Materials Science Forum Vols. 715-716

Abstract: t was shown that ECAP of an Al6%Mg0.3%Sc through route C at a temperature of 325°C (0.6T_m) to a total strain of 8 leads to the formation of partially recrystallized structure; the average size and the volume fraction of new ultrafine grains (UFG) were 1.2μm and 0.7, respectively. Areas of UFG structure alternate with coarse remnant parts of deformed original grains. Effect of this bimodal structure on room-temperature mechanical properties including fatigue behavior is examined. The ECAP processing led to increase in the yield strength and endurance limit accompanied with an insignificant loss of plasticity. Analysis of the surface features indicated that the fracture upon cyclic loading occurred by the transgranular mode in the regions of the unrecrystallized grains; this followed from the presence of striations in the large flat regions of the samples. In contrast, the fracture within regions of UFG grains occurred by the intergranular mode; distinct boundaries were clearly observed between the adjacent facets. It is concluded that the ultrafine grains presenting in the bimodal structure increase the strength characteristics of the alloy whereas the large remnant grains provide its relatively high plasticity. Therefore, the bimodal structure produced by the ECAP may ensure a certain balance between the rate of nucleation and propagation of a fatigue crack. The influence of this microstructure on the fatigue behavior of the material is discussed in detail.

Abstract: Semi-processed nonoriented electrical steels are very attractive products whose magnetic properties are significantly improved through annealing treatment in customers plant. The improvement is due to strong Goss texture formation by strain induced boundary migration (SIBM). In order to the effect of temper rolling reduction on the strengthening of Goss texture, temper rolling reduction was changed in the range of 2% to 8%. The annealing times was changed from 10 minutes to 180 minutes. A mechanism of grain growth during SIBM is suggested from our experimental data. In the specimen temper-rolled by 2%, relatively strong {111}<112> texture develops, whereas in the specimens temper-rolled by 4% through 8%, strong Goss texture develops as a result of SIBM during final annealing. It can be found from observed EBSD data that the Goss grains have the lowest stored energy in all temper-rolled specimens, which is confirmed by average image quality value in EBSD measurements. However, for the Goss grains to grow preferentially, stored energy difference between Goss grains and their neighboring grains may have to be higher than a certain critical value.

Abstract: The current study introduces a thermodynamically correct approach which allows a direct precise measurement of the grain boundary triple line tension. The experimental technique utilizes the measurement of the surface topography of a crystal in the vicinity of a triple junction and grain boundary groove on thin wires by atomic force microscopy. The grain boundary triple line tension of a random triple line was measured to be in the order of 10^-9 J/m.

Abstract: A cellular automaton and a vertex model were used, respectively, for the simulation of recrystallization and grain growth in a Fe-0.374%C-21.64%Mn alloy. The results of the recrystallization simulations revealed that the preferential nucleation during the annealing of the rolled sheet occurs at shear bands, which is corroborated by experimental observations. Subsequently, grain growth simulations were carried out with a 2D vertex model. The model used experimental data as input for its validation in this specific steel. The simulations showed a good agreement with the experimental results.

Abstract: The impact of a magnetic field on texture and microstructure evolution in Zn-1.1%Al was investigated. Specifically oriented cold rolled (85%) sheet specimens were annealed in a magnetic field of 17 Tesla at 340°C for 15, 30 and 90 minutes. X-ray diffraction and EBSD-measurements were utilized to characterize crystallographic texture and grain microstructure. The results confirmed that grain growth in magnetically anisotropic zinc can be substantially affected by a magnetic field. This manifested itself by significant changes in the development of the grain growth texture during magnetic annealing compared to annealing at zero field. The magnetically induced texture changes are caused by the generation of an additional magnetic driving force which arises from a difference in magnetic free energy density between differently oriented grains. The grain microstructure evolution was also essentially affected by a magnetic field such that grains with energetically favoured orientations grow faster and their fraction becomes larger, than that of grains with disfavoured orientations. The results will be also discussed with respect to the magnetically altered grain boundary character distribution.

Abstract: Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V titanium alloy with initial α-colony microstructure during uniaxial compression at 600 and 800°C to a height strain of 70% were studied. It was shown that decrease in deformation temperature considerably influences on the kinetics of globularization of a lamellar microstructure. At the lower temperature stages of strengthening and softening extend that associates with inhibition of globularization. Deformation at 600°C is also associated with a smaller fraction of high-angle boundaries during deformation, smaller fraction of globular grains, increased contribution of shear deformation and more intensive rotation of α-lamellae towards the metal flow direction. In contrast to 800°C, the rate of thinning of α-lamellae at the lower temperature is noticeably higher. The results obtained are related to the change of the type of dislocation slip in α-lamellae due to inhibition of dynamic recovery with decreasing deformation temperature.

Abstract: The microstructural evolution of a quenched medium-C steel during tempering was analyzed by means of Orientation Imaging Microscopy (OIM). The steel was heat treated in order to develop fully martensitic microstructures after quenching with a different prior austenite grain size (AGS). Main results can be summarized as below: A very poor effect of AGS on packet size was found in comparison to bainitic steels. A finer packet was measured at mid-thickness with respect to surface after the quenching process. This phenomenon was attributed to the effect of thermal strain path on phase transformation during quenching. The through-thickness microstructural gradient remains after tempering. High-angle boundary grains do not significantly grow after tempering; on the contrary, low-angle grain boundaries (cells) move, fully justifying the hardness evolution with tempering temperature.

Abstract: The kinetics of static recrystallization in cold rolled ferritic stainless steel sheet tends to slow drastically over the last 10-20% of recrystallization. This has its origins in both the microstructure (deformed grain shape, precipitates) and in the local deformation texture. In this work we have sought to provide a physical explanation for the slow last stages of recrystallization through a texture dependent JMAK model which is informed by the microstructure of the partially recrystallized microstructure. The geometrical assumptions made in developing this JMAK model have been compared to phase field simulations using experimental observations as the source of their starting configuration.

Abstract: Grain size is an important microstructural feature of materials; it contributes to its mechanical and physical properties. Quantitative information of three-dimensional topological evolution during the three-dimensional normal grain growth process is very rare. In this work, an isothermal Monte Carlo procedure is applied to study the kinetics of normal grain growth in three dimensions using the Monte Carlo technique. Monte Carlo simulation of the grain growth in polycrystalline materials employed maps the microstructure onto a discrete lattice. It consists in evaluation of potential energy of a lattice point. Therefore, the choice of the simulation parameters can affect the grain growth characteristics. We attempted to define specific simulation parameters for the three dimensional MC simulation of grain growth using theses lattices. The parameters include the choice of the lattice symmetry, the pixels number and the number of the nearest neighbor lattice sites in the calculation of the system energy. The goal of these computer simulations is to provide insights on the simulated structure and to analyze the grain growth kinetics for the 3D domain through several 2D cross-sections. Compact microstructures were developed and the first results show that the lattice symmetry and the neighbours number have a fundamental influence on the results of grain growth simulation. The saturation is too rapid with only the consideration of the 1st neighbours while acceptable similar evolutions are obtained for the 2nd or 3rd neighbours consideration. Grain size evolution obeys the power law kinetics for the 2D cross-section confirming the quasi-steady state of the 3D normal grain growth. The highest grain growth velocity and grain size were obtained at the cross-sections of the heart of microstructures. One other noted information is the existence of a minority of grains which continue to grow slowly leading up to the abnormal grain growth.

Abstract: A 3D grain growth model is developed based on a generalised mean-field approach. It allows the prediction of the growth history of individual grains of a polycrystal during normal grain growth, which has been compared with results from Monte Carlo Potts simulations showing a good agreement. Based on a stochastic grain growth model the diffusivity of the Brownian-like motion of individual quadruple points and triple junctions in 2D sections can be related to the average growth rate providing a possibility for the determination of the average growth law of the grain ensemble solely from the stochastic growth behaviour of single grains.