Papers by Keyword: Grain Boundary

Abstract: The plastic deformation behavior of polycrystalline copper is strongly governed by microstructural attributes such as grain boundaries and crystallographic orientation. In this study, the individual and coupled effects of grain size and orientation distribution on the mechanical response of copper are systematically investigated using the crystal plasticity finite element method (CPFEM). Uniaxial compression simulations are performed employing an Abaqus UMAT subroutine, and the resulting stress–strain response, slip system activity, and strain localization are examined. Polycrystalline representative volume elements (RVEs) with varying grain sizes and textures are generated to quantify their influence on flow behavior and dislocation density. The results indicate that grain refinement diminishes orientation sensitivity and the influence of individual grains, while enhancing material strength. Moreover, grain boundaries promote heterogeneous slip, localized deformation near grain edges, and increased stress–strain heterogeneity.

Abstract: The effect of grain size on the initiation behavior of exfoliation corrosion in cold-rolled Mg-14mass%Li-3mass%Al alloy was examined. Exfoliation corrosion initiated after 30 minutes in the coarse-grained structure (279μm), whereas it was delayed to 60 minutes in the fine-grained structure (75μm) and further to 75 minutes in the ultrafine-grained structure (39μm). This delay is attributed to the suppression of shear band formation and localization during cold rolling with decreasing grain size, which enhances the uniformity of surface reactions in the corrosive environment and promotes faster and denser formation of the protective film at the early stage. Accordingly, the improved condition of the initial corrosion film is considered the primary factor responsible for the delayed onset of exfoliation corrosion.

Abstract: The study aims to determine the load contributes to changes in the tensile strength of steel P22 at high temperatures. The steel sample was loaded under 95 and 125 N at a temperature of 700 °C for 72 hours. The results showed that the strength of P22 decreased with increasing load. At the temperature of 700 °C, the yield strength (YS) value decreased from 200 to 182 MPa and the ultimate tensile strength (UTS) reduced from 353 to 321 MPa as the load increased from 95 to 125 N. The precipitation of carbide in the matrix of P22 was observed in the steel sample loaded under 125 N at 700 °C for 72 hours. Furthermore, the cavity formation located on the boundary and near the carbide was confirmed when the temperature was 700 °C and the load increased from 95 to 125 N. The cavity was proof of a stress increase near the grain boundary, causing a decrease in the steel’s strength after a certain period of working time at high temperatures.

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.

Abstract: In Cr-rich CoCrFeMnNi alloys, the precipitation of the σ phase at grain boundaries during recrystallization is so fast that ultrafine-grained structure is formed due to the pinning effect of the precipitates. The average grain size of the fcc parent phase is found to be consistent with modified Zener-Smith model. If conventional alloys come to equilibrium, volume fraction of precipitates should approach a saturation value. However, it is interesting to note that the volume fraction of the σ phase in Cr-rich CoCrFeMnNi alloys is inversely proportional to the average grain size of the fcc parent phase. For instance, in Co₂₀Cr₂₅Fe₂₀Ni₁₅Mn₂₀ alloys, the volume fraction changes from 6.5% to 1.2% with increasing average gran size from 14 μm to 210 μm even after annealing at 1273 K for 100 h. It is well known that heterogeneous nucleation of precipitates at grain boundary is energetically favorable and fast diffusion through grain boundary can assist the precipitation. However, they cannot account for the grain size dependence of the volume fraction after reaching equilibrium. Based on stereology, the reciprocal of grain size is proportional to grain boundary area. Thus, chemical fluctuation at grain boundaries (e.g. segregation) is considered to be related to the unusual precipitation at the grain boundaries.

Abstract: Effect of grain boundary morphology on ductility dip cracking (DDC) sensitivity of nickel base alloy inconel52 deposited metal was researched by welding thermal simulation method and high temperature tensile test. The sample was hold at 1300 °C for 2S ~ 10s and then stretched at its DDC sensitive temperature 1050 °C at different tensile rates. The DDC sensitivity was compared by reduction of area (VoA) of tensile test sample. The results show that straight grain boundary reduces VoA, precipitates in grain boundaries increases VoA, and VoA increases with the increase of tensile rate. Straight grain boundary causes stress concentration and strain localization at the trigeminal grain boundary, curved grain boundary decreases the maximum Mises stress which make more uniform stress distribution. Precipitates on the grain boundary can play a role of locking the grain boundary migration and disperse the strain concentration at the trigeminal grain boundary. The lower the strain rate, the longer the deformation time, which will lead to decrease of dislocation movement rate. The smaller the critical shear stress of grain boundary sliding, the smaller the deformation resistance, and the full progress of dislocation movement and climbing. Effect of strain rate on DDC needs more research.

Abstract: In comparison to other Periodic Table elements, rare earth elements demonstrate long-term stability and strong conductivity. Ceria nanomaterial has found many applications in numerous technologies. Doped ceria was prepared by many wet chemical methods. In this paper, we examine the electrical properties of the ceria after adding three dopants, two of which are rare earth elements (Gd and Nd) and one metal (Ca). The compositions, Ce_0.75Gd_0.05Nd_0.2O₂ and Ce_0.75Gd_0.05Nd_0.14Ca_0.06O₂, were formed using the WOWS (without water and surfactant) Sol-Gel method. The X-ray diffraction (XRD) technique was used to investigate the crystallinity of nanostructures. The structure of both samples was cubic. For the electrical measurements, the Precision Analyzer was used for doped Ceria as a function of temperature. With the variation in composition, the electrical properties changes.

Abstract: Aluminum alloys are widely used in automotive industry due to their low density and good corrosion resistance. This category includes alloys based on AlSiMg which are suitable for load bearing parts operating under higher temperatures. This paper deals with analysis of influence of deformation parameters and heat treatment on structure and mechanical properties of EN AW-6082 (AlSi1MgMn) alloy manufactured by horizontal cast module technology. Casted rods were used as a billet, which was formed to defined height by hot open-die forging. Subsequently the precipitation hardening was used as heat treatment. Changes in microstructure were evaluated based on the metallographic analyzes performed by light optical microscopy and scanning electron microscopy using an energy dispersive X-ray spectrometry and electron backscatter diffraction. Mechanical properties were determined by uniaxial tensile test and hardness testing. The results showed, that due to the process parameters, no significant structural changes were observed in the surface layer of forging. However, microstructure is significantly inhomogeneous in the core due to the dynamic softening processes. Mechanical properties are increasing which is significantly influenced by the type and distribution of precipitates emerging during the artificial aging.

Abstract: This paper presents an experimental investigation with the objective to determine the root causes for the cracking of a large size bar made of a medium carbon low alloy steel after open die forging and heat treatments operations. The cracks were observed below the surface during the machining step. In order to understand the mechanisms of crack initiation and propagation, micro-CT tomography and scanning electron microscope (SEM) were employed. Microstructural damage analysis revealed oxidation of different alloying elements, more specifically manganese, chromium and silicon. The presence of defects in the form of cavities and porosities were also observed at the grain boundaries. Some of the above defects were observed along the crack path, while others were on both sides of the cracks without any connection to them and finally, a third group completely isolated from any crack. The characteristics of the defects were thoroughly analyzed and it was found that the crack initiation could be attributed principally to the porosities/cavities formed during solidification. The analysis also showed that crack propagation occurred during solidification and/or forging and heat treatment steps.

Abstract: The 7000 series aluminum alloys suffer from intergranular fracture (IGF) that limits the use of the alloys, although they have highest strength among aluminum alloys. The types of IGF can be classified into two categories: (i) with smooth fracture surface showing practically no plastic deformation that takes place in hydrogen embrittlement and stress corrosion cracking, and (ii) with shallow and fine dimples on the fracture surface showing localized plastic deformation inside precipitate free zones. In this study, attempts have been made to suppress the IGF of both types by (a) controlling precipitate microstructure on grain boundaries by quench control and (b) controlling grain boundary morphology by strain induced boundary migration. The IGF of type (i) (hydrogen embrittlement) was successfully suppressed both by the two controlling processes.