Papers by Keyword: Directionally Solidification

Abstract: This paper presents the results of analysis of superalloys microstructures with different structures: polycrystalline nickel-based superalloy René 80, Mar-M 200Hf directionally solidified with columnar grains and single crystal CMSX-4. Microstructure studies were performed using a scanning electron microscope Hitachi FE-SEM SU-70 and S-3400N equipped with a Thermo Scientific Noran System for analysis of chemical composition by X-ray dispersion. Metallographic microsections of the rods before and after heat treatment was performed, which were electrolytically etched using different reagents. The morphology of γ phase precipitates in the interdendritic areas and dendritic cores was analyzed. Single crystal rods of nickel superalloy CMSX-4 were cast by Bridgman technique in a vacuum furnace of ALD Vacuum Technologies. Rods were withdrawn with rate: 1mm/min and 5mm/min. EDS X-ray microanalysis showed significant differences in chemical composition between the cores dendrites and eutectic regions. Significant differences in the number of dendrites, the shape and length of the arms were observed. A lower speed rate causes that amount of eutectic is lower but value of primary dendrite arm spacing is higher, while a higher withdraw rate increases the amount of eutectic and decreases primary dendrite arm spacing.

Abstract: Ni3Al shows the unique feature of increasing strength with increasing temperature. However, it is too brittle to use as a structural material due to grain boundary weakness. Ductility could be enhanced by controlling grains using directional solidification. In order to increase the ductility or strength of Ni3Al alloys, a ductile γ (Ni-rich) phase of dendrite fibers or a strong β (NiAl) phase of dendrite fibers were arrayed in the γ´ (Ni3Al) matrix by directional solidification. The dendrite spacing could be controlled by varying the solidification rates, and the volume fraction of the γ or β phase could be changed by using alloy compositions, from 23 to 27 at. % Al-Ni alloy. With increasing solidification rates, the dendrite spacing decreased, which caused the tensile strength to be enhanced and the elongation to decrease, evidently due to the phase boundary augmentation. With increasing Al content, the γ dendritic microstructure changed to β dendrites in the γ´ matrix, which resulted in a decrease in elongation as a result of an increase in the volume fraction of the brittle β dendrites in the γ´ matrix.

Abstract: NbSi2 is an attractive material for high temperature applications due to its high melting point, low density and good oxidation resistance. The high-temperature strength of NbSi2 is expected to be further improved by incorporation with Nb5Si3, which performs a high creep resistance and strength at high temperature due to its complex crystal structure. In this paper, directionally solidified NbSi2/ Nb5Si3 in-situ composites have been prepared using an optical floating zone method. Scanning Electron Microscopes (SEM) and X-ray diffraction (XRD) have been used to investigate the phase constitution and microstructure. The orientation relationship between Nb5Si3 and NbSi2 is investigated by transmission electron microscopy (TEM). High-temperature properties of alloys are tested by compression at the strain rate of 1×10-4/s at 1673K and 1773K. It was found that high temperature strength and phase constitution of directionally solidified alloys depended on the addition of Mo.