Papers by Keyword: Zener Pinning

Abstract: A phase field model was presented to investigate the effect of particles-pinning on grain boundary migration in materials containing stored energy differences across the grain boundaries. The accuracy of the phase field framework was examined by comparing the simulated results with theoretical predictions. The pinning effects of coherent and non-coherent second phase particles on the boundary migration were studied in triple-grain models. 2D simulations with second phase particles of different sizes or different area fractions were performed. The effect of stored energy difference across the boundary on the particles-pinning was also investigated. The results showed that the pinning effect could be enhanced by the decrement of the particle size and the increment of particle area fraction. Increasing the stored energy difference across the grain boundary induced higher grain boundary migration velocity and weaker particles-pinning.

Abstract: The objective of this study is to investigate the microstructure and effect of annealing to the hardness properties of oxide dispersion strengthened (ODS) Fe-15Cr-0.3Y₂O₃ alloy. This type of alloy was prepared by mechanical alloying (MA) method followed by compacting and sintering. The microstructure of milled Fe-15Cr-0.3Y₂O₃ alloy powders and pellet was examined by using field emission scanning electron microscope (FESEM). The milled alloy powders consist of nearly spherical shape of powder particles with average size of 10 µm. For the alloy pellet microstructure, the formations of Y₂O₃ nanoparticles with average size of 5 nm were observed indicating the dispersion and incorporation of this nano-scale dispersoids into the alloy matrix. Fe-15Cr-0.3Y₂O₃ alloy pellet was annealed at temperature of 600°C, 800°C and 1000°C, respectively for the Vickers hardness test. The Vickers hardness test was performed by using a micro-Vickers hardness tester with a load of 200 gf. The hardness value (HV) of this alloy pellet started to decrease at temperature of 600°C indicating the grain growth of this material at high temperature

Abstract: The phase field method was applied to study the effect of second-phase particles (SPP) with different geometric orientations and shapes on grain growth. The results show that, in the grain growth process, most of the spherical second-phase particles located at triple junctions, while the stick SPPs located at the grain boundaries along the grain boundary. The second-phase particles are of the strong pinning effect on grain boundary and the limiting grain radius can be expressed by Zener relations. In the condition of the second-phase particles area fraction and size remaining the same, the stick SPPs are of more effective pinning on grain growth than that for spherical SPPs, and the orientation of disk second-phase particles is also an influence factor for pinning effect. Stick second-phase particles with multiple orientations can make a better pining effect than those with only one orientation.

Abstract: Under certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate.

Abstract: Based on cellular automata, a model of simulating grain growth was established and the key technologies of simulation was studied which including second phase particle of single size, multi-size distribution and different shapes generation technologies. The simulation result can accurately reflect the influence law of the second phase particle grain growth and its pinning mechanism. Grain boundaries can therefore more easily break free from the particles than in purely two-dimensional systems, resulting in fewer grain boundaryparticle intersections and a larger final grain size. For a given volume fraction f and size of the particles r, the final grain size increases with film thickness. Moreover, it was found that particles located in the middle of the film are most efficient in pinning grain boundaries. The simulation results are compared with Zener type relations and previous simulation results.

Abstract: Based on cellular automata, a model of simulating grain growth is established to study the effects of the second phase particle’s size distribution on grain growth. The simulation results show that the second phase particles in the matix pin the grain boundary and then inhibit the grain growth. Different size distributions of the second phase particles have different pinning effect on the grain boundary, and the relationship of average grain size for the material with the second phase particles is R_Lognormal＞R_Uniform＞R_Normal. The correlative laws obtained from the simulation is in accordance with the theoretical models.

Abstract: Based on cellular automata, a model of simulating grain growth is established to study the effects of the second phase particle’s size and volume fraction on grain growth. The simulation results show that the smaller is the volume fraction of second phase particle, the finer is the grain of pinned matrix, and the pinning force of bigger second phase particle is stronger than that of smaller one. The correlative laws obtained from the simulation is in accordance with the theoretical models.

Abstract: A simple analytical model is proposed for estimating grain boundary mobility during dynamic recrystallization in metallic alloys. The combined effects of solutes (solute drag) and second phase particles (Zener pinning) on mobility are considered. The approach is based on (and is consistent with) a recently published mesoscale model of discontinuous dynamic recrystallization. The dependence of grain boundary mobility on solute concentration and particle size is summarized in the form of two-dimensional maps.

Abstract: Microstructure formation of CP-Ti and TiB reinforced titanium were in-situ observed during the thermal cycle simulated for Tungsten Inert Gas (TIG) welding, by using laser scanning confocal microscopy. Under the in-situ observation of TiB reinforced titanium, heterogeneous nucleation of α-phase at inclusion was clearly detected and plate growth was shown in high timeresolution. Furthermore, it was observed that grain boundary of β -phase was pinned by the inclusions. Microstructure difference between pure and TiB reinforced titanium was explained based on those in-situ observations.

Abstract: The effects of second-phase particles on the recrystallization kinetics in two-dimensional polycrystalline structures were investigated. Numerical simulations of recrystallization were performed by coupling the unified subgrain growth theory with a phase-field methodology. Simple assumptions based on experimental observations were utilized for preparing initial microstructures. The following results were obtained: (1) The presence of second-phase particles retarded recrystallization speeds. (2) If the mean subgrain size was small enough recrystallized region covered whole system for various values of the particle fraction, f. (3) On the other hand, if the mean subgrain size was not small enough the progress of recrystallization was frozen at some point.