Materials Science Forum Vols. 783-786

Abstract: Development of integrated database including casting shapes with their casting design, technical knowledge, and thermophysical properties of the casting alloys were introduced in the present study. For the shape database, a number of casting products are scanned by using by using industrial computer tomography and compiled to the shape database. Each shape data in the database has also parametric 3D modeling data, not only the product but also its casting design. If casting engineer want to design new casting, similar casting shape with casting design can be easily retrieved from the shape database, and easily modified and adapted to the new casting. Technical knowledge of the various casting processes and their manufacturing process of the castings were compiled and the search engine for the knowledge was developed. Database of thermophysical properties of the casting alloys were obtained by the experimental study or calculation based on CALPHAD approach, and the properties were used for the in-house computer simulation of casting process. The databases were linked with intelligent casting expert system developed in e-design center, KITECH. It is expected that the databases can help non casting experts to devise the casting and its process. Various examples of the application by using the databases were shown in the present study.

Abstract: Competitive growth and grain selection were simulated and analyzed during the directional solidification with the conditions for single crystal superalloy DD6 castings using Commercial software, ProCAST. A three dimensional cellular automaton (CA) model coupled with finite-element (FE) heat flow calculation was applied. Measurements at the grain scale were made using the EBSD method at the cross sections in the starter block and grain selector at an interval of 4 mm from the chill surface. The grain characteristics and the rules of competition growth were obtained. The validity of the simulation results were compared with those of the experiment. It concluded that the model-predicted tendency shows satisfactory agreement with the experiment. Increasing the distance from the chill decreases the number of grains, increases the radius of grains and drives the crystallization orientation of grains to principal stress orientation.

Abstract: During the annealing heat treatment following cold rolling of a 304L austenitic stainless steel sheet material, the material goes through changes in microstructure and mechanical properties. The cold rolling history together with the time/temperature trajectory in the annealing furnace can be used to model the final microstructure. In this work, physically based models for recrystallization and the following grain growth was created for the prediction of the microstructure evolution-both grain size and grain size distribution-, and an artificial neural network, ANN, was added for secondary effects. This is more commonly referred to as a hybrid model. The microstructure hybrid model was tested and validated against cold rolled and annealed production sheet material of various thicknesses and reductions, where the grain size and grain size distribution was measured by Electron Back Scatter Diffraction, EBSD. The recrystallization and grain growth parameters and functionality were fitted for non-isothermal conditions, against experimental tests of cold rolled material. Given process history and time/temperature data from the annealing heat treatment, the model can predict the microstructure, average grain size and grain size distribution with high accuracy and the executing time is short which makes it suitable for in-line use.

Abstract: When producing hot strip, HSLA(High Strength Low Alloyed) products the coiling temperature and the subsequent coil cooling is of great importance for the final mechanical properties. A thermo-mechanically coupled model has been developed where the anisotropic stress dependent thermal properties caused by the layered structure and the asymmetric cooling are included. Additionally the precipitation hardening effect on the yield strength, influenced by the thermal history during cooling was compared with mechanical tensile testing along the strip length at SSAB EMEA works in Borlänge, Sweden. Good agreement between measured and predicted yield stress variations in head and tail was obtained.

Abstract: We review the application of advanced numerical techniques such as adaptive mesh refinement, implicit time-stepping, multigrid solvers and massively parallel implementations as a route to obtaining solutions to the 3-dimensional phase field problem for coupled heat and solute transport during non-isothermal alloy solidification. Using such techniques it is shown that such models are tractable for modest values of the Lewis number (ratio of thermal to solutal diffusivities). Solutions to the 3-dimensional problem are compared with existing solutions to the equivalent 2-dimensional problem.

Abstract: In order to understand the reactivity of Cr₂O₃ surface towards H₂O molecule, the optimized structure, electronic structure, and the behavior of adsorbates were examined using a first-principles calculation based on density-functional theory (DFT). H₂O coverages varying from a quarter to two monolayers (MLs) were considered. At a low coverage, the oxygen atom of H₂O adsorbs on the Cr atom of the outermost Cr₂O₃ surface layer, the entire H₂O molecule is slanted at the direction of a hollow site, and a molecular plane is nearly parallel to the surface. The hydrogen bond is formed between the surface oxygen atom and the hydrogen atom of H₂O molecule. From the optimized structure, the H₂O dissociation mechanism which passes through a transition state is guessed. For 0.5ML coverage the obtained absorption energy is-82.5 kJ/mol. Our results are in good agreement with other reported theoretical and experimental results.

Abstract: A computational method for fast and accurate reconstruction of steels microstructure has been developed. The method makes use of the physical description of phase transition kinetics and mathematical description of grain morphology, implements the spirit of symbolic dynamics for phase transition, and has produced realistic microstructure for steels. Using the known microstructure-property relationship of strong steels, this method enhances the design and optimum the chemical-thermomechanical processing conditions for the production of novel steels.

Abstract: In bulk polycrystals, the first nucleation event in a single grain may induce transformationin neighboring grains, resulting in a cluster of partially transformed grains. This cluster of partiallytransformed grains is normally designated as a ‘spread event’. The collection of these single spreadevents can be defined as the ‘spread’or ‘martensite spread’. In this paper we show how the martensitespreadmay be well described by formal kinetics. Spread taking place as a result of athermalmartensitetransformation, isothermal martensite transformation and even martensite burst are analyzed with thehelp of such a methodology. Formal kinetics is essentially based on a stochastic geometry approach ofthe transformations considered as nucleation and growth (or birth-and-growth ) processes. Therefore,the success of such an analysis strongly suggests the possibility of a computer simulation of the spreadentirely based on probabilistic considerations. Such a simulation has been carried out and the resultsare discussed in this paper with reference to the results obtained by the analytical methods.

Abstract: Traditionally, the discovery of new materials has been the result of a trial and error process. This has resulted in an extremely time-consuming and expensive process. Models for guiding the discovery of new materials have been developed within the European Accelerated Metallurgy project. The application of statistical techniques to large materials datasets has lead to the discovery of unexpected regularities among their properties. This work focuses on mechanical properties. In particular, the interplay between yield strength, ultimate tensile strength and elongation. A methodology based on principal component analysis, and Kocks-Mecking modelling has led to a tool for finding optimal compositional and heat treatment scenarios. The model is first presented for wide ranges of alloys, and the application to the discovery of new magnesium and ferrous alloys is outlined.

Abstract: A fast simulation of the inhomogeneous materials evolution during the deformation steps and its effect on subsequent processes is demanded for the development of new technologies for materials with a homogeneous microstructure. In the paper a layer model for flat hot and cold rolling is presented. A deepened understanding of the influence of inhomogeneities in material state and material flow on the whole process can be reached due to the introduction of a new computational concept for variable layer thickness distributions in the layer model. The concept will be explained and the additional information obtained concerning the influence of shear, local residual stresses as well as inhomogeneous stress and strain states in the roll gap on the microstructure evolution will be presented and discussed.