Papers by Keyword: Initial Microstructure

Abstract: We investigate the effect of the cold reduction rate on ferrite recrystallization behavior during the annealing of low-carbon steel with different initial microstructures. Three types of hot-rolled sheet specimens are prepared: specimens P, B, and M, which consist of ferrite and pearlite, bainite, and martensite, respectively. To evaluate the effect of the cold reduction rate on ferrite recrystallization behavior, hot-rolled sheet specimens are cold-rolled at cold reduction rates of 40% and 67%. The cold-rolled sheet specimens are heated to the target temperature, and then water-quenched to room temperature. Irrespective of the initial microstructures, the ferrite recrystallization is accelerated by increasing the cold reduction rate. In addition, the dislocation densities of specimens P and B increase at the larger cold reduction rate, which accelerates ferrite recrystallization in these specimens. In the case of specimen M, the dislocation arrangement parameter remarkably decreases at the larger cold reduction rate, whereas the dislocation density hardly changes. Thus, we conclude that the accelerated ferrite recrystallization at the larger cold reduction rate for specimen M can be mainly attributed to an increase in the amount of interactions between dislocations in the specimen.

Abstract: The spheroidization mechanism from different initial microstructures during spheroidizing heat treatment was studied in Fe-0.68C-2.33Mn alloy. Two types of initial microstructures, i.e. pearlite and martensite, were obtained by varying the cooling rate. The microstructure and property evolution during spheroidizing annealing was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The DICTRA software, assuming local equilibrium conditions, was used to simulate the carburizing process of different initial microstructures through different cooling rate. The results indicate that the spheroidization mechanism of cementite was related to the initial microstructures and the smaller lamellar spacing of pearlite inhibited the coarsening of cementite, resulting in the size of cementite smaller than that of martensite as the initial structure.

Abstract: The hot working flow stress of as-cast and two different extruded magnesium alloys AZ31was examined by uniaxial compression tests. It was found that the hot deformation behavior was affected by the deformation conditions and initial microstructure. The peak flow stress was sensitive to deformation temperature and strain rate, and the value was decreased with decreasing the deformation rate or increasing the deformation temperature. The extruded samples, instead of as-cast samples, have better ductility at high strain rate and high temperature. The temperature increment for Mg alloy with different extrusion ratios was also investigated. These key features of the deformation behavior were explained in terms of twinning, dynamic recrystallization and grain rotation.

Abstract: A modified Voronoi model is established based on the Richards method to generate 2D non-equiaxed initial microstructure for Monte Carlo simulation. Microstructures produced by the ordinary Voronoi model are isotropic and cannot reflect the effects of the deformed grain shape on the annealing process. The modified Voronoi model based on ellipse set can be used to construct the deformed microstructure. The initial microstructure reflects the mean strain and the grain size distribution follows lognormal distribution.

Abstract: A cellular automaton (CA) model was established to predict and control the microstructural evolution and flow stress characteristic during dynamic recrystallization (DRX). Values of model parameters were identified by a flow stress-based inverse analysis method; their variations with deformation conditions were estimated by the least square regression method and then integrated into the CA model. The effect of initial microstructure and deformation conditions on the microstructural evolution and flow stress behavior were investigated. The simulation agrees well with the experiment, which demonstrates the availability of the CA model.

Abstract: The rheological behavior of two kinds Al-Zn-Mg-Cu aluminum alloy with different extrusion ratio was studied by thermal compression in 300°C~450°C and in strain rate range of 0.01 s-1~10s^-1 on Gleeble1500D simulator. The results show that: (1)the flow stress increases with de^{Superscript text}creasing deformation temperature and increasing strain rate, and the initial microstructure influence the rheological behavior, the flow stress with fine grain is higher than that with coarse one except in strain rate 0.01s-1 and 0.1s-1 in 450°C owing to deformation easily with more grain boundary sliding in high temperature and low strain rate.(2)The flow stress of Al-Zn-Mg-Cu aluminum alloy during hot compression can be expressed as hyperbolic sine constitutive equation with Arrhenius parameter, the apparent steady state activation energy for hot compression with coarse grain is of 181.51kJ/mol, while that with fine grain is of 203.02kJ/mol.(3) The apparent steady state activation energy decreases with increasing temperature and strain rate, and the value of fine initial microstructure is higher than the low extrusion ratio rod commonly.

Abstract: Three kinds of magnesium alloy (AZ31) with different initial microstructure were subjected to tensile deformation. The microstructure before and after tensile deformation were examined. It was found that the deformation mechanisms of the three kinds of magnesium alloy were considerably different. Mg alloy with the coarse grains (about 50mm) along with little dislocation inside deformed by twining and dislocation mechanisms, accompanied with an enormous change of crystal orientation as well as a great increase of hardness. Mg alloy with the fine grains/twins (about 3mm) together with high density of dislocations inside deformed by dislocation mechanisms, accompanied with little change of crystal orientation and hardness. The magnesium alloy with the initial microstructure of fine grains (about 3mm) together with little dislocations inside deformed by dislocation and grain boundary sliding mechanisms, accompanied with little change of crystal orientation, but a large increase of hardness.

Abstract: Nanocrystalline surface layer was fabricated on a quenched and tempered Cr-Si alloy steel by using Surface mechanical treatment. The microstructure features of various sections in the surface layer were characterized by using transmission electron microscopy (TEM). By analyzing the microstructural characteristic at different depths in the treated surface, the effect of the initial microstructures on grain refinement process of quenched and tempered steel was investigated. Experimental evidence showed the initial subgrains with small angle boundary and lower dislocation density were firstly developed into Lamellar-type dislocation cells (DCs) with dense dislocation walls (DDWs). Some initial subboundaries were moved to DDWs by dislocation activities. The width of lamellar-type DCs was 2-3 times of that of initial lathy subgrains. The size of the DCs and subgrains formed in the interim of refinement process was not uniformity. On the top surface the cementite granules were decomposed or fragmented to hyperfine particles, and the size of the grains tended to uniformity. Experimental analysis indicated the configuration of microstructure was affect by the initial microstructure in the initial stage and the interim of the grain refinement process. Surface nanocrystallization of Cr-Si steel can be attributed to dislocation activities.

Abstract: Five samples of Pb-Ca-Sn-Al alloy with different initial microstructures were processed in the same way including cold rolling with thickness reduction of 30% at the temperature of liquid nitrogen and then annealing at 270oC (0.9Tm) for 1 to 10 minutes. Electron back-scatter diffraction (EBSD) analyses indicated the initial microstructure which is at the end of primary recrystallization with a averaged grain size around 10 microns, a fair quantity of annealing twins and a random orientation can definitely result in a final grain boundary character distribution (GBCD) mainly composed of special boundaries (SBs) of which its fraction is higher than 75%, and the averaged SB cluster size exceeds 200 microns, implying the GBCD is optimized in satisfaction. However, the initial microstructures of partial recrystallization or full recrystallization but with coarser grain size are not good at producing high fraction SBs, whereas the initial microstructure of deformation with very strong textures is harmful to the formation of SBs in the processing of rolling followed by annealing. High resolution EBSD measurements revealed that the interactions between mobile incoherent ∑3 boundaries which were probably introduced by the continuous recrystallization from the distorted coherent ∑3 ones could be the formation mechanisms of the GBCD containing high fraction SBs.

Abstract: In order to investigate the effect of the as-cast microstructure on the hot working behavior of an AZ31 magnesium alloy, specimens were cast in copper moulds with and without water cooling. A series of compression tests were performed at a temperature of 350 °C, a strain rate of 0.01 s-1, and at strains up to 1.0. It was found that as-cast microstructure is very sensitive to the solidification conditions, which leads to a significant difference in flow behavior and dynamic recrystallization (DRX) characteristics. It appears that more uniform and refined as-cast grain size promotes dynamic recrystallization and reduces the flow stress. It is also possible that second phases (>1+m in size) contribute to DRX by acting as nuclei.