Basal Orientation Research Articles

Orientation-dependent hardness and nanoindentation-induced deformation mechanisms of WC crystals

The orientation dependence of hardness and nanoindentation-induced deformation mechanisms of differently orientated tungsten carbide (WC) grains in WC–Co hardmetal were studied. Electron backscatter diffraction, atomic force microscopy and scanning electron microscopy investigations were performed to determine the grain orientation, and to study the surface morphology and the resulting deformation fields around the indents. The hardness of the differently orientated WC grains showed significant angle dependence from the basal towards the prismatic directions, but there was only a slight change in hardness between the two types of prismatic orientations ((101¯0) and (21¯1¯0)). Sink-in and pile-up effects, together with highly deformed regions and dislocation steps, were revealed around the imprints in the case of basal and prismatic orientations, respectively. A theoretical model is proposed in which the critical force for slip activation is determined as a function of orientation, based on the possible slip systems of WC. The predictions of the present model concerning the measured hardness values and the deformation field around the indents together with the sink-in effect are in good agreement with the experimental results.

Orientation dependent mechanical properties of commercially pure (cp) titanium

The present investigation is an attempt at correlating the crystallographic orientation and mechanical properties of hexagonal commercially pure titanium (cp-titanium). Annealed cp-titanium sheets are subjected to tensile deformation along the rolling direction, along 45° to the rolling direction and along 90° to the rolling direction respectively. Crystallographic textures and mechanical properties of these cp-titanium samples are investigated in the present study. The hardness of different grains/orientations is estimated through nanoindentation, grain average misorientation, orientation estimated elastic stiffness and Taylor factor measurements. It is observed that the hardness of the grains close to basal orientation is higher compared to non-basal orientations. It is further observed that the estimated bulk mechanical properties of cp-titanium have a direct relationship with the volume fraction of basal grains/orientations.

Microstructure of stir zone in dissimilar friction stir welds of AA6061-T6 and AZ31 alloy sheets

This study was performed to investigate microstructure of dissimilar friction stir welds manufactured with AA6061-T6 and AZ31 alloy sheets. Dissimilar butt joints were fabricated under the ‘off-set’ condition that tool plunge position shifted toward AZ31 from the interface between AA6061-T6 and AZ31. Optimized tool rotating speed and its traveling speed were selected through a lot of preliminary experiments. Electron back-scatter diffraction (EBSD) technique was applied to measure texture in the stir zone (SZ). Grain size distribution and misorientation angle distribution were also obtained. A remarkably fine-grained microstructure was observed in the SZ. Randomized or weaker plane orientations were formed in the SZ of AA6061-T6, while rotated basal plane orientations were concentrated in the SZ of AZ31. Average size of recrystallized grains was measured as just 2.5–4.5 μm. The fraction of high-angle boundary in the SZ of AA6061-T6 increased and that of low-angle boundary in the SZ of AZ31 decreased compared with the base metals.

Effect of Process Variables on the Grain Size and Crystallographic Texture of Hot-Dip Galvanized Coatings

A galvanizing simulator was used to determine the effect of galvanizing bath antimony (Sb) content, substrate surface roughness, and cooling rate on the microstructural development of metallic zinc coatings. Substrate surface roughness was varied through the use of relatively rough hot-rolled and relatively smooth bright-rolled steels, cooling rates were varied from 0.1 to 10 K/s, and bulk bath Sb levels were varied from 0 to 0.1 wt pct. In general, it was found that increasing bath Sb content resulted in coatings with a larger grain size and strongly promoted the development of coatings with the close-packed {0002} basal plane parallel to the substrate surface. Increasing substrate surface roughness tended to decrease the coating grain size and promoted a more random coating crystallographic texture, except in the case of the highest Sb content bath (0.1 wt pct Sb), where substrate roughness had no significant effect on grain size except at higher cooling rates (10 K/s). Increased cooling rates tended to decrease the coating grain size and promote the {0002} basal orientation. Calculations showed that increasing the bath Sb content from 0 to 0.1 wt pct Sb increased the dendrite tip growth velocity from 0.06 to 0.11 cm/s by decreasing the solid–liquid interface surface energy from 0.77 to 0.45 J/m2. Increased dendrite tip velocity only partially explains the formation of larger zinc grains at higher Sb levels. It was also found that the classic nucleation theory cannot completely explain the present experimental observations, particularly the effect of increasing the bath Sb, where the classical theory predicts increased nucleation and a finer grain size. In this case, the “poisoning” theory of nucleation sites by segregated Sb may provide a partial explanation. However, any analysis is greatly hampered by the lack of fundamental thermodynamic information such as partition coefficients and surface energies and by a lack of fundamental structural studies. Overall, it was concluded that the fundamental mechanisms behind the microstructural development of solidified metallic zinc coatings have yet to be completely elucidated and require further investigation.

Enhancing overall tensile and compressive response of pure Mg using nano-TiB2 particulates

A novel attempt is made to synthesize and study the isolated effects of less than two volume fraction TiB2 nanoparticulates (60nm) on pure magnesium. New light weight Mg–TiB2 nanocomposites with superior mechanical properties compared to pure magnesium are synthesized using disintegrated melt deposition technique followed by hot extrusion. The microstructural characterization studies revealed that the samples exhibited fairly uniform distribution of TiB2 nanoparticulates with minimal porosity and good interfacial integrity between Mg matrix and TiB2 particulates. The coefficient of thermal expansion results indicates that the addition of 0.58, 0.97, and 1.98vol.% TiB2 nanoparticulates marginally improves the dimensional stability of pure magnesium. A significant improvement in the room temperature tensile properties of pure magnesium was observed with the addition of less than two volume fraction TiB2 nanoparticulates. The synthesized Mg 1.98vol.% TiB2 nanocomposite revealed the best room temperature tensile properties with a significant increase in the 0.2% tensile yield strength by ~54%, ultimate tensile strength by ~15% and fracture strain by ~79% when compared to pure Mg. The X-ray diffraction studies indicated changes in the basal plane orientation of pure Mg with the addition of nano-TiB2 particulates. A maximum tensile fracture strain of ~16% is achieved with the addition of 0.97vol.% TiB2. The room temperature compressive properties of the nanocomposites reveal that the addition of 1.98 TiB2 increases the 0.2% compressive yield strength of Mg by ~47% and ultimate compressive strength by ~10% with a marginal increase in the fracture strain (~11%). Reduction in tensile–compression yield asymmetry was observed for Mg 0.58 & 0.97vol.% TiB2 nanocomposites which can be attributed to the weakening of the strong basal texture of pure Mg.

Room Temperature Screw Form Rolling of AZ91D Magnesium Alloy through Processing by Extrusion-Torsion Simultaneous Working

Extrusion-torsion simultaneous processing is a very attractive technique for fabricating a rod-shape material with fine grain and random texture. We have proposed a new screw form rolling process combined with preliminary extrusion-torsion simultaneous working. Microstructure evolution and mechanical property change of AZ91D magnesium alloy during extrusion-torsion simultaneous processing was examined through microstructure observation, X-ray diffraction analysis and micro-Vickers hardness measurement. By the addition of torsion, the crystal orientation of AZ91D magnesium alloy workpiece was drastically changed from basal crystalline orientation to the random orientation. Crystal grain occurred through the dynamic recrystallization and tended to coarsen with an increase of extrusion-torsion temperature. Grain refinement under 2 um was achieved at the lowest extrusion-torsion temperature of 523 K. M8 gauge AZ91D magnesium alloy screw was successfully formed at room temperature using the extrusion-twisted workpiece preliminary solution treating at 678 K for 345.6 ks. It was found that the extrusion-torsion temperature of 678 K must be selected to fabricate the good screw without any defects.

Method for the Depletion of the Basal Texture in Rolled AZ31 Magnesium Sheets

The deformation and hardening mechanisms of magnesium usually lead to a typical basal orientation of crystals during a production of sheets by forming techniques. The basal texture related anisotropic property behavior and especially the further decrease in formability at room temperature is disadvantageous and undesired for subsequent rolling and final forming processes.The objective of this work is to find methods to improve these texture-related properties and the cold forming ability of magnesium sheets. Firstly, rolling at different temperatures and pass reductions, with the goal of weakening the basal texture component in semi-finished products is investigated, based on the advantageous initial texture and microstructure of twin-roll-cast (TRC) magnesium strips. In this context, texture and microstructure development is examined after a particular multi-pass rolling and heat treatment processes. Twin-roll-cast magnesium strips of alloy AZ31, with an initial thickness of 4.5 mm, rolled to a final thickness of 1.2 mm, are used as feedstock.Secondly, a new thermo-mechanical magnesium strip treatment has been developed in order to completely disintegrate the basal texture and intentionally generate only non-basal orientations with high Schmid-factors for the easy-to-activate basal slip systems. This process, which is designed as a final strip treatment, has been investigated regarding its texture change effect on rolled 1.2 mm AZ31 sheets, which also originate from TRC feedstock.It has clearly been found that the developed rolling technology for TRC feedstock leads to a significantly reduced basal texture due to grain boundary rotation and recrystallization at those rotated regions. The application of the separately developed strip treatment effects a complete elimination of the basal texture in a large volume of the sheet. Applying both technologies on magnesium sheets results in a tremendous increase in formability at room temperature as a consequence of the altered texture.

Effect of the basal plane orientation on Al and Zn diffusion in hcp Mg

Diffusion couple experiments for Mg–Al and Mg–Zn systems with polycrystalline Mg were performed between 638 and 693K (365 and 420°C) and 553 and 593K (280 and 320°C), respectively, to determine the diffusion of Al and Zn in Mg crystal depending on the orientation of basal plane of hcp Mg. Solute (Al and Zn) diffusion coefficient decreases elliptically in Mg crystal from the basal plane towards normal to the basal plane.

The Extrusion Behavior of Mg Alloys Interpenetrating with Stainless Steel Wire

The interpenetrating magnesium composites reinforced by three-dimensional braided stainless steel wire reinforcement were fabricated. And, the deformation behavior of materials was analyzed in four extrusion velocities by DEFORM-3D software. The results show that with the increases of extrusion velocities, the equivalent stress values exhibit a gradually increasing and then decreasing trend. Owing to the effect of three dimensional reinforcement, the basal plane orientation occur tilt. And, the microstructure turns refined.

Microstructural and textural evolution of pure titanium during differential speed rolling and subsequent annealing

The microstructural and textural evolution of pure titanium during differential speed rolling (DSR) at 500 °C and subsequent annealing were investigated using electron backscattered diffraction analysis. Twinning only occurs in the initial stage of DSR, and further deformation is dominated by dislocation slip. The as-rolled microstructure is characterized by large deformed grains, which form a major component of the material, and some dynamically recrystallized (DRXed) grains mainly existing in shear band regions. A dramatic change in the rolling texture, from the transverse direction (TD) split texture with basal poles largely tilted at ±40° to the single-peak basal texture, occurs during DSR. This change in texture is accelerated during the late stage of DSR and may be attributed to the gradual lattice rotation of deformed grains caused by enhanced basal slip activity. The basal texture changes back to the TD-split texture with basal poles tilted at ±25° after annealing due to consumption of deformed grains with basal orientations from DRXed grains with the TD-split texture. Subsequent extensive grain growth changes the orientation of the a-axis from parallel to the TD to parallel to the rolling direction, which is the result of the preferential grain growth in that direction.

Texture evolution during annealing of AZ31 Mg alloy rolled sheet and its effect on ductility

Magnesium sheet alloys have huge potential in automotive closure applications due to their light weight. However, their limited room temperature formability restricts their widespread applications. Microstructure (both geometric and crystallographic) plays a key role in dictating the formability of an alloy. In this research, a systematic study of the microstructure evolution of a hot-rolled AZ31 sheet during thermal annealing was carried out, and the effect of microstructure on room-temperature deformation was characterized. The initial hot-rolled sheet had a dominant basal texture with equiaxed grains and shear bands. The annealing treatment imparted to the sheet alloy triggered an anomalous, inhomogeneous grain growth in grains with a large deviation from the basal orientation. At 350°C annealing temperature, a bimodal grain size distribution of grains with basal and non-basal orientations was observed, and this combination was shown to significantly enhance ductility resulting from a complex interplay between slip and twinning deformation mechanisms. This key finding is expected to contribute to the development of engineered microstructures for enhanced room temperature formability in this alloy.

Quantitative Analysis on Contribution of Extension Twinning to Plastic Deformation of Mg Alloy by in-situ Tracking on Grains Orientation

The orientation evolution of Mg alloy grains was in-situ tracked during compression at 100°C by the method of electron backscatter diffraction, and meanwhile, quantitative analysis was also conducted on activations of slips and twinning. Effects of extension twinning activation on slips activations were analyzed during deformation based on calculation of variations of Schmid factor for different slips before and after twinning. It is predicted that there are not distinct changes for Schmid factor of basal slips before and after twinning for Mg alloy grains with basal textured orientation, while the Schmid factor of prismatic slips decreases distinctly after twinning and accordingly they are hard to be activated during the subsequent deformation. Simultaneously, the Schmid factor of pyramidal slips increases distinctly after twinning and it is relatively easy for them to be activated during the subsequent deformation.

Integrins and epithelial cell polarity.

Cell polarity is characterised by differences in structure, composition and function between at least two poles of a cell. In epithelial cells, these spatial differences allow for the formation of defined apical and basal membranes. It has been increasingly recognised that cell-matrix interactions and integrins play an essential role in creating epithelial cell polarity, although key gaps in our knowledge remain. This Commentary will discuss the mounting evidence for the role of integrins in polarising epithelial cells. We build a model in which both inside-out signals to polarise basement membrane assembly at the basal surface, and outside-in signals to control microtubule apical-basal orientation and vesicular trafficking are required for establishing and maintaining the orientation of epithelial cell polarity. Finally, we discuss the relevance of the basal integrin polarity axis to cancer. This article is part of a Minifocus on Establishing polarity.

Microstructural evolution and mechanical properties of Mg composites containing nano-B4C hybridized micro-Ti particulates

In this work, the microstructural evolution and mechanical properties of extruded Mg composites containing micro-Ti particulates hybridized with varying contents of nano-B4C are investigated, and compared with Mg-5.6Ti. Microstructural characterization showed the presence of uniformly distributed micro-Ti particles embedded with nano-B4C particulates that resulted in significant grain refinement. Electron back scattered diffraction (EBSD) analyses of Mg-(5.6Ti + x-B4C)BM hybrid composites showed that the addition of hybridized particle resulted in relatively more recrystallized grains, realignment of basal planes and extension of weak basal fibre texture when compared to Mg-5.6Ti. The evaluation of mechanical properties indicated improved strength with ductility retention in Mg-(5.6Ti + x-B4C)BM hybrid composites. When compared to Mg-5.6Ti, the superior strength properties of the Mg-(5.6Ti + x-B4C)BM hybrid composites are attributed to the presence of nano-reinforcements, the uniform distribution of the hybridized particles, better interfacial bonding between the matrix and the reinforcement particles and the matrix grain refinement achieved by nano-B4C addition. The ductility enhancement obtained in hybrid composites can be attributed to the fibre texture spread and favourable basal plane orientation achieved due to nano B4C addition.

Fundamental aspects of HCOOH oxidation at platinum single crystal surfaces with basal orientations and modified by irreversibly adsorbed adatoms

Oxidation of formic acid at Pt(hkl) electrodes with basal orientations and modified by irreversibly adsorbed adatoms is revisited. It was shown that the adatoms of the nitrogen group enhance the electrocatalytic activity through long-range electronic effects in the case of Pt(111) substrates or through shorter third-body effects when adsorbed at Pt(100) electrodes. In both cases, it appears that free platinum sites were required. Special attention is given here to the reactivity at Pt(110) substrates. It is found that maximum electrocatalysis is observed at fully blocked surfaces. This result points out again structure sensitivity effects for this characteristic reaction. Unlike the more compact planes, in which the enhancement of the oxidation rate was explained through essentially rigid models for the adlayer, it is suggested that the resulting enhancement observed at Pt(110) substrates could be explained if some adatom mobility is considered.

Application of Electron Backscatter Diffraction (EBSD) for Crystallographic Characterization of Aluminum-Doped Zinc Oxide Sputtered Films

Zinc Oxide is a wide band-gap compound semiconductor that has been used in optoelectronic and photovoltaic applications due to its good electrical and optical properties. Aluminium has been an efficient n-type dopant for ZnO to produce low resistivity films and high transparency to visible light. In addition, the improvement of these properties also depends on the morphology, crystalline structure and deposition parameters. In this work, ZnO:Al films were produced by d.c. pulsed magnetron sputtering deposition from a ZnO ceramic target (2.0 wt% Al2O3) on glass substrates, at a temperature of 250 ºC.The crystallographic orientation of aluminum doped zinc oxide (ZnO:Al) thin films has been studied by Electron Backscatter Diffraction (EBSD) technique. EBSD coupled with Scanning Electron Microscopy (SEM) is a powerful tool for the microstructural and crystallographic characterization of a wide range of materials.The investigation by EBSD technique of such films presents some challenges since this analysis requires a flat and smooth surface. This is a necessary condition to avoid any shadow effects during the experiments performed with high tilting conditions (70º). This is also essential to ensure a good control of the three dimensional projection of the crystalline axes on the geometrical references related to the sample.Crystalline texture is described by the inverse pole figure (IPF) maps (Figure 1). Through EBSD analysis it was observed that the external surface of the film presents a strong texture on the basal plane orientation (grains highlighted in red colour). Furthermore it was possible to verify that the grain size strongly depends on the deposition time (Figure 1 (a) and (b)). The electrical and optical film properties improve with increasing of the grain size, which can be mainly, attributed to the decrease in scattering grain boundaries which leads to an increasing in carrier mobility (Figure 2).The authors kindly acknowledge the financial support from the Portuguese Foundation for Science and Technology (FCT) scientific program for the National Network of Electron Microscopy (RNME) EDE/1511/RME/2005.

Microstructure Characterization of ZK60 Magnesium Alloys Using TEM and HR-EBSD

ZK60 (Mg-Zn-Zr) alloys exhibited a variation in precipitates with aging, and their mechanical properties also changed. Microindentation tests were carried out on two types of ZK60 alloys of solid solution (T4) and peak aging (T6). Microstructure and texture evolution during indentation was investigated using electron backscatter diffraction. Twinning occurred near the indentation marks in most grains. It was found that tensile twinning was dominant, and two twin variants were usually observed. Texture and microstructure evolution by twinning and slip activation was further examined by uniaxial compression test with strain. The initial random orientation gradually changed into basal fibers with strain. Some grains with nonbasal orientations aligned with the loading direction easily underwent twinning followed by slip deformation. Other grains near basal orientations revealed only slip deformation.

Texture evolution in AZ91 alloy after hot rolling and annealing

A commercial AZ91 magnesium alloy with a random initial texture was subjected to homogenization heat treatment (solution annealing) at 450 °C. This resulted in the appearance of a weak triple (0001)–(10 1¯0)–(11 2¯0) fibber texture. After hot rolling at 400 °C, the deformation texture was typically (0001) basal with splitting of the basal poles in the rolling direction. After annealing at 450 °C for 36 h, the annealing texture was the retained deformation texture with more homogeneous distribution of poles around the ideal basal orientation. There was no trace of abnormal grain growth.

Influence of microstructural evolution on mechanical behaviour of AZ31 alloy sheet processed by flat extrusion container

Influence of microstructural evolution on the mechanical behaviour of AZ31 alloy sheet processed by flat extrusion container is analysed upon annealing treatments. The basal texture is significantly increased as a result of the formation of new grains with a largely altered c axis orientation relative to the initial basal orientation owing to static recrystallisation. The results reveal that the ductility of the alloys without annealing treatments is the highest, but the strength is the lowest, which confirms the strong anisotropy of stress–strain behaviour with respect to the initial textures. Significant differences in the stress–strain curves are observed for the as received and annealed specimens.

Humidity resistant MoS2 coatings deposited by unbalanced magnetron sputtering

MoS2 is a suitable solid lubricant for environments free of oxygen or water vapor (i.e. vacuum). Humid air degrades film properties due to oxidation accompanied by high wear and increasing coefficients of friction. The present study aims at the further development of sputtered pure MoS2 coatings, extending their applicability to varying environmental conditions by increasing the resistance against humidity. The systematic coating development process is supported by using an experimental Box–Behnken design with variations of the deposition parameters cathode voltage, target/substrate distance, temperature and argon gas pressure. In contrast to common one-factor-at-a-time (OFAT) studies, this approach enables a determination of interactions between deposition process parameters and tribological–mechanical MoS2 film properties. The tribological improvement focuses on a maximization of wear resistance in air and vacuum measured in ball-on-disk experiments. The evaluated mechanical properties are hardness, elastic modulus and residual stresses. These stresses were determined by the substrate curvature method. The study reveals that the residual stress state in the films and the hardness-to-modulus ratio are crucial for their tribological performance in humid air and vacuum environments. After a detailed determination of the relationships between deposition conditions and film properties, some selected microstructural analyses are presented which show that a substantially basal orientation of the lattice has positive effects on wear but also causes anisotropic film properties which result in fissile fracture of the coating if strong shock or point loads occur.