J. Y. Huang

Microstructures and dislocation configurations in nanostructured Cu processed by repetitive corrugation and straightening

Abstract The microstructures and dislocation configurations in nanostructured Cu processed by a new technique, repetitive corrugation and straightening (RCS), were studied using transmission electron microcopy (TEM) and high resolution TEM. Most dislocations belong to 60° type and tend to pile up along the {111} slip planes. Microstructural features including low-angle grain boundaries (GBs), high-angle GBs, and equilibrium and non-equilibrium GBs and subgrain boundaries were observed. Dislocation structures at an intermediate deformation strain were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as isolated dislocation cell (IDC), dislocation tangle zones (DTZs), and uncondensed dislocation walls (UDWs).

Microstructure of cryogenic treated M2 tool steel

Cryogenic treatment has been claimed to improve wear resistance of certain steels and has been implemented in cutting tools, autos, barrels etc. Although it has been confirmed that cryogenic treatment can improve the service life of tools, the underling mechanism remains unclear. In this paper, we studied the microstructure changes of M2 tool steel before and after cryogenic treatment. We found that cryogenic treatment can facilitate the formation of carbon clustering and increase the carbide density in the subsequent heat treatment, thus improving the wear resistance of steels.

Nanostructures and deformation mechanisms in a cryogenically ball-milled Al-Mg alloy

An Al-7.6 at.% Mg alloy was ball milled in liquid N2 for 8 h and its microstructures were investigated using transmission electron microscopy. Electron diffraction confirmed that the resulting powder is a supersaturated Al-Mg solid solution with an fcc structure. Three typical nanostructures with different grain-size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small-angle subboundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10-30 nm. In very small crystallites with dimensions less than 10 nm, twinning becomes an important deformation mechanism. The reasons for the different deformation mechanisms were discussed. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated in detail.

Development of repetitive corrugation and straightening

In this paper, we present recent developments in repetitive corrugation and straightening (RCS), a new severe plastic deformation (SPD) technique. Two refinements of the original RCS method are presented and results are shown for commercial purity copper that illustrate the associated improvements in the effectiveness of nanostructuring. Second-generation tooling was implemented using a bench scale rolling mill for continuous processing of sheet and bar. We have found that this design does not produce enough plastic strain per RCS cycle for effective grain refinement prior to the formation and growth of fatigue cracks. Third-generation tooling was designed to process sheet and increase the amount of shear deformation per iteration. The third-generation tooling design introduced significant shear strain and was found to be effective in grain refinement.

Influence of microstructures and crystalline defects on the superconductivity of MgB2

This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at higher fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10u200a000u2009A/cm2 in 50u200a000 Oe (5 T) at 5 K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H>45u200a000u2009Oe(4.5u2009T) and T=5u2009K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2...

Mg(B,O)2 precipitation in MgB2

MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), x-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long e...

Controlling flux pinning precipitates during MgB2 synthesis

MgB2 samples prepared by three different sets of synthesis parameters were investigated using transmission electron microscopy. Results suggest that oxygen dissolved in bulk MgB2 at high synthesis temperatures when the MgB2 samples were exposed to trace amount of oxygen from flowing ultrahigh purity Ar gas. The lower solubility of oxygen in MgB2 at a lower temperature led to the precipitation of nanometer-sized coherent Mg(B,O) in the interior of MgB2 grains during subsequent cooling. The precipitates, which act as effective flux pinning centers, are of composition-modulated structures with the same basic crystal lattice and orientation as the MgB2 matrix. This study has demonstrated the potential of tailoring the size and distribution of Mg(B,O) precipitates through optimizing synthesis parameters for optimum flux pinning.

Grain boundary structure of nanocrystalline Cu processed by cryomilling

The microstructures of cryogenically ball-milled Cu were investigated by high-resolution electron microscopy. It was found that the grain-size reduction is a dislocation-controlled continuous process which consists of the formation of small-angle grain boundaries (GBs), a gradual increase in misorientations as a result of accumulation of more dislocations and, finally, the formation of large-angle GBs. The GBs were generally curved, wavy or faceted, and heavily strained, which are typical characteristics of nanostructured materials. In addition, extrinsic dislocations were found in many GBs, indicating that most are in a high-energy non-equilibrium configuration, which is consistent with observations in equal-channel angular pressing processed Cu, Ni, and Al-Mg, repetitive corrugation and straightening processed Cu and room-temperature ball-milled Cu. These results support a still-disputed concept that GBs in nanostructured metals processed by severe plastic deformation are mostly in non-equilibrium states.

High-resolution transmission electron microscopy study of defects and interfaces in epitaxial TiO2 films on sapphire and LaAlO3

Abstract TiO2 thin films grown on sapphire (α-Al2O3) (1102) and on LaAlO3 (001) at 800° C by pulsed-laser deposition have been characterized by transmission electron microscopy. The TiO2 thin films grown on LaAlO3 (001) crystallize in anatase, while those on sapphire (1102) crystallize in rutile. The epitaxial relationship between the anatase thin films and the LaAlO3 substrate is (004)[110]A//(001)[110]L, where the subscripts A and L indicate anatase and LaAlO3 respectively, and that between the rutile thin films and the sapphire substrate is (011)[111]R//(1102)[202 1]S, where the subscripts R and S indicate rutile and sapphire respectively. The rutile thin films contain a high density of growth twins with (101) and (011)T being the twin planes and [101] being the twinning direction, where the subscript T represents a twinning plane. The atomic structure of the twin boundary, the anatase-LaAlO3 and the rutile-sapphire interfaces were investigated by high-resolution transmission electron microscopy in combination with image simulations. Neither of the two epitaxial systems had a good lattice match between film and substrate. Growth mechanisms are discussed, on the basis of information on the atomic structure of the interfaces. Investigation also revealed that substantial similarity existed in the local atomic patterns of the substrate and the film for both systems, which is attributed to the formation of different TiO2 phases in different substrates.

Photovoltaic response and dielectric properties of epitaxial anatase-TiO2 films grown on conductive La0.5Sr0.5CoO3 electrodes

We have grown epitaxial anatase-TiO2 (001) films on La0.5Sr0.5CoO3 (001) bottom electrodes using pulsed-laser deposition. The small lattice mismatch (0.5%) between the anatase-TiO2 and the La0.5Sr0.5CoO3 makes it possible to grow anatase-TiO2 films with excellent crystallinity on conductive metal oxides. The photovoltaic properties of the epitaxial anatase-TiO2 on the La0.5Sr0.5CoO3 were characterized using a Kelvin probe. The optical band-gap energy was found to be 3.05 eV. The dielectric properties of the epitaxial anatase-TiO2 films were characterized using a capacitor structure of Au/anatase-TiO2/La0.5Sr0.5CoO3 on a LaAlO3 substrate. The dielectric dispersion exhibited a power-law dependence, and the dielectric constant measured at room temperature and 1 MHz was 38.