Materials Science Forum Vols. 783-786

Abstract: Hydrogen is harmful in steel which makes it important to reduce the hydrogen content. Piling slabs after casting gives a slow cooling which increases the diffusion out of the steel. Finite element modeling has been used to simulate this process where hydrogen solubility and phase dependent diffusivity can be taken into account. The hydrogen diffusion model is using STEELTEMP^® 2D for the thermal analysis. Measurements of temperature and hydrogen content in piled slabs have been done and the calculations are in good agreement.

Abstract: The concepts of electronic stress tensor density and energy density give new viewpoints for conventional ideas in chemistry. In this paper, we introduce the electronic stress tensor and energy density and other related quantities such as tension density and kinetic energy density, which are based on quantum field theory, and show their connection to the concepts in chemistry. The topics are: (i) zero surface of the electronic kinetic energy density and size of atoms, (ii) separatrix of the tension field as a boundary surface of atoms in a molecule, (iii) interpretation of energy density based bond order as directional derivative of a total energy of a molecule regarding the bond direction, and (iv) eigenvalues of the stress tensor as tools to classify types of chemical bond.

Abstract: Integrated Computational Materials Engineering (ICME), and Integrated ComputationalMaterials Science (ICMS) are developing fields with an aim of alloy design, by combining physicalmodels describing materials behavior through lengthscales and processing steps. It has beensuspected, however, that uncertainties in input parameters may cumulate in a hereditary way andyield to a high variability in the final output, independently of the quality of models themselves.Such a variability is however rarely quantified. In this aim, an illustrative example is here given,using a set of “cascade models”, each model being voluntarily very simple (grain growth,precipitation, hardening…) whereas assumed to be exact, so that only the effect of parameteruncertainties on the variability of the output (yield stress of a Ni-base superalloy) can be studied. Itis demonstrated that, with usual uncertainty levels in input parameters, the final dispersion (error)can become very high. Additionally, considering that models are not exact themselves would renderthe situation even worse. Besides, global and implicit models, like neural networks or Gaussianprocesses, have been shown to be able to perform reliable predictions and to be used for alloydesign, with acceptable levels of error, the latter being estimated by statistical methods. In addition,unlike ICME or ICMS, predictions are very fast so that automatic alloy composition optimisation ispossible using, for instance, genetic algorithms. Other fast predictive tools, like computationalthermodynamics (Thermo-Calc), can then be used as constraints during alloy optimisation.

Abstract: Due to its high creep strength and oxidation resistance, C-263 is a promising Ni-based alloy for applications in superheater tubes in coal fired thermal power plants. The creep strength is mostly based on finely distributed gamma-prime precipitates. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Selected Area Diffraction Pattern (SADP) and Energy Dispersive Spectroscopy (EDS). Besides, equilibrium and Scheil calculations were carried out using the thermodynamic software MatCalc to analyze the stable phases and the solidification process, respectively. Precipitation calculations during solution annealing and subsequent ageing at 700°C and 750°C up to 10.000h ageing time were performed to predict the phase fraction and precipitates radius. SEM and TEM investigations of aged specimens revealed three different precipitates: M₂₃C₆, γ’ and MX. MatCalc also predicted these precipitates. The calculated phase fraction and mean radius show good agreement with experimental data.

Abstract: In FSW modelling, two major approaches have been used to describe the heat loss from the workpiece to the backing bar. The first method simplifies the heat loss using a convective transfer and has been used by some researchers such as Khandkar et al.[1]. The second method uses a contact gap conductance to represent the imperfect contact at the interface between the workpiece and the backing bar [1-3]. The contact gap conductance, k is defined as: k = Q/(T₀-T_A), where Q is the heat flux from the workpiece to the backing bar, T_O is the temperature of the workpiece and T_A is the temperature of the backing bar. Khandkar et al.[1] found that using the contact gap conductance method was more accurate than the convective heat transfer coefficient. Both Simar et al.[2], Colegrove and Shercliff [3] and Shi et al.[4] have used a variable contact gap conductance in their models. Shi et al.[4] applied a temperature dependent contact gap conductance method where the value increased with temperature to simulate the better contact under the tool. This paper combined the FSW process model with Artificial Neural Network (ANN) models to find the temperature dependent contact gap conductance k which is named as a hybrid models.

Abstract: A model describing the microstructure formation in a directionally solidified immiscible alloy under the convective effect is presented. The microstructure evolution in a directionally solidified Al-Pb alloy is investigated. It is demonstrated that convective flows have great effects on the solidification of immiscible alloys. A convective flow against the solidification direction causes an increase in the nucleation rate while a convective flow along the solidification direction causes a decrease in the nucleation rate. The convective flows lead to a more uneven distribution of the minority phase droplets in the melt. It causes an increase in the size of the largest minority phase droplets and is against the obtaining of the immiscible alloys with a well dispersed microstructure.

Abstract: Utilizing a proper material model for describing the mechanical behavior of any material is key for a successful simulation of friction stir processing (FSP) where temperature, strain, and strain rate gradients vary abruptly within, and when moving away, from the stirring zone. This work presents a comparison of how faithfully do three different constitutive equations reproduce the state variables of strain, strain rate, and temperature in an FEM simulation of a test-case FSP (1000 rpm spindle speed, and 90 mm/min feed). The three material models considered in this comparison are namely: Johnson-Cook (JC), Sellars-Tegart (ST), and Zerilli-Armstrong (ZA). Constants for these constitutive equations are obtained by fitting these equations to experimental mechanical behavior data collected under a range of strain rates and temperatures of twin-rolled cast wrought AZ31B sheets.It is widely recognized that JC-based models over predicts stress values in the stir zone whereas ST-based models are incapable of capturing work hardening outside of the stir zone. Therefore, a ZA model, being a physical based-HCP specific model, is hereby investigated for its suitability as a material model that would overcome such drawbacks of JC-and ST-based models. The equations from the constitutive models under consideration are fed into an FEM model built using DEFORM 3D to simulate the traverse phases of a friction stir process. Amongst these three material models, comparison results suggest that the HCP-specific ZA model yield better predictions of the state variables: strain, strain rate, and temperature, and, consequently, the estimated values for flow stresses.

Abstract: Design and safety assessment of advanced ductile cast iron (DCI) components like windturbines or transport and storage casks for radioactive materials require appropriate material data interms of strength and fracture toughness. Therefore, it is of vital importance to characterize andunderstand the deformation, damage and fracture behaviour of DCI which may substantially changefrom ductile to brittle by increasing loading rate, decreasing temperature and/or increasing stresstriaxiality. This paper reports on recent BAM investigations on different qualities of the widely usedDCI grade EN-GJS-400 with varying pearlite shares (none and 18 % respectively). The focus wason the influences of microstructure, temperature (ambient and -40 °C) and loading rate (quasi-staticto crash) on strength (YS, UTS, flow curve) and fracture mechanical properties (R-curve, crackinitiation toughness, fracture toughness). Systematic metallographical and fractographical analyseswere performed accompanying the whole test program and a systematics of specific damagebehaviour and fracture mechanisms was derived from the results.

Abstract: In the present study, corrosion behavior of a diffusion bonded interface formed between micro-duplex stainless steel (MDSS) and a mixed titanium alloy (Ti6Al4V) formed at 900°C for 60 minutes under 4MPa uniaxial pressure in vacuum has been investigated in 1M HCl and 1 M NaOH solutions using various electrochemical measurements such as Equilibrium Potential (EP), Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PD). For comparison, corrosion behavior of base metal alloys, MDSS and Ti6Al4V have also been also characterized. Bonded interface has been characterized in light optical microscopy and scanning electron microscopy using back scattered electron. The layer wise σ phase and λ+FeTi phase mixture has been observed at the bond interface and the bond tensile strength and shear strength were ~556.4MPa and ~420.2MPa, respectively. The corrosion rates of the bonded joint are intermediate to the corrosion rates of MDSS and Ti6Al4V alloy.

Abstract: The carbon content in steel of the most produced grinding balls, 3 and 5 inch diameter, varies between 0.6% and 1.1% and, due to the presence of other addition elements, after their standard heat treatment, all the ball zones –external and central –reach high hardness values: over 60 RC in the case of the 3 inch diameter balls. The internal zone, hard and with low tenacity, produces a notorious diminution of the working life of the balls with respect to their theoretical potential. In this context, this work has as its objective to improve the quality of the grinding balls of 3 inch diameter, giving them highe r superficial hardness and a central zone with less martensite, and as a consequence, with moderate hardness and adequate tenacity. To perform this, the temperature time transformation (TTT) curves for the steel of the balls was determined by means of calorimetric (DSC) and metallographic analyses. Afterwards, a mathematical model (FDM) for the temperature distribution of the balls during their quenching and equalization treatment was done and experimentally validated. Along with the TTT curves and the math ematical temperature model, new heat treatment conditions were established: a reduced ball quenching time of 55 seconds (instead of the current 80 seconds), with a final central temperature of 500°C (instead of 273°C) and an equalization time of 200 seconds (instead of 40 seconds), and a final ball temperature of 203°C (instead of 139°C). With this new treatment, less martensite was obtained in the central zones of the balls, with an associate hardness on this area of 53-55 RC, while the standard hardness is maintained in their external zones.