Y. Y. Tse

On the Texture Formation of Selective Laser Melted Ti-6Al-4V

Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of α/β titanium alloys. The relationship between the SLM process parameters and the microstructure of titanium alloys has been the object of several works, but the texture formation during the SLM process has yet to be understood. In the present study, the texture formation of Ti-6Al-4V components was investigated in order to clarify which microstructural features can be tailored during the SLM process. The microstructural characterization of the as-built components was carried out using various microscopy techniques. Phase and texture analysis were carried out using backscattered electron imaging and diffraction. It was found that as-built components consist exclusively of α′ martensitic phase precipitated from prior β columnar grains. The texture of the prior β phase was reconstructed and discussed in relation to the used SLM process parameters. It was found that the β grain solidification is influenced by the laser scan strategy and that the β phase has a strong 〈100〉 texture along its grain growth direction. The α′ martensitic laths that originate from the parent β grains precipitate according to the Burgers orientation relationship. It was observed that α′ laths clusters from the same β grain have a specific misorientation that minimizes the local shape strain. Texture inheritance across successive deposited layers was also observed and discussed in relation to various variant selection mechanisms.

Optimization of the BaZrO3 concentration in YBCO films prepared by pulsed laser deposition

YBCO films were prepared by pulsed laser deposition from nanocrystalline targets doped with different concentrations of BaZrO3 ranging from c = 0.9 to 9.0 wt%. The critical temperature of the films decreases almost linearly with increasing BaZrO3 content whereas the critical current density shows a maximum near 3.9 wt%. In comparison with undoped YBCO films the accommodation field B* is considerably enhanced and the critical current density is improved in high fields in the films doped with BaZrO3, e.g. for c = 3.9 wt% by factor of 4.5 in a field of 5 T at 5 K. In the doped films the BaZrO3 particles grow epiaxially with YBCO. Transmission electron microscopy results show that the density of the BaZrO3 particles increases with increasing doping but the particle size remains practically the same (5–10 nm).

Structural Properties of YBCO Thin Films Deposited From Different Kinds of Targets

The accommodation field of YBa2Cu3O6+infin (YBCO) a thin films depends on the microstructure of the films and specifically the amount of pinning sites, which can be controlled by the type of the target used in the laser deposition. Typical values for the accommodation field at 5 K are 40-100 mT, 160 mT and 500 mT for films deposited from a micrograined, nanograined and optimally BaZrO3 (BZO) doped targets, respectively. We present structural data on YBCO thin films prepared by laser deposition from a nanograined target, a micrograined target and nanograined targets with added BZO. The characterizations were done with XRD and TEM and it was found that the main difference in the undoped films (nano and micro) was the stiffer lattice of the nanofilms. The BZO inclusions were found to induce a large amount of dislocations to the YBCO lattice surrounding the BZO. This can explain the increase observed in the accommodation field.

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111} «uvw» oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.

Microstructure of Ti-6Al-4V produced by selective laser melting

Ti-6Al-4V is the most widely used titanium alloy. Manufacturing of Ti-6Al-4V components using novel additive processing techniques such as selective laser melting is of great interest. This study focuses on the microstructure characterisation of Ti-6Al-4V components produced by selective laser melting (SLM) with full (Ti-6Al-4V base plate) and partial (Ti-6Al-4V needle-shaped bed) support. The starting material, a plasma atomised powder, and the component products are studied using various microscopy techniques including optical, scanning electron and transmission electron microscopy and electron backscattered diffraction (EBSD). Powder particles are fully dense, possess a spherical shape and are composed of acicular α phase. The as-built material shows oriented acicular martensitic phase with well defined columnar grains. The morphology of martensitic phase and microstructural evolution will be discussed in relation to the SLM processing parameters employed and the different cooling rates experienced by the components.

Integrated nanotechnology of pinning centers in YBa2Cu3Ox films

A controlled pinning change from the ab-plane dominant to the c-axis dominant has been achieved in a novel method of nanostructured YBa2Cu3Ox (YBCO) growth. The method is a synchronous self-assembly of BaZrO3 (BZO) and Ag-assisted YBCO nanothreads. The formation of entangled nanothreads increases the critical current density while keeping the critical temperature close to that in pure YBCO films. The nanothreads extend through the whole thickness of thick films, making the method suitable for increasing total critical current density per centimeter of width (Ic − w). Two growth mechanisms, the formation of BZO nanorods and YBCO nanocolumns, complement each other, form a coherent structure and produce samples with strong correlated pinning. In addition to the increase in Ic − w, correlated pinning leads to an increase in vortex melting temperature in a wide range of magnetic fields. The films grown by this method have high Ic − w both in low magnetic fields along the c axis and high magnetic fields in the ab plane of YBCO. Such a superconductor would be suitable for both cable and magnet applications.

Deformation and Recrystallization of Interstitial Free (IF) Steel

The rolling texture of Body Centered Cubic (BCC) low carbon and interstitial free (IF) steel consists of two crystallographic fibers, the α fiber (⟨110⟩ //RD) and the γ fiber (⟨111⟩ //ND). Cells, subgrains, and microbands, where these are not associated with significant lattice curvature, provide the driving forces for recrystallization. However, when these microstructures are associated with significant short range lattice curvature, as they are in deformation or shear bands, they can provide the nucleation sites. The process of recrystallization in IF steel is shown to occur in two stages: the first where the nuclei are contained in the original rolled-out grains belonging to the γ fiber, and where the essential lattice curvature is derived from shear and deformation bands. The second stage involves the impingement/coalescence of several recrystallized grains in the as-rolled envelope of an original hot band grain which provides a super-nucleation event, in which the as rolled boundaries bow out to give an equiaxed microstructure. An elegant proof is provided for this process, along with a simple model based on Strain Induced Boundary Migration (SIBM). The well-known optimal cold rolling, beyond which the drawability is known to degrade, is explained by deformation banding of a particular subset of the grains belonging to the α fiber. Finally, a novel experiment is described, based on the observed deformation banding behavior of crystals belonging to the γ fiber, which leads to very strong γ recrystallization textures, which are desirable in deep drawing.

Two-dimensional growth of SrTiO3 thin films on "001… MgO substrates using pulsed laser deposition and reflection high energy electron diffraction

Two-dimensional epitaxial growth of SrTiO3 is achieved on MgO, a substrate presenting a large lattice mismatch of 7.9%. In situ reflection high energy electron diffraction (RHEED) is used to monitor and control the growth. An interval deposition technique is implemented to force the two-dimensional growth. It is necessary to deposit a single atomic layer of TiO2 before SrTiO3 to prevent the formation of high energy SrO/MgO interfaces and so allow the film to completely wet the substrate surface. Defect formation is inhibited. The resulting film exhibits a similar surface to the substrate and shows a streaky RHEED pattern.

Effect of growth defects on microwave properties in epitaxial Ba0.5Sr0.5TiO3 thin films grown on (001) MgO by pulsed laser deposition

Ba0.5Sr0.5TiO3 (BSTO) films have been grown heteroepitaxially on (001) MgO substrates by pulsed laser deposition (PLD) to fabricate microwave phase shifters for the wide frequency range 45 MHz–50 GHz. Both as-grown and ex situ annealed films have a cube on cube epitaxial relationship with ⟨100⟩BSTO//⟨100⟩MgO. Threading dislocations are the dominant defects, mostly with Burgers vectors b = ⟨101⟩. Growth at 10−1 mbar oxygen pressure, compared to 10−4 mbar, resulted in significantly better properties. Ex situ annealing of the film grown at 0.1 mbar resulted in a reduction of 40% in threading dislocation density and a 40% increase in dielectric tunability.

Deformation banding and origins of rolling and annealing textures in low carbon and interstitial free steels

Abstract This paper is dedicated to the memory of Professor Sir Robert Honeycombe, who examined the first authors PhD thesis on this subject in 1972. The paper reviews some of the very large improvements in the understanding of the formation of textures and microstructures in drawing quality steels in both their deformed and recrystallised states made since the 1970s, focusing in particular on deformation bending in interstitial free steels.