Papers by Keyword: Eutectic In Situ Composite

Abstract: Ni₃Si compound is one of the excellent high temperature structural materials, because it possesses the attributes of high melting point, high strength, low density, excellent oxidation resistance at elevated temperatures, and magnificent corrosion resistance in acid environments, particularly sulfuric acid solutions, while the application of this compound is limited due to poor ductility at ambient temperatures and lack of fabricability at high temperatures. The incorporation of a ductile phase into the intermetallic materials has become an attractive means to modify the ductility. In this paper Ni-Ni₃Si hypoeutectic in situ composites are obtained by Bridgman directional solidification technology. Microstructure of the Ni- Ni₃Si hypoeutectic in situ composites are regular lamellar eutectic structure at the lower solidification rates, whereas eutectic cells or dendrites can be found with the increase of the solidification rate, due to the increase of the composition undercooling. Moreover, the directional solidification mechanism was investigated as well.

Abstract: In situ composite of Al2O3/GdAlO3(GAP) ceramic eutectic prepared by directional solidification is an interesting candidate for the manufacture of turbine blades because of its excellent mechanical properties. In the present paper, directionally solidified Al2O3/GAP eutectic in situ composite ceramics are manufactured by the laser zone remelting technique to investigate the rapid solidification process. The laser power and scanning rate necessary to carry out the ceramic melt growth is determined. The characteristic microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The as-solidified Al2O3/GAP eutectic presents an elongated colony structure consisting of only -Al2O3 and GAP phases with an oriented growth array. The eutectic spacing is strongly dependent on the laser scanning rate, rapidly decreasing to the sub-micron range for the samples grown at the highest rate. Besides, the formation condition and evolution of the particular microstructure of the composite during rapid solidification are discussed.

Abstract: Directionally solidified (DS) oxide eutectic in situ composites are attracting increasing attention because of their unique properties and potential applications to high temperature structural materials, optical or electronic devices. Among the alumina-based eutectic composites, DS Al2O3/Er3Al5O12(EAG) eutectic is considered to be promising candidate for use as selective emitter at high temperature. In this work, eutectic in situ composites of Al2O3/EAG rods having smooth surface and full density are successfully prepared by directional solidification using the laser zone remelting method, aiming to investigate the growth characteristic of this novel binary eutectic under high temperature gradient. The microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The Al2O3/EAG eutectic presents a very fine irregular network structure consisting of only -Al2O3 and Er3Al5O12 phases without grain boundaries and amorphous phases between interfaces. The eutectic interphase spacing is strongly dependent on the laser scanning rate, rapidly decreasing at the sub-micron levels for the samples grown at high rate. Furthermore, the microstructural formation and evolution of the composite are analyzed and discussed.

Abstract: Directionally solidified oxide ceramic eutectic composites with superior strength, oxidation resistance, creep resistance, structural stability and low sensitivity to crack at high temperature have aroused much attention in recent years, and various preparation techniques have been developed. In situ fabrication of ceramic eutectic composites by laser rapid solidification is a cheap and quick method compared to conventional multi-step fabrication methods of fiber reinforced composites for high temperature use. In this paper, Al2O3/YAG/ZrO2 ternary eutectics are rapidly prepared from melt by directional solidification using laser zone remelting technique, the growth characteristic and fracture toughness are investigated. The results show that: (1) Laser rapidly solidified Al2O3/YAG/ZrO2 ceramic eutectic in situ composite presents a fine interpenetrating network structure, in which Al2O3, YAG and ZrO2 phases are continually interconnected and finely coupled without pores, colonies and grain boundaries between interfaces. (2) Laser scanning rate and power density strongly affect the eutectic growth. With the processing parameters adjusted properly, the eutectic shows homogeneous and coupled lamellar microstructure. The characteristic dimensions of the microstructure are around 2~3 1m for Al2O3 and YAG phases, and around 0.2~1 1m for ZrO2 phases, respectively. (3) The hardness and fracture toughness of the rapidly solidified Al2O3/YAG/ZrO2 eutectic are 16.7 GPa and 8.0 MPa.m1/2, respectively.

Abstract: Due to excellent oxidation and corrosion resistance at elevated temperature, oxide ceramic could be the preference served at high temperature oxidizing atmosphere over a long period of time. In recent years, alumina-based eutectic in situ composite prepared by various solidification techniques, which has superior properties even close to the melting point about 2100K, has been paid much attention. In this paper, Al2O3/YAG/ZrO2 ternary eutectic and hypoeutectic ceramics are prepared from melt by laser zone-remelting technique, the rapid solidification characteristic and the mechanical property of the composites are investigated. The results show that: (1) Compared to sintered composite with the same composition, laser zone-remelted Al2O3/YAG/ZrO2 eutectic in situ composite has different microstructure showing fine interpenetrated network with Al2O3, YAG and ZrO2 phases continuously intergrown, while none of pores, grain boundaries and amorphous phases is found. (2) The scanning rate and the power density of the laser beam has strong effect on the microstructure morphology. When the power density is determined, the eutectic spacing is reduced with the scanning rate increased. The characteristic eutectic spacings and phase sizes of YAG and Al2O3 are about 2~3μm, and the characteristic dimension of ZrO2 is less than 1μm. (3) The hardness and the room-temperature fracture toughness of Al2O3/YAG/ZrO2 eutectic are respectively Hv=16.7±2.0 GPa and KIC=8.0±2.0 MPa.m1/2, and those of hypoeutectic are respectively Hv=15.8±2.0 GPa and KIC=3.9±1.0 MPa.m1/2.

Abstract: The directionally solidified specimens of Nb-13.52 Si-22.60 Ti–6.88 Hf–2.54 Cr–2.24 Al alloy were prepared in an electron beam floating zone melting furnace at the withdrawing rate of 0.1, 0.3, 0.6, 1.0, 2.4 and 6.0 mm/min. All the primary Nb solid solution (Nbss) columns, Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies and divorced (Nb)3Si/(Nb)5Si3 plates or chains align well along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the microstructure is gradually refined, and the amount of Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies increases. Both the room temperature ultimate tensile strength σb and fracture toughness KQ are improved for the directionally solidified specimens. The tensile fracture occurs in a cleavage way.