Papers by Author: Qing Chang Meng

Abstract: Amorphous Si-B-C-N ceramics obtained by high energy ball milling and hot pressing using hexagonal boron nitride (h-BN), graphite (C) and amorphous Si as starting materials have been studied. The mechanical milling with high energy resulted in the generation of large amounts of amorphous composites only milled for 5 h. Si-B-C-N powders were consolidation by hot pressing at 1850 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that small amount of BN and SiC crystal lies in the amorphous matrix. The flexural strength reached the maximal value of 137.2 MPa at a mole ratio of BN/(Si+C) being 0.6.

Abstract: The Al2O3/LiTaO3 (ALT) composite ceramics were fabricated by hot pressing method and polarization treated at different temperatures along height and width directions. Effects of polarization treatment on mechanical properties of the ALT composite ceramic were investigated. Flexural strength decreased with the increase of polarization temperature. Meanwhile, the flexural strength of samples polarized in height direction is higher than that polarized in width direction. The composite ceramic fractures intragranularly, and many rupture steps in polarization direction were observed on fractographs of the composite ceramics. Domain switching in LiTaO3 particles increased the resistance of crack propagation and improved the mechanical properties of the polarized ALT composite ceramics.

Abstract: The TiB/Ti metal matrix composites (MMCs) with different volume fractions of in situ TiB reinforcements were spark plasma sintered at 1000 °C with a pressure of 20 MPa for 5 minutes in vacuum. The in situ synthesized TiB is whisker shape with a hexagonal transverse section and distributes uniformly and randomly in the Ti matrix. The Young’s modulus of TiB was back-calculated from the elastic properties of the composites using the Halpin-Tsai model. The Young’s moduli of all the composites were found to increase with the increase of TiB volume fraction. The calculated value of TiB Young’s modulus is about 489±83GPa. Values of Young’s moduli of TiB whisker obtained according to different methods were compared and discussed.

Abstract: Effects of different sintering methods such as pressureless sintering and hot press sintering on relative density and microstructure of the Al2O3p/LiTaO3 (ALT) composite ceramics were investigated to obtain a preferable sintering process. Relative densities of all ALT composites are below 90% when sintered with the cold isostatical pressing followed by pressureless sintering at temperatures of 1250 to 1350°C. The relative densities and microstructure of ALT composite ceramics with the hot press sintering process in a N2 atmosphere at 1150 and 1300°C were investigated. The relative density of ALT composite hot pressed at 1150°C is only 77%, and almost theoretical density at 1300°C. This indicates that sintering pressure plays an important role in the densification of ALT composite ceramics in temperature range of 1150 to 1350°C. Investigation on morphologies of the composites shows that the Al2O3 particles distributed along grain boundaries of LiTaO3, which leads to a fine-grained microstructure in the ALT composite ceramics

Abstract: The in situ TiB whisker reinforced titanium matrix composites were prepared by mechanical alloying followed by spark plasma sintering. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure of the TiB whiskers. The effect of sintering temperature on morphologies of in situ TiB whiskers was evaluated. With the increase of spark plasma sintering temperature, the average diameter of in situ TiB whiskers increased. The in situ TiB whiskers exhibited a hexagonal shape with (100), (101) and (10 1 ) planes at the transverse section and a growth orientation of [010]TiB direction.

Abstract: Amorphous nano-sized silicoboron carbonitride (Si-B-C-N) powders with average grain size <50 nm were fabricated by high energy shaker mill using hexagonal boron nitride, graphite and amorphous silicon powders as starting materials. The powders were consolidated by spark plasma sintering at 1900° and 1950°C. Amorphous phase were partially retained in ceramic sintered at 1900°C. For ceramic sintered at 1950°C, amorphous Si-B-C-N ceramic transferred to hexagonal BN and cubic SiC.

Abstract: The densified α-sialon ceramics with the compositions RE0.333Si10Al2ON15 (RE= Yb,Y, Dy, Sm and Nd) were prepared by a two-step hot-pressing sintering. The ceramics doped with smaller cations (Yb3+, Y3+ and Dy3+) are fully composed of α-sialon, while the larger cation-doped ceramics (Sm3+ and Nd3+) exist a few M′ (RE2Si3-xAlxO3+xN4-x) phases. A small amount of β-sialon phases are also found in the Nd-doped sialon. Microstructure observation indicates that Yb-α-sialon consists of equiaxial grains, but when increasing the radius of the doped cations, the elongated α-sialon grains form and the aspect ratio of grain increases slightly. TEM observation indicated that almost no intergranular phases exist in the α-sialon doped with smaller cations, but a few exist in Sm- and Nd-doped α-sialon. Some elongated α-sialon grains with grain core were found. EDX results suggest that the compositions of the grain shell and that of the core are different. Surrounding the grain core, some misfit dislocations were seen.

Abstract: In situ TiB whiskers reinforced Ti metal matrix composites have been fabricated by spark plasma sintering (SPS). Microstructure and stacking faults of in situ TiB whiskers in the composites were investigated in detailed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The TiB whiskers exhibit a hexagonal shape at the transverse section and grow along the [010]TiB direction. The crystallographic planes of the TiB whiskers at transverse section are always of the (100), (101) and (10 1 ) planes. The interfaces between the in situ TiB and Ti matrix are clean and faceted along (100)TiB side. However, small lateral growth steps are observed along (101) and (10 1 ) planes of TiB. The stacking faults in TiB whiskers are typically with a stacking fault plane of (100)TiB and displacement vectors of 0.5a±0.254c. The formation mechanism of stacking faults has been proposed and verified by HREM observations.

Abstract: The investigation aimed at the invalidation mechanism analysis and reliability evaluation of aluminum alloy welded joint under space environment will provide spaceflight designing epartments and manufactures with important technology gist and data so as to greatly reduce paceflights manufactured cost and improve spaceflight working reliability. In this paper, the tensile properties and microstructures of LF6 aluminum alloy welded joint under vacuum thermal cycling have been studied by means of vacuum thermal cycling system. The influence of vacuum thermal cycling on the tensile properties of the welded joint has investigated. Results show that the strength and the enlongation reach their maximum values at about 75 times cycling. The changes of the tensile properties are regarded as resulted from the changes of grain and the motion of dislocation.

Abstract: In this paper, pure aluminum and Al-Cu (1%, 3% and 5% mass.) alloy matrix composites reinforced with 40Vol% Al2O3-SiO2 sort fiber were fabricated by Squeeze-casting technique. Microstructure and tensile properties of the composites were investigated by means of SEM, EPMA, TEM and tensile tests. It was found from the experimental results that when pure aluminum, which was added into Cu elements, is used as matrix, chemical reaction occurred at the interface between amorphous SiO2 and matrix due to the Cu element diffusion and enrichment towards the sort fiber surface. Results indicated that with the increasing of Cu content the ultimate tensile strength decreased firstly then increased continuously.