Solid State Phenomena Vol. 353

Abstract: The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro-alloyed steels (HSLA). Following repeated heating cycles in in with temperature ranging Ac₁ /Ac₃, the ICHAZ will face with an evident toughness and fatigue behavior reduction especially due to martensite-austenite constituent (MA) formation. Microalloying in high strength steels causes the generation of some phases in the matrix able to increase the mechanical properties of the joint. In this paper we report an investigation related to 1000 ppm vanadium addition in the welded joint of a structural S355 steel. The inter-critical zone of ta double pass welded joint is here reproduced by dilatometer, with second peak temperature ranging 720°C-790°C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.

Abstract: The combination of advection and migration of grain boundaries is analyzed on the basis of a simple mesoscale model, where parallelepipedic grains are considered under uniaxial compression straining. Strain hardening and dynamic recovery are described by the classical Yoshie-Laasraoui-Jonas equation. Grain-boundary migration is driven by the difference in dislocation densities between one representative grain and the average over the material. Finally, nucleation is assumed to occur at grain boundaries. Special attention is paid to the aspect ratio, which starts from unity (infinitely small cubic nucleus) and tends to zero when the grain disappears. In spite of the role of migration, the average shape of the grains is determined as a first approximation by their lifetimes.

Abstract: The temperature dependence of the diffusion coefficient in metallic glass-forming systems do not follow the Arrhenius behavior over a wide temperature range. Instead, it exhibits a kink behavior at around the glass transition temperature. Some researchers associate this behavior to the difference in the diffusion mechanism operating in the glassy and the supercooled liquid state, whereas others do not support this view. In addition, usually, the temperature dependence of the diffusion coefficient is analyzed by splitting the temperature range into two regions, above and below the glass transition temperature. In the present study, we developed an analytical theory that describes the continuous variation of the diffusion coefficient across a temperature where the kink behavior is observed. According to the theory, the kink behavior arises from the freezing of free volume available for diffusion by lowering the temperature. A connection to the vacancy mechanism of diffusion has been also pointed out.

Abstract: It is now well established that the grain size is the fundamental microstructural feature of all polycrystalline materials. In practice, a very wide range of grain sizes will be needed in order to fully evaluate the effect of grain size on the mechanical properties of metals. For many years this was a significant limitation because it was not possible to use conventional thermomechanical processing to produce materials with submicrometer or nanometer grain sizes. Recently, this problem has been addressed by developing alternative processing techniques based on the application of severe plastic deformation. This overview demonstrates that, although the flow stress increases with decreasing grain size at low temperatures and decreases with decreasing grain size at high temperatures, this clear dichotomy in behavior may be adequately explained by using a single theoretical flow mechanism based on the occurrence of grain boundary sliding.

Abstract: In sintering simulation, there are basically two approaches: microscale simulation, in which distinct particles or pores are regarded, and macroscale, where the porous body is regarded as continuum with variable density.Material parameters of the latter can be determined by experiment or by microscale models.Current microscale sintering models mainly use circular resp.~spherical particle geometries to represent the actual shape of real particles.However, sintering behavior is heavily dependent on the morphology of the powder particles, since sintering progress is driven by reduction of the bound surface energy.So current models neglect the influence of local contact morphology.Here, a finite differences based microscopic sintering model is presented, which is capable to work with irregular particle geometries.Asymmetric particle contacts in shape and substance are possible within.The differences between circular particle contacts and asymmetric ones are investigated.Furthermore, a statistical way of describing the morphology of powder particles and its inclusion into sintering simulation using Monte Carlo techniques are shown.Morphology data are obtained from microscopic imaging by extracting the 2D contours.The particles' contour lines are fitted to a parameterized shape function including ovality and first order waves to obtain a description of the particles' fine shapes.From the statistical distribution of the shape parameters, randomized particle groupings are sampled as input for microscopic sintering simulation.Statistical analysis of the samples' sintering behaviors leads to statements about the powder's.Comparisons to classical spherical modelling are given.

Abstract: Three main rheological laws are found in the literature to describe the strain hardening of materials at high temperatures. The choice of the most suited law to describe a flow stress curve is often discussed as a function of the nature of the material; but it still remains difficult to choose the most appropriate one. These semi-empirical laws systematically comprise two main terms linked either to the dislocations generation or their annihilation.The objective of this paper is to determine by an inverse method which law appears to be the most suited. It is finally demonstrated that the application of one law is mostly equivalent to another. The various laws are overall equivalent and do not help to describe some peculiar physical mechanism of plasticity.

Abstract: A novel procedure for material quality assessment developed for castings like ductile irons and Al alloys, is based on the analysis of tensile strain hardening through dislocation-density-related constitutive equation, and consists of plotting the Voce equation parameters found through modeling the tensile flow curves with the Voce constitutive equation. In sound materials the Voce parameters have a regular trend, consistent with the physical meaning of the dislocation-density-related Voce constitutive equation. The Voce parameters identify a regular trend also in defective materials, even if defects and metallurgical discontinuities might be expected to add a random and unpredictable component to the plastic behavior. This unexpected regular behavior in defective materials has been called as Defects-Driven Plasticity (DDP), and its rationalization seems to be possible by coupling the concepts of Notch Strengthening (NS) of defects, and stable ductile fracture propagation of the Continuous Damage Mechanics (CDM). The rationalization of DDP and the experimental findings to support, are here reported in high Silicon strengthened ductile irons.

Abstract: The standard textbook analysis of dislocations is generally limited to the case of infinitely straight screw or edge dislocations, which do not exist. This is due to the complexity of the formulas for arbitrary dislocation loops, i.e., Burger’s equation for the displacement field, the Peach-Köhler equation for the stress field and Blin’s equation for the interaction energy, which involve line integrals along the dislocation loop. The integrands are complex, and integration often involves non-elementary functions. Elaboration of the integrands with symbolic mathematical software produces tensor formulas which can be reused at will. By formulating convenient parametric expressions for the configuration studied and using superposition, mathematical software can be used to perform the integrations for arbitrary Burgers vectors. Often, the resulting expressions for the tensorial fields are very long, but they can be easily incorporated as user-defined formulas for plotting, parametric analysis, and incorporation into routines for energy minimisation or the non-linear equations for force equilibrium. The effectiveness of this approach will be illustrated by the example of short straight dislocations, circular dislocations, the interaction between a pileup and dissociated dislocations in the grain boundary, and the nucleation of dislocations at grain boundaries.