Wen Bo Han

Superplasticity and Sinter-Forging of Fine-Grained Si3N4-Si2N2O Composite

The Si3N4- Si2N2O composites are fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering(LPS) method in this article. XRD analysis shows that the sintered body consists of β-Si3N4 and Si2N2O. SEM experiment conforms that the average grain size of sintered body is less than 300nm. The superplastic deep-drawing forming can be proceed at a low temperature of 1550°C with a forming velocity of 0.2mm/min. There are only a few small sintered defects before forming, but there are a lot of cavity groups after forming. Cavitation failure occurs by nucleation, growth and interlinkage of cavities. The complex-shape gears can be formed by a sinter-forging technology when the sintering temperature is 1600°C and the superplastic forging temperature is 1550°C.

Superplasticity and Diffusion Bonding of IN718 Superalloy

The superplasticity and diffusion bonding of IN718 superalloy were studied in this article. The strain rate sensitivity index m was obtained at different temperatures and various initial strain rates using the tensile speed mutation method; m readied its maximum value 0.53 at an inititil strain rate of 1×10−4s−1 at 1253K. The diffusion bonding parameters, including the bonding temperature T, pressure p, and time t, affected the mechanism of joints. When the bonded specimen with 25μm thick nickel foil interlayer was tensile at room temperature, the shear fracture of the joints with nickel foil interlayer took place at the IN718 part. Microstructure study was carried out with the bonded samples. The microstructure shows an excellent bonding at the interfaces. The optimum parameters for the diffusion bonding are: T=1273-1323K, p=20-30MPa, t=45-60min.

Superplastic Extrusion of Ultra Fine-Grained 3Y-TZP/Al2O3 Ceramic for Turbine Disk

Superplastic extrusion of axial forcing and radial flowing under different conditions was adopted to form a turbo-rotor, using hot-press sintered compact of zirconia (3Y)-toughened alumina composite that was prepared at 1450 °C for 1 hour with relative density of higher than 96%. Subsequent superplastic extrusion was attempted at temperature of 1500 °C-1650 °C. The results indicate that 3Y-ZrO2 plays an important role as a second-phase pinning agent and inhibits grain growth. The material shows good deformability and potential of near-net-shape forming. Comparing with undeformed sample, extruded sample was observed apparent coarsening in grain size and the remaining mechanical properties after deformation are not improved, irrespective of improved density. The dominating deformation mechanism is grain sliding and rotation accommodated with zirconia coordinated deformation.

The Dispersion of SiC Nanopowders in Ethanol

The dispersion of SiC nanopowders in ethanol solution was studied by sedimentation test, particle size measurement and TEM analysis. The dispersion behavior of SiC nanopowders in ethanol solution was strongly dependent on the pH values, types and amounts of dispersant. PEI was found to be effective for the dispersion of SiC nanopowders in ethanol solution. With the addition of PEI, the isoelectric points of SiC nanopowders in ethanol solution were at pH 9.5, and shift to pH 12.3. The stability of SiC suspension increased with the dispersant content increasing until reached 2.5 wt% PEI. The suitable pH value for the dispersion of SiC nanopowders should lower than 10.

Microstructure and Mechanical Properties of Hot Pressed ZrC-SiC-ZrB2 Composites

Ternary ZrC-SiC-ZrB2 ceramic composites were prepared by hot pressing at 1900 °C for 60 min under a pressure of 30 MPa in argon. The influence of ZrB2 content on the microstructure and mechanical properties of ZrC-SiC-ZrB2 composites was investigated. Examination of SEM showed that the microstructure of the composites consisted of the equiaxed ZrB2, ZrC and SiC grains, and there was a slight tendency of reduction for grain size in ZrC with increasing ZrB2 content. The hardness increased considerably from 23.3 GPa for the ZS material to 26.4 GPa for the ZS20B material. Flexural strength was a strong function of ZrB2 content, increasing from 407 MPa without ZrB2 addition to 627 MPa when the ZrB2 content was 20vol.%. However, the addition of ZrB2 has little influence on the fracture toughness, ranging between 5.5 and 5.7 MPam1/2.

Effects of Carbon Black on Microstructure and Mechanical Properties of Hot Pressed ZrB2-SiC Ceramics

Abstract. ZrB2-SiC ultra-high temperature ceramics (UHTCs) was hot-pressed at a temperature of 1900°C with the addition of carbon black as a reinforcing phase. Microstructure and mechanical properties were investigated. Analysis revealed that the amount of carbon black had a significant influence on the sinterability and mechanical properties of ZrB2-SiC ceramics. When a small amount ( < 10 vol.%) of carbon black was introduced, it may react with oxide impurities (i.e. ZrO2, B2O3 and SiO2) present on the surface of the starting powder, thus promote the densification and grain refining of ZrB2-SiC ceramics. As a result, the mechanical properties including flexural strength and fracture toughness were improved. However, with the further adoption of carbon black, mechanical properties were not improved much, which could be attributed to the redundant phase at grain boundaries. The results presented here point to a potential method for improving densification, microstructure and mechanical properties of ZrB2-based ceramic composites.

Synthesis and Superplastic Deep Drawing of TiAl Alloy

TiAl alloys are powerful candidates for light weight high temperature structural materials because of their excellent high temperature strength, low density and good oxidation resistance. Unfortunately, TiAl alloys are difficult to machine and hot working due to ordered structure, which impede large-scale application. Synthesis and superplastic deep drawing of a TiAl alloy were studied. Mechanically synthesized fine crystalline Ti/Al composite powders as precursor of TiAl alloy. The TiAl alloy with fine grain via reactive sintering was obtained. The deformation behavior under biaxial stress by means of deep drawing was achieved. The optimum temperature is 1100°C≤T≤1150°C. The microstructures corresponding to deep drawing in the part were studied.

Superplastic Bulging Capability of Ti-6Al-4V Butt-Welded Plate by High Energy Beam Welding

The superplastic bulging capabilities of Ti-6Al-4V butt-welded plates with 0.8mm in thickness with high energy beam welding methods namely plasma arc welding (PAW), electron beam welding (EBW) and laser beam welding (LBW) are studied in virtue of superplastic bulging tests. Superplastic bulging tests are performed at the superplastic forming temperature 925°C under 1MPa gas pressure. The superplastic bulging capability is represented by the maximum relative bulging height h after fracture. Experimental results suggest that all of butt-welded plates with high energy beam welding methods possess good superplastic bulging capability. Among them, the maximum relative bulge height of LBW is the highest, that of EBW is slightly lower and that of PAW is the lowest. The higher the input energy density is, the bigger the bulge height will be. Furthermore their microstructure evolutions of various weld metals during superplastic bulging were systematically analyzed via metallographical tests. The relation between the microstructure of weld metal and its superplasticity is found. Metallographical analysis shows that the microstructure of Ti-6Al-4V weld metal with high energy beam welding methods is composed of fine acicular martensite. The higher the input energy density is, the finer the martensite structure will be. Upon heating, this martensite changes to a basketweave-like structure and upon bulging, the martensite structure have the trend of transforming to fine equiaxed grain. This can explained the reason why the Ti-6Al-4V butt-welded plates with high energy beam welding methods have excellent superplastic bulging capability.

A Study on Concrescence of Inert Nano Bioceramic Scalpel Wounds

Over the last 100 years innovative techniques in the fabrication of ceramics have led to their use as high-tech materials. Inert bioceramics, such as ZrO2, have inherently low level of reactivity compared with other materials such as polymers and metals as well as surface reactive or resorbable ceramics. The aim of this study was to validate the effect of incision using a zirconia inert bioceramic scalpel to incise animal body and to prove incision property of tissues before clinic. The experiments of incisions concrescence using a sterilized Zirconia inert bioceramic scalpel were investigated in this paper. We used 4-month-old inbred line SD rats as experiment animals. They were divided into three groups to undergo incision on the back skin and subcutaneous tissue. The subcutaneous tissue and musculature samples were obtained and analyzed by optical microscopy at 3, 7 and 14 days for histopathological evaluation respectively. The experimental results showed that no wound dehiscence was observed after suture removal at 14 days after the operation. The experiments proved that an inert bioceramic scapel was nontoxic, nonallergenic, and noncarcinogenic for incisions that obtained normal concrescence criteria two weeks after operation.

Interface of Graphene/ZrB2 Ceramics Structure by Molecular Dynamics Simulation

With honeycomb lattice of sp2 hybridized carbon atoms, graphene has demonstrated excellent electrical and mechanical properties. One of its promising applications is to fabricate graphene-ceramic composite to improve the mechanical properties. In order to quantify the strength between graphene-ZrB2 interactions, molecular dynamic method was utilized to simulate typical interface of graphene/ZrB2 ceramic structure. Berendsen method was used to control the temperature and pressure during the whole simulation process. Universal potential function was employed to simulate the force filed between graphene and ZrB2 structure. The binding structures of graphene/ZrB2 (0001) interface were analyzed in detail and the bonding energy of the interface was calculated. The influence of numbers of graphene layer and sandwich structures on the bonding energy of interface is discussed. The study helped to understand the influence of graphene on mechanical properties of ZrB2 ceramic.