Papers by Keyword: Microstructural Evolution

Abstract: TiC base high manganese steel-bonded carbide was manufactured by powder metallurgy technique, and the effect of adding mode of WC additive on microstructural evolution and properties of the alloy was studied. SEM microstructure showed that surrounding structure appeared obviousely with WC additive; while microstructure of the alloy was well-distributed and particle shape was distinct very much. The shape of TiC particles became irregular with pure WC powder addition, and irregular with further increase of WC content, and the shape of some TiC particles became angular because of excessive reaction between TiC particles and WC additive. microstructure of the alloy was refiner and the strength, toughness were improved with the increase of WC addition. (Ti, W)C solid solution is superior to form on the surface of TiC particles through dissolution precipitation mechanism because the activity energy of WC was higher than that of TiC-WC compound carbides when WC was added in the alloy, and the particle shape of TiC became more irregular and the properties were improved significantly.

Abstract: The hot deformation behavior of aluminum alloy 5E61 was studied by hot compressive tests using a Gleeble-1500 thermal simulator. The tests were performed at temperatures varying from 250°C to 500°C and strain rates ranging from 0.001 s^-1 to 10s^-1. The results achieved in the present study showed that the steady flow stress increases with decreasing temperature and increasing strain rate, in accordance with the Zener-Hollomon parameter. The related microstructure is sensitive to deformation temperature, strain rate and strain. The constitutive equation based on true stress-true strain curves has been developed by hyperbolic sine equation with the hot deformation activation energy of 153.907KJ/mol. The softening mechanism could be ascribed dominantly to dynamic recovery. The results of TEM observation suggested that Mn-containing particles and Al₃(Er,Zr) phase have precipitated in the aluminum matrix and pin the dislocations, which could effectively inhibit the dynamic recrystallization.

Abstract: Different cold rolling and recrystallization annealing of commercially pure titanium (CP-Ti) sheet was conducted. Microstructural evolution in commercially pure titanium sheet after cold forming and recrystallization annealing were investigated. Cold forming properties using a microstructural analysis, Vickers microhardness and erichsen value measurements were performed. Alpha grain sizes of processing 1 and 2 are 38.67μm and 40.12μm, respectively. Erichsen values of the specimens of processing 2 reaches the high value. This is attributed to recrystallization annealing. Equiaxed alpha grains are formed from the elongated grains and cold forming properties of CP-Ti were improved. The effect of the first recrystallization annealing was remained. Forming properties were a moderate value for processing 1 due to the presence of equiaxed grains.

Abstract: Duplex Stainless Steel (DSS) combines good mechanical strength and ductility with moderate to good corrosion resistance in a variety of environments. DSS are prone to the formation of various intermetallic phases (σ, χ, π, R), carbides (M23C6) and nitrides (Cr2N), when it exposed to temperatures lower than 1000°C. This study focuses on effect of secondary phase precipitation on impact toughness of ASTM A182 DSS. Cylindrical bar of DSS was open die forged in the temperature range of 1200-1050°C. After the forging, the bar was slow cooled by covering it with ceramic wool. This leads to the formation of σ phase at the ferrite/austenite interfaces and significantly reduced the impact toughness of the material. Solution treatment was done at different temperature and effect on toughness studied. The microstructural changes produced as consequences of the distinct test condition have been analyzed by means of optical and electron microscopy, X-ray diffraction.

Abstract: Effects of welding and long-term service on the microstructural evolution of superheater tubes of TP347H stainless steel used in power plants were investigated by optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscope (TEM). Analyses after welding or long-term service, showed fine NbCs in grains, which will precipitaion strengthen the matrix. When TP347 was long-term serviced in power plants, M₂₃C₆ formed preferentially on the grain boundaries and on twin boundaries, which was attributed to the embrittlement and the intergranular corrosion and fracture. The steam side had less recrystallization rate and more oxide compared to the fire side, which is part of the reason for the cracking from steam side to the fire side. And HAZ is more brittle than the matrix, because of α-Fe phase and coarse grains, due to which, cracks tend to initiate in the steam side of HAZ and propagate to the fire side.

Abstract: A semi-empirical approach to the modeling of the microstructural evolution during the hot rolling of austenite including grain growth, hardening and softening has been discussed in the frame of a generalized energetic expression for related structural processes. The current concept suggests the activation energy of iron self-diffusion in austenite and its dependence upon the chemical composition of the steel for prediction of the particular phenomena. Additionally, the precipitation sequences, the size distribution, Oswald ripening and interaction with softening are also included in the model of the microstructural evolution. The simulation results are reliable to the structural evolution of the low carbon steels microalloyed with Nb and Ti during hot rolling.

Abstract: This work deals with the microstructural evolution of creep resistant martensitic/ferritic 11% Cr-steel during thermomechanical treatment from an experimental as well as modeling point of view. The creep resistance of this material group is highly dependent on the precipitate status. The initial precipitate status is controlled by the chemical composition of the alloy and the heat treatment after casting or hot rolling. It is therefore of utmost interest to understand and model the precipitate kinetics during this process. Once the microstructural evolution has been modeled successfully, only minimum effort is required to computationally test variants in the composition or heat treatment in order to optimize the process. In this work, the material was hot rolled, austenitized and subsequently annealed. All heat treatments have been performed during dilatometry tests. In order to investigate the microstructural evolution during the process, specimens were extracted at definite stages of the treatment. The specimens were then investigated applying various microscopical techniques in order to quantify the microstructural features (grain size, martensite lath width and precipitate data). The experimental data were then compared to thermodynamic simulations (MatCalc). General data such as nucleation sites for precipitates were taken from literature, grain size and martensite lath widths from the experimental data. Simulations include equilibrium calculations and precipitate kinetic simulations. In general, the simulations showed good agreement with the experimental findings, with minor room for improvements. The work thus lays a solid ground for future improvements of the heat treatment process.

Abstract: Effect of solution treatment on the properties and microstructural evolution of Al-Zn-Mg-Cu-Er-Zr alloy was investigated by hardness test, conductivity test, exfoliation corrosion test, scanning electron microscope (SEM), transmission electron microscopy (TEM). The results show that the hardness of single-stage solution treated samples first increase and then decrease, the conductivity and volume fraction of the residual phase particles decrease with increasing solution temperature. Compared to conventional multi-stage solution, slow heating process solution treatment can dissolve plenty of residual phases and further improve the hardness and decrease conductivity. Besides, the effect of solution treatment on EXCO is discussed, the result of exfoliation corrosion of SHT8 sample test is judged as EA. Moreover, the effect of the presence of nanometer-sized Al₃(Er,Zr) particles on the microstructural evolution during solution treatment has also been discussed in detailed.

Abstract: The aim of this study is to reveal the influence of ultrasonic treatment in the molten state on microstructural evolution of the magnesium alloy AZ31 with and without the addition of SiC particles. Therefore, a heatable wedge-shaped mold for holding the magnesium melt on the required temperature was constructed with the possibility to insert an ultrasonic horn into the melt. Previously ingots of AZ31 were molten in an electrical furnace under a protective gas atmosphere. SiC particles with an average diameter of 2 μm were added to the melt. Mechanical stirring was conducted to ensure homogeneous distribution of the particles. The molten Mg was subjected to ultrasonic sound with constant frequency of 20,000 Hz and amplitude of 12.4 μm. The time of ultrasonic treatment was defined on 60 seconds. The material solidified quickly due to the cooling with water nozzles after withdrawing of the ultrasonic horn. AZ31 without SiC particles was treated under the same conditions. The resulting microstructure was observed by optical and scanning electron microscopy. Depending on resulting intensity and the effective area a refinement in grain size and also more homogeneous distribution of precipitations in the material is achieved. The resulting microstructures of AZ31 with and without SiC addition were compared and discussed.

Abstract: The present study details the results of finite element analysis (FEA) based predictions for microstructure evolution in ATI 718Plus^® alloy during the hot deformation process. A detailed description of models for static grain growth and recrystallisation is provided. The simulated average grain size is compared with those experimentally measured in aerofoil parts after forging trials. The proposed modified JMAK model has proved to be valid in the main body of the forging. The results predicted for the surface are less accurate. The recrystallised grain size on the surface is smaller than in the centre of the part which corresponds to the experimental results and reflects the main trend.