Evolution Of Grain Shape Research Articles

Transformation of coercivity mechanism and evolution of microstructure in high performance Nd-Dy-Fe-Co-B thick films

It is important to understand the relationship between coercivity and microstructure for the development of high performance rare-earth permanent magnets. In this paper, the effects of film thickness on perpendicular magnetic anisotropy, coercivity mechanism, magnetization reversal process and microstructure of Nd-Dy-Fe-Co-B thick films are systematically investigated by varying film thickness from 1 to 12 µm. It is found that the coercivity mechanism transforms from a mixed type dominated by pinning to another mixed type dominated by nucleation. In addition, the evolution of grain shape from irregular spherical crystals to columnar ones occur at film thickness of 3 µm. The columnar crystals form on the capping layer and grow toward the buffer layer. When the film is thicker than 6 µm, Nd elements accumulate on the Ta buffer layer. With increasing the film thickness, the columnar crystal structure can grow throughout the whole thick film, and the Nd-rich phase is extruded onto the buffer layer, while the pinning effect weakens and the nucleation mechanism is dominated. This work can help us better understand the relationship between coercivity mechanism and microstructure of thick rare-earth permanent magnetic films.

Copper doping effect on the properties in ZnO films deposited by sol–gel

In this work, an aqueous sol–gel route involving zinc acetate dihydrate and cupric chloride as precursors was used to obtain intrinsic and Cu-doped ZnO thin films onto SiO2/n-Si and glass substrates. Various concentrations of Cu ranging from 1 to 5% were used to modify the ZnO crystal structure. The effect of Cu-doping was evaluated by means of various microscopic and spectroscopic techniques. Scanning electron microscopy (SEM) proved increased grain density and evolution of grain shape from circular to hexagonal with Cu incorporation in the ZnO host lattice. Furthermore, atomic force microscopy (AFM) revealed that grain size increases with increasing Cu concentration. Moreover, X-ray diffraction (XRD) investigations reported a small influence of Cu-doping on the lattice constant value. However, Cu-doping level impacts the mean crystallite size, and consequently, the amount of dislocations emerging from the boundaries of the mosaic blocks. Finally, optical measurements proved the decrease of film transparency, as well as bandgap narrowing effect with increasing dopant concentration from 1 to 5%. As a result, copper incorporation into ZnO films using the proposed low-cost sol–gel method has the ability to tune the bandgap of ZnO-based materials and to promote extra charge carrier generation, which is highly beneficial for UV optoelectronic applications.

Numerical investigation of the effect of rate-sensitivity, non-octahedral slip and grain shape on texture evolution during hot rolling of aluminum alloys

Based on texture simulations with a viscoplastic self-consistent polycrystal algorithm, this contribution presents a comprehensive investigation of the effect of the rate-sensitivity exponent, the relative strength of non-octahedral slip modes as well as the grain shape on texture evolution. The simulations are validated with texture measurements of samples from the industrial hot rolling process at different rolling reductions. Simulated textures at different hot rolling temperatures are compared with experimental findings from the literature. The present investigation indicates, that texture simulations must include three non-octahedral slip systems, use temperature dependent rate-sensitivity exponents and account for the evolution of grain shape.

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

The strength of foliations defined by shape preferred orientation of plagioclase in troctolitic cumulates from the Layered Series of the Skaergaard intrusion, and the Rum Eastern Layered Intrusion, increases as the grains become more tabular, due either to the greater propensity of highly non-equant grains to be re-arranged by magmatic currents or tectonic disruption of poorly consolidated mush, or by the effects of a pre-existing shape preferred orientation on final grain shape in fully solidified rocks. The stratigraphic evolution of grain shape, microstructures and fabrics in the lowest 320 m of the Skaergaard Layered Series records the progressive inflation of the chamber to its final size. During the earliest stages of solidification, the extent of in situ nucleation and growth on the chamber floor decreased upward through the stratigraphy, due to the development of a thermally insulating blanket of mush on the floor. An upward increase in foliation strength as the chamber inflated to its final size was a result of the increasing strength of convection of the bulk magma and an increasing contribution to the floor mush of crystals derived from the walls of the enlarging magma chamber. Plagioclase in the troctolites in the open-system magma chamber of the Rum Eastern Layered Intrusion is generally more equant than that in the Skaergaard intrusion, perhaps related to the slower crystal growth on the margins of the continuously replenished Rum chamber. Significant sub-solidus modification of original igneous microstructures is observed in Rum troctolites from parts of the stratigraphy recording frequent replenishment events.

Size Effect in the Uniaxial Compression of Polycrystalline Ni Nanopillars with Small Number of Grains

Molecular dynamics (MD) simulations are used to investigate the compression of nickel nanopillars in the ranges from 3 to 18 nm grain size (d) and from 12 to 30 nm specimen size (D). The results reveal that grain size play a more significant role than specimen size. There is a strong grain size effect—smaller is weaker—on the yield as well as the flow stress. The deformation is mainly governed by grain boundary (GB) motion for d < 12 nm, while for $$ d \ge 12\;{\text{nm}} $$ , it is governed by dislocation activity. When the grain size is small enough, the deformation of nanopillar is specimen size independent. For larger grain sizes, an irregular fluctuation in flow stress is observed resulting from sensitivity to grain orientation, fracture, and coalescence. Extended dislocations and twins play important roles in grain-shape evolution during compression. Due to the high fraction of surface grains, the “theory based on GB-shear and surface layer” is found inapplicable to explain the size effect in the plasticity of specimens with small number of grains across their diameter. Consequently, an expression of flow stress is proposed that includes effects of grain and specimen sizes, and the specimen-to-grain size ratios.

Nano-Heteroepitaxy: An Investigation of SiGe Nano-Pillars Coalescence

Si/SiGe heterostructures are used in novel device architectures such as stacked nanowires [1] or neuromorphic memories [2]. Epitaxial growth of SiGe on Si is not lattice matched (4.2% difference between Si and Ge), so controlling strain relaxation in thick layers is important. We have recently shown that nano-heteroepitaxy yielded smooth and fully relaxed SiGe layers with a thickness of only a few hundreds of nm [3]. These could then be used as templates for tensile strained Si layers. However, the generation of defects such as stacking faults or twins was inherent to the process itself regardless of the nature of the nano-pillars (Si or SiGe). It is therefore likely that these defects occur during the coalescence process itself, during which adjacent pillars merge in order to form a 2D layer. In this work, we show the characteristics of our SiO2 based nano-template and investigate SiGe nano-pillars at different stages of the coalescence process. We have studied the coalescence of Si0.75Ge0.25 nano-pillars in a 300 mm industrial Reduced Pressure-Chemical Vapour Deposition tool. Dichlorosilane and germane were used to grow these structures at 700°C and a pressure of 20 Torr. The mean growth rate was around 47 nm/min. An integration scheme based on diblock copolymer patterning was used in order to fabricate a nano-template with a honeycomb pattern, with a pitch of 35 nm (see Figure 1). Selective epitaxy using a chlorinated chemistry of SiGe nano-pillars was investigated for different thickness, with a surface preparation based on a Siconi NH3/NF3-based remote plasma treatment [4]. SiGe nano-pillar merging was first examined with Atomic Force Microscopy (see Figure 2). Starting at 30 nm, the shape of the grains takes various forms depending on the number of merging nano-pillars, making the coalescence process heterogeneous in terms of shape evolution. As expected, an increase in average grain diameter and a decrease in the number of grains per unit surface area occurs as the deposited thickness increases (see Figure 3). The coalescence degree (i.e. the inverse of the number of grains) allowed us to predict a full coalescence thickness at ~80 nm. X-Ray Diffraction was performed, with omega-2theta scans performed around in-plane and out-of-plane orders to investigate the structural properties of the nano-pillars at various degrees of coalescence. This showed an increasing macroscopic degree of strain relaxation with thickness, confirming that growth from nano-cavities induces a fast relaxation. Scanning Spreading Resistance Microscopy showed a good sensitivity concerning the electrical activity of the nano-pillars (see Figure 4). At 20 nm, the electrical distinction between nano-pillars and the oxide mask was straightforward. The resistance of individual pillars was otherwise rather uniform. At 35 nm, we detect local resistivity variations within the coalescing nano-pillars, which are likely due to electrically active defects. We used cross-sectional Transmission Electron Microscopy to better understand the generation of structural defects during coalescence. Imaging of the 20 nm thick sample showed that individual nano-pillars were facetted and defect free (see Figure 5). The analysis of the 35 nm thick sample showed various cases ranging from defect free nano-pillar merging to the generation of stacking faults and twinning at the early stages of coalescence (see Figure 6). In conclusion, SiGe nano-pillars coalescence was investigated using a 300 mm industrial Reduced Pressure-Chemical Vapour Deposition tool. An integration scheme based on diblock copolymer patterning was used to provide nanometer-scaled templates for the epitaxy of SiGe nano-pillars. In order to study their merging, growth of nano-pillars with thicknesses ranging from 20 to 35 nm was performed and samples characterized by AFM, XRD, SSRM and TEM. Results showed evolution of grain shape, size and number, and high degrees of macroscopic strain relaxation were obtained even in the emerging nano-pillars. Defects such as stacking faults and twins were identified at the early stages of nano-pillars coalescence.

Effect of the grain shape on the q-value evolution of steel sheets

Two polycrystalline plasticity models, namely the ALAMEL model and the viscoplastic self-consistent (VPSC) model were used to study the effect of the grain shape on the evolution of plastic anisotropy. The latter was characterized by the q-value in this paper. The two models both predict the evolution of grain shape and texture and then can predict the effect of the initial grain shape on the texture. The final anisotropy of the materials was also assessed with the help of these models. Data of tensile tests in three directions for three low carbon steel sheets with either equiaxed or a pancake grain shape were used to evaluate the model predictions. Both models give the same trend of the grain shape effect on the evolution of the q-values. Results demonstrate that the effect of the grain shape can be clearly reflected in the x-value distribution as defined and discussed in this paper. But it is not so obvious in the predicted textures for the investigated three materials according to both models.

Evolution of grain grading and characteristics in repeatedly reconstituted assemblages subject to one-dimensional compression

One-dimensional compression tests were carried out on repeatedly reconstituted assemblages that were uniformly graded prior to the first compression. Four stress levels were adapted to observe the interplay among grain shapes and grading, overburden stress and the number of remouldings. It was found that a stable limiting grading can be obtained after several loadings on repeatedly remoulded specimens. The position of this limiting grading is a function of the stress level. The evolution of grain shape with remoulding was found to depend on the magnitude of the applied stress. The grading with a fractal dimension of 2·65 can only be achieved by loading the repeatedly reconstituted specimen at an elevated stress. Stable shape characteristics of the grains were observed at the fractal grading. The amount of breakage of an assemblage is described by the relative breakage Br and the evolution of Br of the virgin compressed and repeatedly compressed specimens with stress is discussed.

Polycrystal plasticity models based on crystallographic and morphologic texture: Evaluation of predictions of plastic anisotropy and deformation texture

Two micro–macro scale transition models, namely the ALAMEL and visco-plastic self-consistent (VPSC) models (both predict the evolution of grain shape and texture), were used to study the effect of the initial grain shape. The evolution of plastic anisotropy was characterized by the q-value and was simulated in three directions for three low carbon steels assuming either equiaxed (hypothetical one) or a pancake (measured one) grain shape. In addition, texture evolution during rolling was investigated for one of the materials. Comparisons between the model predictions and the measured values demonstrate that the grain shape effect can be predicted well by both models. In most cases, the initial q-value can be nicely estimated by both models. VPSC is better than ALAMEL on q-value evolution prediction for simulations which consider the measured aspect ratios. For texture evolution of rolling, the effect of initial grain shape is only significant at large deformation.

Orientation control of textured SrTiO3 thin films on platinized α-Al2O3 (0 0 0 1) by an ion beam sputter deposition method

SrTiO3 (STO) thin films were grown on Pt/Al2O3 substrates without a Ti adhesion layer by an ion beam sputter deposition method in a range of growth temperatures between 600 and 750 °C. The effect of growth temperature on the film orientation was investigated by high-resolution x-ray diffraction, pole figure measurements and in-plane grazing incidence x-ray diffraction. The film orientation showed a strong dependence on the growth temperature. The films exhibited a predominant (1 1 0) orientation at a low growth temperature of 600 °C. With an increase in growth temperature to 750 °C, a highly (1 1 1)-textured STO film with two different orientation variants was achieved on the Pt/Al2O3 substrate. A narrow full-width at half-maximum of 0.12° for the rocking curve measured on 2 2 2STO reflection and a six-fold symmetry from {1 0 0}STO and {1 1 0}STO pole figures were observed. Three-dimensional island growth mode was observed on the surfaces of all films, as investigated with an atomic force microscope. The evolution of grain shape and size was apparently found with an increase in growth temperature. The root-mean-square roughness of the STO film grown at 750 °C was raised to be about 4 nm due to the surface faceting of (1 1 1)-oriented grains. The growth mechanism of the (1 1 0)- and (1 1 1)-textured STO films on the Pt/Al2O3 substrates was also explained and discussed in detail.

Phase field modeling the growth of Ni 3Al layer in the β/γ diffusion couple of Ni–Al binary system

The growth behavior of Ni 3Al phase layer in the β/γ diffusion couple of Ni–Al binary system, including the shape evolution and growth kinetics, have been simulated by using the KKS multiphase field model. Simulation results indicate that, when Ni 3Al layer growth is controlled completely by volume diffusion, it could be regarded as parabolic growth. However, if the fast grain boundary diffusion is taken into account, the growth rate of Ni 3Al phase is accelerated, and the growth kinetics deviates from parabolic growth, which is consistent with experiments and other simulation results. Simulation results also demonstrate some details of shape evolution of grains, such as the uneven Ni 3Al/γ and Ni 3Al/β interfaces and the grain boundary migration of Ni 3Al grains caused by fast grain boundary diffusion.

Morphology of Carbonized Rice Grains Excavated at Non Yang Site, Thailand, and Evolution of Grain Shape from a Historical Perspective

Morphology of Carbonized Rice Grains Excavated at Non Yang Site, Thailand, and Evolution of Grain Shape from a Historical Perspective

Formation of Grain Boundaries in Liquid‐Phase‐Sintered WC–Co Alloys

The origin of WC/WC grain boundaries in liquid‐phase‐sintered WC–Co alloys has been investigated in a WC–Mo2C–Co model system using coarse WC polygrain powder. The evolution of grain shape during liquid phase sintering was able to be identified by observing a growth layer that contained Mo. During liquid phase sintering, most of the grain boundaries in the powder were penetrated by a Co liquid but some of them were not. Electron backscattered diffraction analysis confirmed that some boundaries in the powder, in particular, Σ2 twist boundaries and Σ97 special boundaries, remained intact during liquid phase sintering. These experimental results confirm that the grain boundaries of WC grains in liquid‐phase‐sintered WC–Co alloys originated from those present in the starting powder.

Evolution of grain shape and size during high-temperature creep of a yttria-doped fine-grained alumina

A semi-automatic method of image analysis was used to characterize the evolution of the microstructure of a polycrystalline alumina in terms of grain size and grain shape during different thermomechanical treatments. This study showed that for this material normal grain growth occurs during compressive creep and annealing; in addition, during creep under certain conditions grains flatten in the direction of the applied stress. Three-dimensional estimation of the grain shape was performed by analysing differently oriented plane sections of the specimens.