Randomly-oriented Grains Research Articles

Phase-field simulations of ferro-electro-elasticity in model polycrystals with implications for phenomenological descriptions of bulk perovskite ceramics

We investigate the role of polycrystalline disorder on the effective ferro-electro-elastic behavior of perovskite ferroelectric ceramics under electro-mechanical loading. Assuming random initial grain orientations, we use high-resolution phase-field simulations and periodic homogenization of two-dimensional model polycrystals to study the evolution of the domain microstructure within and across grains as well as the resulting effective, macroscopic polarization and strain fields under loading. The number of randomly-oriented grains in simulations, at fixed grain size and fixed numerical resolution per grain, is used to control the polycrystalline disorder. Results indicate that, when the polycrystalline samples are sufficiently disordered (i.e., when sufficiently many randomly-oriented grains are considered), their effective electromechanical response under uniaxial compression is stable, and the concomitant polarization and deformation are always aligned with the mechanical load. Thus, the present study supports the viewpoint that polycrystalline disorder in bulk perovskite ceramics stabilizes the overall ferro-electro-elastic response despite the underlying nonconvex polarization energy landscape.

Significantly enhanced reversibility and mechanical stability in grain-oriented MnNiGe-based smart materials

Materials that undergo a magnetostructural transition (MST) usually exhibit fascinating magnetoresponsive properties, making them an important class of smart materials. However, practical application of this class of smart materials has been hindered by structural degradation as well as large irreversibility of the MST during consecutive thermal and field cycles. Here we report a significant improvement of the reversibility and mechanical stability in grain-oriented MnNiGe-based alloys that were fabricated using a directional solidification method. The preferred grain orientation enables synergistic deformations between neighboring grains during the MST, leading to a substantial reduction in the transition-induced stress concentration. As a result, in situ and ex situ microscopic observations demonstrate a good mechanical stability of the textured alloys across the MST, in strong contrast to conventional MM'X (M, M’ = Mn, Fe, Co, Ni; X = Si, Ge) materials with randomly-oriented grains. The detailed transition stages of the MST have also been observed at the microscopic scale, and are reported for the first time in the MM'X family. Besides, a low thermal hysteresis (ΔThys) of ∼ 4 K was obtained in the textured alloys, which is the lowest ΔThys in the MM'X material family. Textured MnNiGe-based alloys show a large reversible isothermal entropy change (≥ 35.9 Jkg−1K−1) in a 5 T field change, which is the highest among typical magnetocaloric materials. Consequently, this work provides a promising strategy for enhancing the cyclic stability of materials with a MST, which may boost their practical applications in solid-state refrigerators, energy harvesters and high-precision actuators.

A deep learning method for predicting microvoid growth in heterogeneous polycrystals

In heterogeneous polycrystals, microvoid growth presents inherent randomness and dispersion, which generally follows a statistical law. This statistical characteristic intrinsically arises from randomly distributed grain-orientations around microvoids. It remains a huge challenge to explicitly depict the inherent correlation between microvoid growth and grain-orientation distribution by conventional deterministic damage models. In recent years, deep learning has gained in popularity in materials science and has been demonstrated to exhibit excellent data-mining abilities. To our best knowledge, deep learning has not been applied to investigate the statistical damage-evolution issues hitherto. In this work, a novel microvoid growth model based on deep neural network is creatively designed, incorporating both convolutional and long short-term memory components. The former extracts the spatial grain-orientation information, and the latter captures the causal effect of strain history on the microvoid growth. Moreover, to train and test the deep learning-based model, a microvoid-growth database is generated through a large number of crystal plasticity-based finite element simulations, incorporating randomly-oriented grains and different void locations. All the sample data (i.e., the grain-orientation distributions, microvoid locations and microvoid-growth curves) are processed by specific methods (e.g., the pixel-based method) to be amenable for the training process. Our results show that this novel model well captures the statistical characteristic of the microvoid growth in heterogeneous polycrystals. It is expected that the deep learning-based method can provide a new way to predict the microvoid growth at the grain-level.

Molecular dynamics study on friction of polycrystalline graphene

Graphene, as a one-atom thick 2-dimensional material, is an ideal solid lubricant for small length scale devices such as NEMS/MEMS (nano/micro-electro-mechanical systems) and is often synthesized using chemical vapor deposition (CVD). While CVD-grown graphene consists of a large number of randomly-oriented grains, the effects of this polycrystalline structure on graphene friction still remain far from completely understood. In this study, we investigate the tribological properties of the multigrain structure against a pristine (defect-free single-crystal) graphene surface using molecular dynamics (MD) simulations. The MD simulations presented here test the friction of multiple such configurations created by a novel method mimicking the natural growth of grains in two dimensions. The simulation results reveal that most multigrain configurations exhibit persistent negligible friction without transiting to high friction states. However, there also exist several configurations having relatively large friction forces compared with those from perfectly-aligned single-crystal graphene interactions. A systematic analysis of the independent effects of the grain orientation and grain boundary explains these observations. In particular, it is found that there exists a critical range in the misalignment angle of grain (0.3–1.5°, to the pristine surface) where the shear stress (friction force per unit area) is two or three times larger than the perfectly-aligned grain. Thus, configurations which contain grains with a small misalignment angle in this critical range exhibit large friction forces, despite their substantially smaller grain area. Moreover, for those configurations where all grains have misalignment angles greater than the critical angle, grain boundaries act as the primary source of friction.

Effective improvement in the distribution of single crystalline diamond nanorods fabricated from the randomly-oriented polycrystalline films

By using the bias-assisted reactive ion etching technique and the etching masks of Au nanodots, the high-density single crystalline diamond nanorods have been obtained from the polycrystalline diamond films with randomly-oriented grains grown on silicon substrates. The inhomogenous distribution of the nanorods mainly results from the existence of grain boundaries in the films, and can be effectively improved by adding the graphite sheet under the substrate. Furthermore, the fabrication mechanism of the nanorod array is carefully investigated in this paper. The morphological evolution is observed by using a scanning electron microscopy technique. Raman measurements are carried out to monitor the phase change during the etching process. It is found that the non-diamond phases are present during the experimental process, and then are completely removed away in the end.

Microstructure of organometallic derived PLZT ceramics

The PLZT powders with the formula Pb 0.905 La 0.095 (Zr 0.65 Ti 0.35 ) 0.976 O 3 + 3.5 wt.% PhO were prepared by the organometallic precursor method (Pechini and partial oxalate processes). The microstructure of sintered 9.5/65/35 PLZT ceramics obtained from a partial oxalate procedure shows that the outstanding feature of this microstructure is its fairly uniform grains of about 1.8 μm. The microstructure of sintered PLZT ceramics obtained by the Pechini process consists of uniform small randomly-oriented grains tightly bonded together in the central part of the sample with a grain size of about 1.2 μm. Cubic and elongated grains are formed at the sample's border. The microstructures of hot pressed PLZT ceramics obtained from both processes are dense and rather uniform. After a double stage of hot pressing (2 plus 20 h) the microstructure of PLZT is fully dense uniform and homogeneous with a grain size of approximately 2.5 μm.

Texture evolution in NiAl

We have investigated texture evolution in stoichiometric NiAl and Ni-49at.%Al–1at.%Ti using orientation imaging microscopy (OIM) and X-ray diffraction. Grain size and orientation were studied after extrusion and after heat treatment, leading to the following model of texture evolution. During extrusion, dynamic recrystallization occurs with a continuing cycle of deformation to produce a 〈110〉 fiber texture, nucleation of nearly randomly-oriented grains, and preferential growth of 〈111〉 grains. Upon cooling from the extrusion temperature, nucleation continues until the temperature drops below the recrystallization temperature. A 〈110〉 fiber texture observed after extrusion suggests the specimen did not fully recrystallize during cooling and the deformation texture is retained. A 〈111〉 fiber texture indicates a fully recrystallized microstructure in which significant grain growth has occurred. It has been found that titanium addition increases the recrystallization temperature of NiAl and retards grain growth resulting in the retention of the 〈110〉 fiber texture.

Initial results of a novel pre-deposition seeding technique for achieving an ultra-high nucleation density for CVD diamond growth

Abstract The unique thermal properties of diamond, along with the ability to deposit it over large areas, make it an appropriate candidate for electronic packaging and other thermal management applications. In a recent exploration, our group observed defects in diamond films, referred to as “microcavities”. Microcavities are essentially voids in CVD diamond resulting from the growth of randomly-oriented grains. These microcavities present serious challenges to the fabrication of high-density electronic packages when they appear at the surface of the film/substrate. Two approaches for solving the microcavity problem were explored; (1) “planarization by filling” and (2) enhancement of the nucleation density. Results of the first approach have been reported elsewhere. The latter approach, which is the subject of this paper, is based on an electrostatic seeding technique for uniformly depositing nanometer size diamond particles over the entire surface of any material of any shape to serve as nucleation sites for diamond growth. This approach to initiating diamond growth is inexpensive and simple. The resulting films are very dense, and both the number and size of the microcavities are significantly reduced. Details of the seeding technique and resulting diamond films, characterized using SEM, will be discussed.

Microstructure and stability of melt spun INCONEL 713 LC

The alloy IN-713LC* was used in an investigation of the effect of process parameters on the microstructure of a rapidly solidified melt-spun material. The resultant ribbon microstructure consisted of several distinct regions, each of which corresponds to a different thermal history during processing. A chill zone of equiaxed randomly-oriented grains exists in a region of the foil which was in contact with the wheel during casting. This zone develops into a dendritic growth morphology with distance away from the lower ribbon surface. Dendrites inclined in the direction of wheel rotation result from growth into a flowing stream. TEM studies showed that a cell structure formed, the cell size decreasing with increasing wheel speed. Aging studies indicated that the cell structure plays an important role in γ’ precipitation. Results relating to heat treatments (as would be encountered in compaction and use) and the stability of the melt-spun structure are considered.