation and ferrite–to–austenite transformation on heating and austenite–to–ferrite transformation on cooling. In this model the austenite–to–ferrite transformation is described by a mixed–mode approach, which implies that the transformation kinetics is controlled by both the interface mobility and the diffusivity of the partitioning elements. This approach also allows incorporation of the ferrite nucleation occurring on structural defects. The developed CA algorithm, in which the transformation rules for the grain boundary and interface cells are controlled by the growth kinetics of the forming phase, allows three-dimensional systems to be treated within relatively short simulation times. The simulated microstructure reproduces quite well the microstructure observed in experimental samples. A good agreement is obtained between the experimental and simulated ferrite recrystallisation and ferrite and austenite transformation kinetics. The present approach also models the development of the carbon concentration profile in the austenite, which is, for instance, essential for subsequent martensite formation.
Authors: Guilhem Martin, Muriel Véron, B. Chéhab, R. Fourmentin, Jean Denis Mithieux, S. K. Yerra, Laurent Delannay, Thomas Pardoen, Yves J.M. Bréchet
Abstract: Duplex stainless steels (DSS) are alloys made of ferrite and austenite, with a proportion of each phase around 50%. Their main advantage in comparison with other austenitic and ferritic stainless steels is the attractive combination of high strength and corrosion resistance together with good formability and weldability. Unfortunately, DSS often present a poor hot workability. This phenomenon can stem from different factors associated to the balance of the phases, the nature of the interface, the distribution, size and shape of the second phase, and possibly also from difference in rheology between ferrite and austenite. In order to determine the specific influence of phase morphology on the hot-workability of DSS, two austenite morphologies (E: Equiaxed and W: Widmanstätten) with very similar phase ratio have been generated using appropriate heat treatments. It was checked that the latter treatments generate stable microstructures so that subsequent hot mechanical tests are performed on the microstructures of interest. One microstructure consists of a ferritic matrix with austenitic equiaxed islands while the other microstructure is composed of a ferritic matrix with Widmanstätten austenite. The latter morphology corresponds to the morphology observed in as-cast slabs.
Authors: M.J. Ribeiro, João Carlos de Castro Abrantes, Joao A. Labrincha
Abstract: Al-rich sludge produced from industrial anodising and surface treatment processes had been tested in the fabrication of mullite-based materials, by using unidirectional dry pressing as shaping technique. Mixture with common natural silica and magnesium-containing materials, like ball clay, kaolin and diatomite were prepared in order to achieve interesting final fired refractory and/or electrical insulating materials. Microstructural changes upon sintering and crystalline phase evolution are detailed and their relationship with the electrical behaviour is also studied, by the use of impedance spectroscopy. Finally, different models were used to fit the experimental electrical resistivity values and to estimate the percolation limit of the bi-phase material.
Authors: Vinicius André Rodrigues Henriques, Elisa B. Taddei, Cosme Roberto Moreira Silva
Abstract: Ti-35Nb-7Zr-5Ta alloy is considered an attractive material for implants manufacture due to an excellent combination of properties, including high mechanical and corrosion resistance, beyond the lowest elastic modulus among the titanium alloys. The alloy processing by powder metallurgy (P/M) eases the obtainment of parts with near-net shape forming and low production costs. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 800-1600 °C, in vacuum. The isochronal sintering demonstrated to be efficient for the study of the microstructural evolution. The samples presented high densification and adequate microstructure. The results show that a beta-homogeneous microstructure is obtained in the whole sample extension when sintered at high temperatures beyond that P/M technology allows an effective porosity control.
Authors: Michael J. Hoffmann, Stefan Holzer
Authors: H. Schubert, J. Deuerlein, U. Neidhardt, H. Lange, Fritz Aldinger, E. Muschelknautz
Authors: J.N. DuPont
Abstract: The primary factors that effect solid state diffusion during solidification are described and binary solute redistribution equations that permit estimation of the significance of solid state diffusion are discussed. Model calculations suggest that solid state diffusion of substitutional alloying elements in FCC alloys is insignificant under most processing conditions, while that of interstitial alloying elements is likely to be complete. Experimental data that supports these results are presented. Several cases that highlight the practical importance of microsegregation on performance of engineering alloys are described as well as methods for avoiding or minimizing microsegregation for improved properties. A solute redistribution model for handling the limiting cases of solute diffusion in ternary alloys is presented and model calculations are reviewed to reveal the strong influence diffusion can have on the solidification path and resultant microstructure.
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