Seeding of single crystal superalloys--Role of seed melt-back on casting defects

Single crystal seeds of the nickel-base superalloy CMSX-4 have been partially melted in a temperature gradient and then quenched. Small islands of random orientation are observed throughout the melted-back semi-solid. These random orientations appear to be pinched-off secondary dendrite arms, but there is no evidence that they are transported ahead of the dendrite tips to nucleate stray grains during directional solidification.

Microstructural development during hot working of Mg-3AI-1Zn

he microstructural evolution is examined during the hot compression of magnesium alloy AZ31 for both wrought and as-cast initial microstructures. The influences of strain, temperature, and strain rate on the dynamically recrystallized microstructures are assessed. Both the percentage dynamic recrysallization (DRX) and the dynamically recrystallized grain size were found to be sensitive to the initial microstructure and the applied deformation conditions. Lower Z conditions (lower strain rates and higher temperatures) yield larger dynamically recrystallized grain sizes and increased percentages of DRX, as expected. The rate with which the percentage DRX increases for the as-cast material is considerably lower than for the wrought material. Also, in the as-cast samples, the percentage DRX does not continue to increase toward complete DRX with decreasing Z. These observations may be attributed to the deformation becoming localized in the DRX fraction of the material. Also, the dynamically recrystallized grain size is generally larger in as-cast material than in wrought material, which may be attributed to DRX related to twins and the inhomogeneity of deformation. Orientation maps of the as-cast material (from electron backscattering diffraction (EBSD) data) reveal evidence of discontinuous DRX (DDRX) and DRX related to twins as predominant mechanisms, with some manifestation of continuous DRX (CDRX) and particle-stimulated nucleation (PSN).

Non-Schmid behaviour during secondary twinning in a polycrystalline magnesium alloy

The present work combines electron backscattering diffraction and Schmid analysis to investigate secondary twinning in the magnesium alloy Mg–3Al–1Zn. Inspection of the misorientations between the parent and ^{{1011} - {1012}} doubly twinned volumes reveals that there are four possible variants. One of these variants (the one that forms a misorientation with the matrix characterized by 38°^⟨1210⟩ ) is favoured much more than the others. This variant involves the activation of secondary twinning systems quite inconsistent with Schmid-type behaviour. For the secondary twin to grow significantly it must take on a shape enforced by the primary twin. This is non-optimal for strain compatibility. It is argued that the 38°^⟨1210⟩ variant occurs most because it provides the closest match between the primary and secondary twinning planes, thus minimizing the compatibility strain.

Cobblestone mesotexture in a nanocrystalline Ni–20Fe electrodeposit

A distinct mesotexture seen in a nanocrystalline Ni–20Fe electrodeposit is described. The texture is characterized by a ^⟨0 0 1^⟩fibre axis perpendicular to the local curvature of the nodule growth surface. Each nodule contains of the order of 10⁸ grains over its growth interface. The texture shows some similarity to traditional cobblestone patterns. Similar forms of mesotextures are likely to be ubiquitous in nanocrystalline electrodeposits and can be expected to affect the homogeneity and, possibly, anisotropy of the mechanical response.

Texture selection mechanisms in uniaxially extruded magnesium alloys

Extrusion textures in magnesium alloys are of high interest due to their influence on yield asymmetry. This data supports work describing three mechanisms of texture selection that may play a role during extrusion. These mechanisms involve localized differences in deformation at the grain level, the change in local environment experienced by grain boundary bulges and shear banding. The work employs visco-plastic crystal plasticity and electron backscattering diffraction.

The five-parameter grain boundary character and energy distributions of a fully austenitic high-manganese steel using three dimensional data

The three-dimensional interfacial grain boundary network in a fully austenitic high-manganese steel was studied as a function of all five macroscopic crystallographic parameters (i.e. lattice misorientation and grain boundary plane normal) using electron backscattering diffraction mapping in conjunction with focused ion beam serial sectioning. The relative grain boundary area and energy distributions were strongly influenced by both the grain boundary plane orientation and the lattice misorientation. Grain boundaries terminated by (1 1 1) plane orientations revealed relatively higher populations and lower energies compared with other boundaries. The most frequently observed grain boundaries were {1 1 1} symmetric twist boundaries with the Σ3 misorientation, which also had the lowest energy. On average, the relative areas of different grain boundary types were inversely correlated to their energies. A comparison between the current result and previously reported observations (e.g. high-purity Ni) revealed that polycrystals with the same atomic structure (e.g. face-centered cubic) have very similar grain boundary character and energy distributions. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Heat affected zone microstructures and their influence on toughness in two microalloyed HSLA steels

Microstructures and Charpy impact properties have been examined in two microalloyed steels following heat treatments to simulate weld heat affected zone (HAZ) structures over a range of heat input conditions, characterised by the cooling time from 800 to 500°C (Δt8/5). The base materials were low carbon structural steel plates microalloyed with vanadium and nitrogen (V-N) and niobium (Nb), respectively. The toughnesses of the HAZs displayed remarkably different behaviours as shown by their impact transition temperatures. For the V-N steel, the toughness improved with increasingly rapid cooling (low heat input conditions) whereas the Nb steel showed an opposite trend. Some of this behaviour could be explained by the presence of coarse ferrite grains in the slowly cooled V-N steel. However, other conditions where all the structures were bainitic and rather similar in optical micrographs gave widely different toughness values. The recently developed method of five dimensional boundary analysis based on electron backscattering diffraction has been applied to these cases for the first time. This showed that the lath boundaries in the bainite were predominantly on {1 1 0} planes of the ferrite and that the average spacing of these boundaries varied depending on steel composition and cooling rate. Since {1 1 0} is also the slip plane in ferrite, it is considered that close spacing between the lath boundaries inhibits general plasticity at stress concentrations and favours initiation of fracture. The differences between the two steels are believed to be due to their transformation behaviours on cooling where precipitation of vanadium nitride in austenite accelerates ferrite formation and raises the temperature of the phase transformation in V-N steels.

Texturas e propriedades reológicas dos minérios de ferro do quadrilátero ferrífero (MG) e sua utilização em beneficiamento

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Microanalysis of epitaxially grown diamond tip array

Electron backscattering diffraction has been applied on polycrystalline diamond films grown using microwave plasma assisted chemical vapour deposition on silicon substrate, in order to provide a map of the individual diamond grains, grain boundary, and the crystal orientation of discrete crystallites. The nucleation rate and orientation are strongly affected by using a voltage bias on the substrate to influence and enhance the nucleation process, the bias enhanced nucleation process. In this work, the diamond surface is mapped using electron backscattering diffraction, then a layer of a few microns is ion milled away exposing a lower layer for analysis and so on. This then permits a three dimensions reconstruction of the film texture.

Grain size measurements in Mg-Al high pressure die castings using electron back-scattered diffraction (EBSD)

Optical metallographic techniques for grain-size measurement give unreliable results for high pressure diecast Mg-Al alloys and electron back-scattered diffraction mapping (EBSD) provides a good tool for improving the quality of these measurements. An application of EBSD mapping to this question is described, and data for some castings are presented. Ion-beam milling was needed to prepare suitable samples, and this technique is detailed. As is well-known for high pressure die castings, the grain size distribution comprises at least two populations. The mean grain size of the fine-grained population was similar in both AZ91 and AM60 and in two casting thicknesses (2 mm and 5 mm) and, contrary to previously published reports, it did not vary with depth below the surface.

NANOSCALE ELECTROCHEMISTRY BY IN-SITU TRANSMISSION ELECTRON MICROSCOPY

Nanoscale research in energy storage has recently focused on investigating the properties of nanostructures in order to increase energy density, power rate, and capacity. To better understand the intrinsic properties of nanomaterials, a new and advanced in situ system was designed that allows atomic scale observation of materials under external fields. A special holder equipped with a scanning tunneling microscopy (STM) probe inside a transmission electron microscopy (TEM) system was used to perform the in situ studies on mechanical, electrical, and electrochemical properties of nanomaterials. The nanostructures of titanium dioxide (TiO2) nanotubes are characterized by electron imaging, diffraction, and chemical analysis techniques inside TEM. TiO2 nanotube is one of the candidates as anode materials for lithium ion batteries. It is necessary to study their morphological, mechanical, electrical, and electrochemical properties at atomic level. The synthesis of TiO2 nanotubes showed that the aspect ratio of TiO2 could be controlled by processing parameters, such as anodization time and voltage. Ammonium hydroxide (NH4OH) treated TiO2 nanotubes showed unexpected instability. Observation revealed the nanotubes were disintegrated into nanoparticles and the tubular morphology was vanished after annealing. The nitrogen compounds incorporated in surface defects weaken the nanotube and result in the collapse of nanotube into nanoparticles during phase transformation. Next, the electrical and mechanical properties of TiO2 nanotubes were studied by in situ TEM system. Phase transformation of anatase TiO2 nanotubes into rutile nanoparticles was studied by in situ Joule heating. The results showed that single anatase TiO2 nanotubes broke into ultrafine small anatase nanoparticles. On further increasing the bias, the nanoclusters of anatase particles became prone to a solid state reaction and were grown into stable large rutile nanoparticles. The relationship between mechanical and electrical properties of TiO2 nanotubes was also investigated. Initially, both anatase and amorphous TiO2 nanotubes were characterized by using I-V test to demonstrate the semiconductor properties. The observation of mechanical bending on TiO2 nanotubes revealed that the conductivity would increase when bending deformation happened. The defects on the nanotubes created by deformation helped electron transportation to increase the conductivity. Lastly, the electrochemical properties of amorphous TiO2 nanotubes were characterized by in situ TEM system. The direct chemical and imaging evidence of lithium-induced atomic ordering in amorphous TiO2 nanotubes was studied. The results indicated that the lithiation started with the valance reduction of Ti4+ to Ti3+ leading to a LixTiO2 intercalation compound. The continued intercalation of Li ions in TiO2 nanotubes triggered an amorphous to crystalline phase transformation. The crystals were formed as nano islands and identified to be Li2Ti2O4 with cubic structure (a = 8.375 Å). This phase transformation is associated with local inhomogeneities in Li distribution. Based on these observations, a new reaction mechanism is proposed to explain the first cycle lithiation behavior in amorphous TiO2 nanotubes.