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The effect of grain boundary (GB) misorientation on hot tearing susceptibility of directionally solidified (DS) nickel-based superalloys was explored. We found that the castability of second generation nickel-based superalloy CMSX-4 is inferior to DS superalloy IN792, an alloy well known for bad castability. The castability of CMSX-4 is somewhat improved at a higher solidification rate. The hot tearing tendency increases with increasing GB misorientation angle. As feeding tendency becomes greater with increasing misorientation, this points to the importance of GB cohesion for solidification cracking in the alloy. Microstructure investigation reveals that hot tearing is associated with formation of continuous gamma and gamma prime eutectic films at the GB in CMSX-4. We assume that the gamma and gamma prime eutectic, which reflects the remaining liquid at the end of solidification, prevents the impinging dendrite arms from touching and in this way decreases cohesion.
High-current-density electropulsing was applied to a coarse-grained Cu–Zn alloy with two phases of α-phase and β′-phase. It was found that with an electropulsing treatment, ultrafine-grained (UFG) microstructure could be formed in the α-phase, but could not be formed in the β-phase. The results indicated that the formation of UFG microstructure was dependent on solid-state phase transformation. The main reason for the formation of UFG microstructure by electropulsing treatment resulted from the effect of a decrease in thermodynamic barrier and enhancement of nucleation rate in a current-carrying system, but not from the high heating and cooling rate during electropulsing treatment. The bulk UFG samples prepared by electropulsing treatment were free of porosity and contamination and had no large microstrain. It was reasonable to anticipate that a new method might be developed to produce ideal bulk UFG samples directly from the conventional coarse-grained materials by application of electropulsing.
Phase transformation about precipitation in a Cu–Zn alloy was studied. It was found that with an electropulsing treatment the number of nuclei during phase transformation could be dramatically enhanced and nucleation of precipitates was more homogeneous. The phenomena did not result from the effect of rapid heating or rapid cooling during electropulsing but resulted from the electric current itself. The results were in good agreement with the theoretical model that electric current can increase nucleation by decreasing the thermodynamic barrier during phase transformation.
High current electropulsing was applied to a low-carbon steel in the solid state. The relationship between grain size and experimental conditions was revealed. It was found that the ultrafine-grained (UFG) microstructure could be formed when electric current density, heating rate, and cooling rate all were high. The UFG samples prepared by applying electropulsing were free of porosity and contamination, and had no large microstrain. Also, their tensile strength was dramatically enhanced over that of their coarse-grained counterparts, without a decrease in ductility. The mechanism for grain refinement and formation of the UFG microstructure was discussed. It is proposed that the effect of a decrease in thermodynamic barrier and enhancement of nucleation rate in a current-carrying system cannot be neglected.
The microstructure of a low-carbon steel after high current density electropulsing treatment was characterized by high-resolution transmission electron microscopy. It was found that nanostructured γ-Fe could be formed in the coarse-grained steel after the electropulsing treatment. The mechanism of the formation of a nanostructure was discussed. It was thought that change of the thermodynamic barrier during phase transformation under electropulsing was a factor that cannot be neglected. It was reasonable to anticipate that a new method might be developed to produce nanostructured materials directly from the conventional coarse-grained crystalline materials by applying high current density electropulsing.
Specimens of 1045 steel with quenched crack were treated under electropulsing. Scanning electron microscopy was employed to examine the change of specimens before and after the treatment. The results showed that the crack can be healed under electropulsing without melting, and the healing could be produced within a very short time. The original microstructure of specimens was not changed during the healing. It is thought that the temperature and the transient thermal compressive stress caused by high-rate heating are the main factors causing the healing of the crack.
The influence of electropulsing on the damage of 1045 steel was studied. The results show that electropulsing can substantially increase the strength and ductility of the damaged material, with a little decrease of the Martensite hardness. After the treatment of electropulsing, the microstructure around the crack is modified, and the new structure can prevent the crack from growing. The healing effect of electropulsing on crack is discussed. It is thought that the temperature and thermal compressive stress caused by electropulsing are the main factors causing the healing effect.
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