Y.D. Wang

Tailoring size and structural distortion of Fe3O4 nanoparticles for the purification of contaminated water.

Fe(3)O(4) magnetic nanoparticles with different particle sizes were synthesized using two methods, i.e., a co-precipitation process and a polyol process, respectively. The atomic pair distribution analyses from the high-energy X-ray scattering data and TEM observations show that the two kinds of nanoparticles have different sizes and structural distortions. An average particle size of 6-8 nm with a narrow size distribution was observed for the nanoparticles prepared with the co-precipitation method. Magnetic measurements show that those particles are in ferromagnetic state with a saturation magnetization of 74.3 emu g(-1). For the particles synthesized with the polyol process, a mean diameter of 18-35 nm was observed with a saturation magnetization of 78.2 emu g(-1). Although both kinds of nanoparticles are well crystallized, an obviously higher structural distortion is evidenced for the co-precipitation processed nanoparticles. The synthesized Fe(3)O(4) particles with different mean particle size were used for treating the wastewater contaminated with the metal ions, such as Ni(II), Cu(II), Cd(II) and Cr(VI). It is found that the adsorption capacity of Fe(3)O(4) particles increased with decreasing the particle size or increasing the surface area. While the particle size was decreased to 8 nm, the Fe(3)O(4) particles can absorb almost all of the above-mentioned metal ions in the contaminated water with the adsorption capacity of 34.93 mg/g, which is approximately 7 times higher than that using the coarse particles. We attribute the extremely high adsorption capacity to the highly-distorted surface.

Determination of microstructure and twinning relationship between martensitic variants in 53 at.%Ni–25 at.%Mn–22 at.%Ga ferromagnetic shape memory alloy

A recent study by high-resolution neutron powder diffraction provided accurate crystallographic information for the newly developed ferromagnetic shape memory alloy 53 at.%Ni–25 at.%Mn–22 at.%Ga. This made it possible to study by high-resolution electron backscatter diffraction the local microstructures and the twinning relationships between martensitic variants. The twin interfaces were also investigated and they are found to be coherent on the {112} planes.

Texture and microstructure development in cold-rolled interstitial free (IF) steel sheet during electric field annealing

Abstract The effects of electric field annealing on the development of recrystallization texture and microstructure in a cold-rolled IF steel sheet were studied by means of ODF analysis and optical microscopy. Results showed that the applied external electric field might retard recrystallization, and enhance the γ-fiber recrystallization texture that is beneficial to the deep-drawability.

Crystal structures and textures of hot forged Ni48Mn30Ga22 alloy investigated by neutron diffraction technique

Abstract A ferromagnetic shape memory alloy of Ni48Mn30Ga22 prepared by induction melting was successfully hot forged. Strong textures and a large anisotropy of in plane plastic flow were developed during the hot forging process. The crystal structures, both in austenitic and martensitic states, were investigated by means of neutron powder diffraction technique. The result suggests that Ni48Mn30Ga22 has a cubic L21 Heusler structure at room temperature, the same as that in the stoichiometric Ni2MnGa. When cooled to 243 K, the Ni48Mn30Ga22 alloy changes into a seven layered orthorhombic martensitic structure. No substantial change of the neutron diffraction pattern was observed upon further cooling to 19 K, indicating that there is no intermartensitic transformation in the investigated alloy, which is different from the transformation processes in the Ni–Mn–Ga alloys with higher martensitic transformation temperatures.

Micromechanical behaviors of duplex steel : in situ neutron diffraction measurements and simulations

Recently, much attention has been paid to studying the micromechanical behavior of multiphase materials by virtue of their extensive engineering applications. A strong heterogeneity in stress exists in two-phase materials during loading due to the different thermal expansion coefficients and the respective mechanical properties of each individual phase. For one duplex stainless steel under uniaxial compression, the distributions of microstrains were characterized with the strain response of multiple reflections to the applied external stress for each phase by employing in situ neutron diffraction experiments. Based on these, the anisotropic elastoplastic properties of the duplex steel on microscales, i.e. phase size and grain size, were modeled using a two-phase viscoplastic self-consistent (VPSC) model involving lattice rotation (texture evolution). Good agreement between predictions and neutron diffraction measurements was found. The stress partition between phases and orientated grains was discussed to characterize the phase stress and the grain-orientation-dependent stress, thus the particular micromechanical properties of two-phase materials may be explored.

Quantitative Texture Analysis from X-ray Diffraction Spectra

A method to obtain the orientation distribution function (ODF) of a polycrystalline material directly from X-ray diffraction spectra is presented. It uses the maximum-texture-entropy assumption to reduce the diffraction data needed for the ODF analysis. The validity of this new method is illustrated through two model examples.

Vacuum brazing of NiTi alloy by AgCu eutectic filler

Abstract Joining of NiTi alloy to itself has been realised by vacuum brazing process using AgCu28 eutectic as filler metal. Microstructures, mechanical and shape memory behaviour have been investigated. The shearing strength of the brazed joint exceeds 100 MPa, and rupture occurs at the diffusion layer of parent metal beside brazing metal. The brazed joint will be stronger than parent metal on condition of the specimen with a joint of lap length 10 times of plate thickness. The brazed specimen shows a good shape memory behaviour. From the point of view of practice, the brazing joint design principle and brazing quality improvement have been discussed.

Strain analysis of misfit dislocations in α-Fe2O3/α-Al2O3 heterostructure interface by geometric phase analysis

The α-Fe2O3/α-Al2O3 heterostructure interfaces have been studied using transmission electron microscopy (TEM). The interface exhibited coherent regions separated by equally spaced misfit dislocations. The misfit dislocations were demonstrated to be edge dislocations with dislocation spacing of ∼4 nm. The strain fields around the misfit dislocation core were mapped using a combination of geometric phase analysis and high-resolution transmission electron microscopy images. The strain measurement results were compared with the Peierls-Nabarro dislocation model and the Foreman dislocation model. These comparisons show that the Foreman model (a=2) is the most appropriate theoretical model to describe the strain fields of the dislocation core.

Neutron diffraction study on crystal structure and phase transformation in Ni-Mn-Ga ferromagnetic shape memory alloys

Crystal structure and phase transformation behaviors in two Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) with compositions of Ni48Mn30Ga22 and Ni53Mn25Ga22 (at. %) as a function of temperature were investigated by in situ neutron diffraction experiments. Neutron diffraction technique proves to be highly efficient in characterizing structural transformation in Ni-Mn-Ga FSMAs, which consist of nearby elements in the periodic table. Our neutron results show that Ni48Mn30Ga22 has a cubic, L-21 Heusler structure from 373 to 293 K. Its crystal structure changes into a seven-layered orthorhombic martensitic structure when cooled to 243 K, and no further transformation is observed upon cooling to 19 K. Neutron diffraction results also show that Ni53Mn25Ga22 has a tetragonal 14/mmm martensitic structure from 20 to 403 K. A pre-transformation around room temperature is observed from an abrupt jump in unit-cell volume of Ni53Mn25Ga22, which corresponds with an endothermic peak detected in a heated DSC curve.

Mechanical behaviours of workhardening and worksoftening bulk metallic glasses

Abstract The workhardening Cu47·5Zr47·5Al5 and the worksoftening Zr52·5Cu17·9Ni14·6Al10·0Ti5·0 bulk metallic glasses before and after precompression deformation were characterised for thermal and mechanical behaviours. The predeformation introduces excessive free volume in both glasses. Cu47·5Zr47·5Al5 and Zr52·5Cu17·9Ni14·6Al10·0Ti5·0 exhibit substantial workhardening and worksoftening behaviours respectively. For Cu47·5Zr47·5Al5, the precompression has a negligible effect on serrations in the plastic flow during nanoindentation, which is related to the hardening of a shear band, while for Zr52·5Cu17·9Ni14·6Al10·0Ti5·0, the precompression moderates serrations in the plastic flow during nanoindentation, which is associated with the softening of a shear band. Strengthening from mechanically induced nanocrystallites at shear bands is responsible for the workhardening of Cu47·5Zr47·5Al5, which overwhelms softening due to the introduction of excessive free volume.