Zhimeng Guo

Preparation and properties of biomedical porous titanium alloys by gelcasting

Porous titanium alloys have been prepared by gelcasting in this study. The elastic solid green body was first polymerized and then vacuum sintered to porous titanium alloys with low contamination by controlling sintering conditions. The microstructure and the total porosity of the vacuum sintered porous Ti-Co and Ti-Mo alloys were analyzed by using scanning electron microscopy and x-ray diffraction. Moreover, compression and bending tests were conducted to investigate their mechanical properties. The results show that open and closed three-dimensional pore morphologies and total porosity ranging from 38.34% to 58.32% can be achieved. In contrast to porous Ti by gelcasting, the compression and bending strengths of porous titanium alloys were significantly increased by adding Mo and Co with Youngs modulus ranging between 7-25 GPa, which is close to that of human cortical bone, therefore being suited for potential application in load-bearing implants.

Effect of yttrium substitution on magnetic properties and microstructure of Nd-Y-Fe-B nanocomposite magnets

Abstract The phase evolution, microstructure and magnetic properties of Nd9–xYxFe72Ti2Zr2B15 (x=0, 0.5, 1, 2) nanocomposite ribbons were investigated. It was found that substitution of Y enhanced glass forming ability of the over-quenched ribbons and stabilized the amorphous phase during post annealing treatment. Appropriate content of Y substitution effectively refined the microstructure and enhanced the remanence of the annealed samples. The residual amorphous intergranular phase in the annealed sample improved the squareness of loops, resulting in enhanced maximum energy product. The optimum annealing treatment significantly improved magnetic properties of the Y substituted ribbons. Best magnetic properties, Jr=0.78 T, Hci=923.4 kA/m, (BH)max=98.5 kJ/m3 were obtained from Nd8YFe72Ti2Zr2B15 ribbon spun at Vs = 4 m/s, and annealed at 700 °C for 10 min.

Consolidation and properties of ultrafine binderless cemented carbide by spark plasma sintering

Abstract Owing to the absence of metal binder, binderless cemented carbides have higher wear, corrosion, and oxidation resistance. WC-0.3VC- 0.5Cr3C2 powders with an average particle size of 200 nm and a little amount of active element were consolidated by spark plasma sintering. The sintered microstructure revealed that the average WC grain size was 0.24 μm, which was almost consistent with the initial fine powder. The results of XRD showed that W2C phase was formed. Nearly complete densification of ultrafine binderless cemented carbide was achieved by sintering at 1400°C for 120 s under 50 MPa. The resulting hardness and the fracture toughness were 28.18 GPa and 6.05 MPa·m½, respectively.

Gelcasting of porous titanium implants

Abstract Porous titanium implants with large size and complex shape were successfully fabricated by a novel near net forming process, gelcasting, and the effects of processing parameters on the viscosity of the Ti slurry based on acrylamide system, dry green strength, porosity and mechanical properties of sintered body were investigated. The viscosity of Ti slurry with 34 vol.-%Ti and 20 wt-% monomer based on the premix dispersed by 0˙4 wt-%NH4OH is <1 Pa s when the shear rate is >20 s-1. A green body with the strength of 30 MPa can be obtained from the 34 vol.-%Ti slurry with the ratio of monomer to cross-linker from 3 to 120, and when sintered at 1373 K for 1˙5 h, the total porosity and open porosity of porous titanium were 46˙5 and 40˙5% respectively, and its mechanical properties are compressive strength ∼192 MPa, Youngs modulus ∼8˙5 GPa, similar to those of human cortical bone and appropriate for implanting purpose.

In situ fabrication and properties of AlN dispersion strengthened 2024 aluminum alloy

Nanoscaled aluminum nitride (AlN) dispersion strengthened 2024 aluminum alloy was fabricated using a novel approach in which Al-Mg-Cu compacts were partially nitrided in flowing nitrogen gas. The compacts were subsequently consolidated by sintering and hot extrusion. The microstructure and mechanical properties of the material were preliminarily investigated. Transmission electron microscopy and X-ray diffraction results revealed that AlN particles were generated by the nitridation of Al-Mg-Cu compacts. The material exhibited excellent mechanical properties after hot extrusion and heat treatment. The ultimate tensile and yield strengths of the extruded samples containing 8.92vol% AlN with the T6 heat treatment were 675 and 573 MPa, respectively.

Near-net shape processing of spherical high Nb-TiAl alloy powder by gelcasting

Spherical Ti-45Al-8.5Nb-(W,B,Y) alloy powder prepared by an argon plasma process was near-net shape by gelcasting. In the non-aqueous system, methaerylate-2-hydroxy ethyl, toluene, benzoyl peroxide, and N,N-dimethylaniline were used as the monomer, solvent, initiator, and catalyst, respectively. To improve sintering and forming behaviors, many additives were included in the suspension. The concentrated suspension with a solid loading of 70vol% was prepared. The high Nb-TiAl powder was analyzed by electron microscopy and X-ray diffraction. It was found that the green bodies had a smooth surface and homogeneous microstructure, exhibiting a bending strength as high as 50 MPa. After sintering at 1480°C for 2 h in vacuum, uniform complex-shaped high Nb-TiAl parts were successfully produced.

Influence of nano-Al2O3-reinforced oxide-dispersion-strengthened Cu on the mechanical and tribological properties of Cu-based composites

The mechanical and tribological properties of Cu-based powder metallurgy (P/M) friction composites containing 10wt%–50wt% oxide-dispersion-strengthened (ODS) Cu reinforced with nano-Al2O3 were investigated. Additionally, the friction and wear behaviors as well as the wear mechanism of the Cu-based composites were characterized by scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS) elemental mapping. The results indicated that the Cu-based friction composite containing 30wt% ODS Cu exhibited the highest hardness and shear strength. The average and instantaneous friction coefficient curves of this sample, when operated in a high-speed train at a speed of 300 km/h, were similar to those of a commercial disc brake pad produced by Knorr-Bremse AG (Germany). Additionally, the lowest linear wear loss of the obtained samples was (0.008 ± 0.001) mm per time per face, which is much lower than that of the Knorr-Bremse pad ((0.01 ± 0.001) mm). The excellent performance of the developed pad is a consequence of the formation of a dense oxide composite layer and its close combination with the pad body.

Densification and properties of ODS-iron based alloy

Y2O3 dispersion strengthened iron-based powders and oxide dispersion strengthened (ODS) alloys were prepared by hydrothermal synthesis and spark plasma sintering (SPS), respectively. The effects of Y2O3, vibratory milling treatment, and Ti element on the microstructure and mechanical properties of the materials were investigated by scanning electron microscope (SEM) and micro-electronic universal tester. The results show that the best mechanical properties are obtained with 1 wt.% Y2O3 addition. The size of agglomerated particles can be decreased to a certain degree by vibratory milling treatment. With the addition of Ti element, the tensile strength and hardness of the samples were improved.

Characterization of nanometer tungsten powders

Abstract Three types of tungsten powders were prepared by hydrogen reduction of three precursor powders at low temperature, which were used as samples, and were then characterized by Brunauer-Emmer-Teller (BET) method, scanning electron microscopy (SEM), transmission electronic microscopy (TEM), small angle X-ray scattering (SAXS), and field-emission scanning electron microscopy (FESEM) respectively. The results showed that although BET and SEM could not characterize the particle size of nanometer powders, they were important means of assistance to exclude non-nanometer powders. TEM and FESEM could directly measure the particle size of nanometer powders, but this needs a lot of time, to count the average particle size and particle size distribution. SAXS could not describe the state of agglomeration. By the combination of FESEM and SAXS, the particle size, particle size distribution, and particle shape of nanometer powders could be precisely characterized.

Plasma preparation and low-temperature sintering of spherical TiC-Fe composite powder

A spherical Fe matrix composite powder containing a high volume fraction (82vol%) of fine TiC reinforcement was produced using a novel process combining in situ synthesis and plasma techniques. The composite powder exhibited good sphericity and a dense structure, and the fine sub-micron TiC particles were homogeneously distributed in the α-Fe matrix. A TiC–Fe cermet was prepared from the as-prepared spherical composite powder using powder metallurgy at a low sintering temperature; the product exhibited a hardness of HRA 88.5 and a flexural strength of 1360 MPa. The grain size of the fine-grained TiC and special surface structure of the spherical powder played the key roles in the fabrication process.