Materials Science Forum Vol. 553

Abstract: Residual stress fields can cause creep damage in thermally aged components, even in the absence of working loads. In order to study this issue, the authors present a numerical study on the development of triaxial residual stresses in stainless steel specimens. A mechanical model dedicated to the analysis of heat treatment problems is described. The presented formulations are implemented incrementally with a non-linear constitutive model, adequate to the simulation of a wide range of thermal processes. The flow rule is a function of the equivalent stress and the deviatoric stress tensor, of the temperature field and of a set of internal state variables. The thermomechanical coupled problem is solved with a staggered approach. Spray water quenching was used to generate residual stress fields in solid cylinders and spheres made from 316H stainless steel. Finite element simulations were performed to find out how process conditions and specimen geometry influence the resulting residual stress distributions. The results show that compressive residual stresses are developed near the surfaces of the cylinders and spheres while tensile residual stresses occur near the centre. The level of residual stresses was found to be dependent on the heat transfer coefficient.

Abstract: High silicon steel is used for electrical applications because its electrical resistivity is increased and the magnetostriction is reduced. A silicon content up to 6.5 wt.-% gives excellent magnetic properties. The improvement of the magnetic properties stays in contrast with the lack of ductility of these alloys, making their thermo-mechanical processing difficult. The optimum final microstructure and texture depends on the final application of the material: extremely big grains with a Goss orientation ({110} <001>) are desired in transformers and grains with an average size of 100 -m and cube component ({100} <001>) are used in electrical motors. A series of plane strain compression (PSC) tests were performed on 3 electrical steels, with a silicon content from 1.8 to 4.1 wt.-%, in a temperature range of 800 to 1100°C, strain rates between of 0.5 and 5 s-1. Reductions and time between deformation and quenching were also varied in order to study the recrystallisation progress. Apparent activation energies for hot working, calculated using the hyperbolic sine equation, was in good agreement with literature and higher than the activation energy for self diffusion in iron. These values increase with the silicon content. The high temperature texture evolution was investigated by means of electron back scattering Diffraction (EBSD) technique, which allows the quantification of important texture components in function of the thermo-mechanical parameters applied during hot rolling and the plane strain compression tests. The hot rolled microstructures have shown an average grain size of 140 -m and a texture with a maximum on the cube fibre ({001} <-1-10>). The conventional α (<110> // RD) / γ (<111> // ND) fibre texture was developed after plane strain compression and their intensities depend on the deformation temperature and reduction. A similar tendency was observed for the fraction of static recrystallised grains.

Abstract: The fracture toughness of the Fe2B phase was evaluated in this study. Formation of the Fe2B boride is carried out though paste boriding process applied on AISI 1045 steel surface. The treatment was carried out at temperatures of 1193, 1223 and 1273 K for 6 h using a 5 mm thick boron paste. A Vickers microhardness tester was used to generate microcracks at a load of 200g. The indentations were made across the thickness of the iron boride layer at four different distances from the substrate. The experimental results show that the critical stress intensity factor KIC for the Fe2B phase shows a potential law dependence on crack length; this contradicts the concepts of Linear Elastic Fracture Mechanics, which establish that the fracture toughness value is a constant of the material.

Abstract: In this study, the evaluation of interfaces on iron boride Fe2B growth obtained by paste boriding process was carried out. Fractal geometry is used like a powerful tool for the roughness analysis present during iron boride growth. Experiments were performed in AISI 1045 steel at temperatures of 1193K for exposure times of 2, 4 and 6 h, and 1223K for treatment times of 2, 4, 5 and 6 h, varying the boron paste thicknesses in the range of 1 – 5 mm for each temperature and time. The fronts of the interfaces in iron boride coatings were characterized and digitized with mean of an optic microscope and Scion Image software. Self-affine methods were applied to the interface growths for validate the fractality of the system. It was established that the interface width, ω , scales to ω (L) ∼ L H , where H represents the roughness exponent of the boride layers.

Abstract: The complete and simultaneous simulation of the overall filling and curing processes is presented. Fluid flow in the porous medium is described by the Brinkman-Forchheimer flow model, and fluid flow in the clear fluid domain is described by the Navier-Stokes equations. The flow front is captured using the volume fraction concept and a compressive convective scheme. Energy conservation equation and resin conversion equation give the equations to obtain the temperature and degree of cure, respectively. The physical model is solved using a control volume based finite element method. A limited set of results is presented, showing the usefulness of the information obtained from the complete and simultaneous simulation of the overall real process.

Abstract: This paper is on the geometrical effective thermal conductivity of hollow metal sphere structures. Two different technologies of joining, namely adhesive bonding and sintering, are considered. The spheres are arranged in the nodes of a cubic primitive lattice and connected by an adhesive layer, respectively directly joined by sintering. Furthermore, the influence of the cell wall thickness of the spheres on the thermal conductivity is investigated.

Abstract: Hollow sphere structures (HSS) constitute a group of innovative materials which are characterised by more constant material properties compared to classical cellular metals [1]. Their big potential lies within multifunctional applications where combinations of their proper- ties yield symbiotic advantages. In the scope of this paper their effective thermal conductivity is investigated. In addition to the analysis of the dependency of this material parameter on the conductivities of the base materials and the sphere wall thickness, special focus is given to the influence of the morphology of joining.

Abstract: In this paper, the Finite Element and lattice Monte Carlo methods are used to calculate the effective thermal conductivity of two models of a composite: circular and square inclusions arranged in a square planar arrangement. A new lattice Monte Carlo method based around Fick’s First Law is also presented. Excellent agreement is found between these quite different methods. It is also shown that the results are in excellent agreement with the century-old Maxwell Equation.

Abstract: In the present study we elaborated a thermodynamical model for analysis of isothermal phase transformations under high pressure. Our study was provoked by the necessity to characterise the behaviour of MTe2 chemical compounds (M = Pd, Pt) while subjected isothermally to high pressure. As known [1] MTe2 powders are representatives of the CdI2 structure type. This structure type is a bi-dimensional one and as such is atypical for the big family of lamellar MQ2-type dichalcogenides (M = Pd, Pt; Q = S, Se, Te). Specific of lamellar structure is the strong ionicity of the bonds. One of the most interesting points stands on the possibility for realising interactions between the layers of different types of ions. That could be done under high pressure by any of the following transformation processes: (i) phase transition to the typical pyrite structure; (ii) phase rearrangement changing the parameters of the crystal cell but keeping the 2D-type structure. In this framework our aim was to elaborate a thermodynamical model for analysis of such isothermal phase transformations under high pressure. Our analysis model is designed to answer the following questions: (i) if the treated compound undergoes a classical phase transition or a phase rearrangement; (ii) which is the order of the phase transition or the phase rearrangement, respectively; and (iii) what is the degree-of-stability of the treated compound under high pressure. To detect if the transformation process is a phase transition or a rearrangement, we compute both volumetric and longitudinal Gibbs free energies and their partial derivatives. We recognise the transformation to be: (i) a phase transition when it affects the volumetric Gibbs free energy and its partial derivatives; (ii) a phase rearrangement if it affects the longitudinal Gibbs free energy and its partial derivatives. The order of the transformation process (phase transition or rearrangement, respectively) is determined by the order of the partial derivative of the Gibbs free energy (volumetric or longitudinal, respectively), which is discontinuous in the transformation point. Hence, we compute the two first partial derivatives (i.e., the first one and the second one) of the Gibbs free energy (both volumetric and longitudinal). For characterising the degree of stability of the treated compound under high pressure we calculate its entropy generation (volumetric and longitudinal, respectively) during the treatment process. The established model was further applied to PdTe2 and to PtTe2 while subjected isothermally to high pressure.