Y. T. Zhu

Effect of lattice strain and defects on the superconductivity of MgB2

The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variations in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (∼1%) and the presence of Mg vacancies (∼5%) resulted in lowering the Tc by only 2 K.

Partial-dislocation-mediated processes in nanocrystalline Ni with nonequilibrium grain boundaries

The partial-dislocation-mediated processes have so far eluded high-resolution transmission electron microscopy studies in nanocrystalline (nc) Ni with nonequilibrium grain boundaries. It is revealed that the nc Ni deformed largely by twinning instead of extended partials. The underlying mechanisms including dissociated dislocations, high residual stresses, and stress concentrations near stacking faults are demonstrated and discussed.

Predictions for Partial-Dislocation-Mediated Processes in Nanocrystalline Ni by Generalized Planar Fault Energy Curves: An Experimental Evaluation

Generalized planar fault energy (GPFE) curves have been used to predict partial-dislocation-mediated processes in nanocrystalline materials, but their validity has not been evaluated experimentally. We report experimental observations of a large quantity of both stacking faults and twins in nc Ni deformed at relatively low stresses in a tensile test. The experimental findings indicate that the GPFE curves can reasonably explain the formation of stacking faults, but they alone were not able to adequately predict the propensity of deformation twinning.

Microstructures and mechanical properties of ultrafine-grained Ti foil processed by equal-channel angular pressing and cold rolling

We processed coarse-grained Ti and equal-channel angular pressing (ECAP) processed ultrafine-grained (UFG) Ti into 20-μm-thick Ti foils by cold rolling and intermediate annealing. The foils produced from rolling the UFG Ti exhibit a homogeneous nanostructure, while foils produced from rolling the coarse-grained Ti exhibit heterogeneous structures with a mixture of nanostructured regions and coarse-grained regions. The former foils also have higher strength and ductility and exhibit uniform deformation over a larger strain range at room temperature than the latter ones. This work demonstrated the advantage and viability of producing nanostructured Ti foil by rolling ECAP-processed UFG Ti stock.

Variation of oxygen content and crystal chemistry of YBa4Cu3O8.5+δ

Abstract All phases discovered so far in the Y–Ba–Cu–O system have layered (2-dimensional) structures with the exception of YBa4Cu3O8.5+δ (Y143), which has a cubic (3-dimensional) structure. In an attempt to understand the crystal chemistry of the Y143 phase and to make it superconductive, the Y143 compound has been treated at 450°C in oxygen and subsequently annealed at several different temperatures in a nitrogen atmosphere. The samples have been characterized by X-ray diffraction, thermogravimetric analysis (TGA), iodometric titration, and SQUID magnetometer measurements. The Y143 phase was found to be nonsuperconductive for all annealing conditions. The oxygen content was found to decrease with increasing annealing temperature in two steps, corresponding to the loss of oxygen from two different lattice sites. The lattice parameter decreased during the first step and increased during the second step with increasing annealing temperature or with decreasing oxygen content. Rietveld refinement of X-ray powder diffraction data showed that the O3 and O4 sites, which are geometrically identical within the crystal lattice, have an equal occupancy of 51.5% in samples not annealed in N2. However, when samples were annealed in N2, an oxygen redistribution from the O4 site to the O3 site occurred during the low temperature (

Crystal structure and chemistry of four new RBa4Cu3O8.5+δ (R=Ho, Er, Tm and Yb) compounds

Abstract Four new RBa 4 Cu 3 O 8.5+ δ (R143) compounds, where R=Ho, Er, Tm, and Yb, were synthesized from precursors Ho 2 O 3 , Er 2 O 3 , Tm 2 O 3 , Yb 2 O 3 , BaO 2 , and CuO at 1000°C in an oxygen atmosphere. The oxygen stoichiometric value δ was found to be 0.50 for Ho143, 0.53 for Er143, 0.79 for Tm 143 and 1.01 for Yb143 by iodometric titration. Rietveld refinement of X-ray powder diffraction data showed that all the compounds belong to the space group Pm3. The compounds have a cubic unit cell with lattice parameters of 8.09485±0.00007 A for Ho143, 8.08236±0.00005 A for Er143, 8.07670±0.00008 A for Tm143, and 8.06198±0.00005 A for Yb143. SQUID measurements indicated that none of the compounds was superconducting above 5 K.

Powder diffraction data of SmBa 4 Cu 3 O 8.5+δ

Rietveld analysis of X-ray powder diffraction data was performed on SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, which was synthesized from precursors Sm{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000{degree}C in an oxygen atmosphere. SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} has a cubic perovskite-related structure that is isostructural with YBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, and a doubled perovskite unit cell parameter of 8.17790{plus_minus}0.00004{Angstrom}. {copyright} {ital 1997 International Center for Diffraction Data.}

Powder diffraction data of SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}

Rietveld analysis of X-ray powder diffraction data was performed on SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, which was synthesized from precursors Sm{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000{degree}C in an oxygen atmosphere. SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} has a cubic perovskite-related structure that is isostructural with YBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, and a doubled perovskite unit cell parameter of 8.17790{plus_minus}0.00004{Angstrom}. {copyright} {ital 1997 International Center for Diffraction Data.}

Microstructure and Mechanical Properties of Long, Ultrafine-Grained Ti Rods

In this paper, we report our recent studies on the effect of processing routes on the microstructure and mechanical properties of long ultrafine-grained Ti rods. CP Ti (Grade 2) rods were processed into long, ultrafine-grained (UFG) rods with a dimension of 6.5 mm in diameter and more than 1000 mm long), via equal-channel angular pressing followed by thermo-mechanical treatment. This paper presents the UFG microstructures in the processed rods and evolution of mechanical properties at various stages of the processing. It was established that the formation of homogeneous UFG structure with a grain size about 70 nm enhanced the ultimate strength by 2.5 times in comparison with the initial annealed state, while maintaining a good ductility of 12%.

The Development of Ultrafine-Grained Ti for Medical Applications

Bulk samples of commercially pure (CP) titanium were processed by severe plastic deformation (SPD), namely, equal channel angular (ECA) pressing in combination with thermal mechanical treatment, to produce ultrafine-grained (UFG) microstructures. It is shown that by altering SPD processing parameters one can form distinct types of UFG microstructures, which differ in grain shape, size and intragrain defect density and defect distribution. These UFG microstructures in CP titanium result in strength and fatigue properties similar to those of Ti-alloys. We are thus able produce CP titanium suitable for replacing Ti-6A1-4V in biomedical and orthopaedic applications such as dental implants, medical instruments, and trauma fixation devices (nails, plates, screws, and wires).