Papers by Author: Lian Zhou

Abstract: A nanostructured surface layer was induced on zircaloy-4 by the method of surface mechanical attrition treatment (SMAT). X-ray diffraction and microhardness tester were applied to identify the average grain size and hardness of specimen processed for different duration, transmission electron microscopy and high-resolution transmission electron microscopy were adopted to observe the microstructure of specimen. The results showed that the surface hardness enhanced gradually and then stabilized with the processing duration increasing, while the average grain size declined gradually, to the minimum 20 nm at 15 min, then increased. The formation of nanocrystalline was due to the mechanical twins and dislocation slip.

Abstract: The effect of specific surface area on the growth and the porosity of oxide ceramic superconductor YBCO single domain are investigated. The enhancement of growth depth depends on the large specific surface area σ. When the specific surface area σ is more than the critical value 200mm2/g, the YBCO single domain is full-growth in the sample of size Ф20mm×18mm. It reveals that a large specific surface area providing an effective tunnel of oxygen diffusion for the crystal growth. On the other hand, this tunnel makes oxygen gas emitting out resulting in porosity diameter decreased. The mean porosity diameter is observed as 5μm in hole sample smaller than 80μm in plain sample. The porosity distribution is calculated as 1/(100μm)2 in plain sample and 8/(10μm)2 in hole sample.

Abstract: Electron beam cold hearth melting process is an efficient method to produce the premium quality titanium alloys, especially to eliminate inclusions. A simulation work was carried out to study the process, concerning the flow field and particle trajectory at three different melt rates. The simulation results show that, when there is an overheat zone near the outlet zone, the molten metal flows to the sidewall of the cold hearth, and from the outlet zone to the inlet zone at the top surface which avoids the inclusion particle flows out the cold hearth. At the bottom of the liquid pool, the fluid flows to the outlet directly along the center plan, which forms a short circuit, decreases the residence time of the inclusion particles; there is a critical density range of inclusion particles, which have more probability to flow out of the cold hearth. The inclusion particles, whose density lower than it, will flow to the sidewall. The inclusion particles, whose density higher than it, will sink into the bottom mushy zone. Both cases let the inclusion have higher probability to eliminate the inclusions.

Abstract: A kind of metastable β type Ti-30Nb-13Zr-0.5Fe (wt.%) alloy for biomedical application was newly designed and developed. In order to exam the phase transformation during aging and its effects on the mechanical properties, the alloy was β solubilized and aged at 350°C-550°C for 4 hours. The microstructures were observed by OM and TEM, and the phase structures were identified by XRD. The tensile tests were performed with various aged microstructures. The results show that a lot of ω phase precipitates during aging at 350°C, leading to the increase of strength and elastic modulus and drastic decrease of plasticity. Aging at 450°C, dot α phase uniformly precipitates from metastable β phase. The good combination of high strength 、high plasticity and low elastic modulus was obtained under this aging condition. With increasing aging temperature and aging time α precipitations coarsen and precipitation free zones (PFZ) along prior β grain boundaries form, which are the main reasons to lower the strength and plasticity.

Abstract: Hydroxyapatite/TiO2 composite material was coated onto Ti25Nb3Mo2Sn3Zr (TLM) alloy substrate. To study the effects of hydroxyapatite/TiO2 composite coatings on bone-related protein expression, the osteoblast were cultured with composite coatings for different times. The phase transformation and compound formation of the HA/TiO2 coatings were investigated using XRD (X-ray diffraction). The mRNA expression of Type I collagen, alkaline phosphatase (ALP) and osteocalcin were studied by RT-PCR (reverse transcriptional polymerase chain reaction). The titania delayed the crystallization of HA. The mRNA expressions of Type I collagen are decreased as the increasing of TiO2 percentage. The mRNA expressions of osteocalcin are approached. The ALP expression on H4 coating (HA/TiO2 mol ration is 5) after the osteoblast cultured with composite coating for 6 days is the highest. The increasing of TiO2 amount decreases the bioactivity of the composite coatings.

Abstract: The unidirectional solidification technology by a zone melting method was performed to obtain the large single domain YBCO. The interface morphology and chemical composition at the growth front of the YBCO crystal were investigated in order to make clear the growth characteristic of the YBCO crystal during melting growth by unidirectional solidification. It was found that YBCO crystal would cease growing when yttrium was depleted in the liquid phase at the YBCO crystal growth front. For maintaining the continuous growth of YBCO crystal, compositions of Y, Ba and Cu in raw samples have to be adjusted so as to make yttrium rich in the liquid phase at the YBCO crystal growth front during the melting growth process. It is very useful for the study on the mechanism of the YBCO crystal growth.

Abstract: Y123 and Y211 powders were synthesized by solid-state reaction method. YBCO superconductor specimens were prepared by unidirectional solidification technique. The microstructure and the structural characteristics were observed by means of the optical microscope, X-ray diffraction (XRD), scanning electron microscope (SEM). The results showed that the specimen have a highly aligned structure, it was observed obvious laminated structure of Y123 crystal. Crystal growth mechanism of YBCO was investigated. Jackson's factor of Y123 crystal was calculated. Interface form was judged tentatively theoretically. The specimen was rapidly quenched by quick pulling. Solid/liquid interface structure was observed. It was concluded that a-b plane was faceted grow forms and layer growing along c-axial on a-b plane.

Abstract: The effects of BaF2 on thermal behavior and microstructure of melt-processed YBCO superconductors have been investigated. The differential thermal analysis (DTA) results indicated that the peritectic decomposition temperature of YBCO precursor powder was lowered when BaF2 was added (from 1020
to 976
). The peritectic decomposition temperature was lowest at the content of 5 wt.% BaF2, and the variation of melting temperature was not significant above 5 wt.% BaF2. The microstructures of the doped samples have been observed by scanning electron microscopy (SEM). The results showed that the    of BaF2        microstructure  .

Abstract: Bulk samples with the stoichiometry of Mg:B=1:2 and 1:4 were prepared by solid state reaction method. The microstructure and constituent of the samples were investigated by using the scanning electron microscope (SEM) and the x-ray diffraction (XRD). XRD results showed that the MgB2 phase was first formed in all the samples with different stoichiometry of Mg and B, which indicated that the MgB2 was the most thermodynamically stable phase in the Mg-B binary system. For the samples of Mg:B=1:2, the MgB2 single phase was formed very well when these samples were sintered at 650°C~700°C. For the samples with later added Mg reaching to the stoichiometry of Mg:B=1:2, a longer reaction time or higher reaction temperature was required for the formation of the MgB2 single phase. The SEM results showed that the samples with later added Mg had dense microstructures, suggesting that the later addition of Mg could reduce the porosity of the sample. A small increase of the superconducting transition temperature, Tc, in the Mg addition sample resulted from the dense microstructures.

Abstract: Nb3Sn superconductors are widely used in high magnetic field application. Internal tin processed Nb3Sn wires used for ITER coils (at 4.2K, 12T) were heated by two steps, local heat treatment and reaction heat treatments. The superconducting properties of Nb3Sn were investigated as a function of reaction heat treatment (HT) for strands during 625°C~665°C.To study the heat treatment effects on Jcn and n-value of Nb3Sn strands, different HT-parameters, i.e., annealing temperature and times, were applied on the Nb3Sn multifilament strands.