Haijun Su

Preparation and characterization of Al2O3/Y3Al5O12/ZrO2 ternary hypoeutectic in situ composites by laser rapid solidification

The directionally solidified oxide eutectic in situ composite is one of the most promising high-temperature structural materials in oxidizing environments. Pore-free rods and plates of ternary Al2O3/Y3Al5O12 (YAG)/ZrO2 hypoeutectic in situ composites with hypoeutectic composition (71u2002molu2009% Al2O3, 17u2002molu2009% Y2O3, and 12u2002molu2009% ZrO2) are prepared rapidly by the laser zone remelting technique. The hypoeutectic growth and microstructure characteristic of grown crystals are studied using the scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy, aiming to understand well the rapid solidification behavior of the ternary oxide system of Al2O3–Y2O3–ZrO2. The rapidly solidified Al2O3/YAG/ZrO2 hypoeutectic shows a refined, interpenetrating, and irregular lamellar structure with a reticular distribution of coarse Al2O3 and yttrium aluminum garnet (YAG) and smaller ZrO2 phases. The fine ZrO2 phases are partially embedded at the Al2O3/YAG interfaces. Moreover, the typical dendrite microstru...

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000u2005h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.

Growth Characteristic and Fracture Toughness of Laser Rapidly Solidified Al2O3/YAG/ZrO2 Ceramic Eutectic In Situ Composite

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.

Laser Zone-Remelted Alumina-Based Eutectic In Situ Composite

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.

Microstructure and Mechanical Property of Al2O3/Y3Al5O12 (YAG) Eutectic Ceramic In Situ Composite Prepared by Laser Zone Melting

Directionally solidified Alumina-based eutectic ceramic in situ composite is a kind of promising candidate for high temperature structural material applied at elevated temperature above 1923K because of its excellent properties. With laser zone melting directional solidification, Al2O3/Y3Al5O12 (YAG) eutectic ceramics are successfully prepared. The relationship between the eutectic microstructure and the processing parameter is studied, and the mechanical property of the composite is measured. The results show that: (1) Laser power density and scanning rate strongly affect the eutectic microstructure. With proper processing parameters adjusted, the binary lamellar eutectic microstructure is obtained, in which Al2O3 and YAG phases are three-dimensionally coupled and continuously connected without grain boundaries and amorphous interface phases. (2) The eutectic spacing decreases to about 1μm with increasing scanning rate. (3) The maximum hardness of 19.5GPa and the room fracture toughness of 3.6MPa.m1/2 are obtained by Vickers indentation measurement.