Grain Boundary Distribution Research Articles

Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature.

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10-9 K m2 W-1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics.

Effect of hot band annealing on texture and grain size distribution of primary recrystallization in grain-oriented silicon steel

During final high-temperature annealing in grain-oriented silicon steel, a few Goss grains grow exclusively fast and consume the matrix grains, which are related to the quality of the final product. The optimization of texture and microstructure in primary recrystallization is crucial to achieve the sharp Goss texture in grain-oriented silicon steel. The driving force for this phenomenon is the reduction of grain boundary area, which can be decribed as a reduction in grain boundary energy. The distribution of grain boundary and its energy determined by texture and microstructure of primary recrystallization are very important data to understand the optimization point. The hot band annealing is known to be a prerequisite for the high quality product in grain-oriented silicon steel. It can affect the evolution of microstructure and texture in primary recrystallization. In this study, the influence of the hot band annealing temperature on the texture and grain size distribution in primary recrystallization was investigated in detail.

Microstructure evolution of the TiB2/Al composites fabricated by powder metallurgy during hot extrusion

The microstructure evolution near the central axis during hot extrusion for the fine-grained TiB2/Al-Zn-Mg-Cu composites fabricated by powder metallurgy is studied by experimental observation at different locations in the remaining die. The significant increase of the fraction of ultrafine grains and texture evolution indicates the extensive dynamic recrystallization at the narrowest position of the die. The deformed grains are distinguished from the undeformed grains with the parameter of grain orientation spread (GOS). And the deformed grains are generally rotated around the extrusion direction. Additionally, the introduced submicron TiB2 particles have the potential to stimulate the dynamic recrystallization associated with progressively increased misorientation of subgrain boundaries, and even change the spatial distribution of the low angle grain boundaries. The preferential activation of {111} 〈1−10〉 system and non-octahedral (NOC) slip system {001} 〈110〉 contributes to the formation of strong deformation fiber texture according to the analysis of the Schmid factor. It is supposed that grain boundary sliding accommodated dislocation slip mechanism controls the microstructure evolution.

Gradient structure regulated plastic deformation mechanisms in polycrystalline nanotwinned copper

Molecular dynamics simulations are performed to investigate the effects of grain size gradient and twin thickness gradient on uniaxial tensile deformation behaviors of gradient-structured polycrystalline copper. Simulation results reveal that there exist an inverse Hall–Petch effect and gradient structure regulated plastic deformation mechanisms. That is, the average flow stress of the gradient nanograined (GNG) model or the gradient nanotwinned (GNT) model decreases with decreasing the grain size or twin boundary (TB) spacing, exhibiting the breakdown in the Hall-Petch relationship when the grain size or TB spacing is smaller than a critical size. The dominant plastic deformation mechanism is found to transform from the grain boundary (GB)-mediated one to the dislocation-based counterpart with the increase of the applied strain in the GNG model. The TB migration and annihilation dominate the plastic deformation in the grains with small TB spacing; while in the grains with large TB spacing, the dislocation multiplication and cutting across TB are mainly responsible for accommodating the deformation in the GNT model. Furthermore, the gradient distribution of strain and incompatibility of deformation induced by GB or TB gradient distribution are observed by monitoring the microstructural evolution.

High-strength and anti-corrosion of Al-Cu-Mg alloy by controlled ageing process

ABSTRACTThe evolution of strength and corrosion behaviour induced by ageing treatment in Al-Cu-Mg alloy is poorly understood. In this work, correlations among strength, intergranular corrosion and pitting corrosion induced by precipitates were studied by various corrosion performance tests and hardness tests. The susceptibility of intergranular corrosion is decreased and pitting corrosion sensitivity is increased as the ageing temperature and time are increased. Retrogression and re-ageing treatments not only transform the size of the intragranular precipitates from greater than 500 nm to be less than 10 nm but also lead to the discontinuous distribution of grain boundary precipitates. The microstructural changes in the grain interior and grain boundary together promote the Al-Cu-Mg alloys to have high-strength and excellent resistance to intergranular corrosion and pitting corrosion.

Ni nanocrystals tuning low-frequency colossal permittivity of epitaxial BaTiO3 matrix

The ceramic-metal composites (CMCs) with colossal permittivity have great potential to be widely applied into embedded capacitors. However, the colossal permittivity of these composites could not be explained in the low-frequency region (<100 Hz) due to the uncontrollable grains and grain boundaries in the polycrystalline ceramic matrix. In this work, Ni nanocrystals (NCs) were embedded in epitaxial BaTiO3 films. According to modulate the interfacial stress, the epitaxial growth of BaTiO3 was not disturbed by the NCs, avoiding the influence of grains and grain boundaries in polycrystalline oxides matrix. By tuning the spacing of NCs, the controllable distribution of grain (Ni NCs) and grain boundary (contact interface between the Ni and BaTiO3) was realized. The effects of grain and grain boundary on the low-frequency colossal dielectric response of CMCs were revealed. This study provides an ideal CMCs model for designing and application of new thin-layer capacitors.

Low-energy grain boundaries in WC-Co cemented carbides

The influencing factors, distribution characteristics and evolution mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were demonstrated. Based on characterizations from various angles, the correlations of grain shape aspect ratio, WC contiguity and distribution of low-energy grain boundaries were analyzed for cemented carbides with addition of different grain growth inhibitors. It was found that the ∑2 grain boundaries have higher fraction in the cemented carbides with stable complexions, finer grain size and larger WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbides with anisotropic surface energy by coalescence of WC platelets with large shape aspect ratio. The results suggest that the fraction and distribution of low-energy grain boundaries in the cemented carbides can be modulated by matching grain growth inhibitor and sintering temperature of the initial WC-Co composite powder.

Experimental and simulation study of grain boundaries in UO2

EBSD measurements performed on polycrystalline UO2 samples were analyzed to obtain the linear fraction distribution of grain boundaries as a function of their Coincidence Site Lattice (CSL) indexes. In parallel, molecular dynamics simulations of 26 CSL grain boundaries were performed to calculate their formation energies using four different empirical potentials. Comparing calculated formation energies and measured linear fractions allowed us to select the best suited empirical potential for the study of grain boundaries and to evidence a decrease in the formation energy of a given grain boundary when its length fraction increases. Cleavage energies were calculated using the selected potential and the toughness of a grain boundary was estimated, since this property corresponds to the energy needed to open it. An interesting relation is observed: the cleavage energy seems to decrease when the misorientation angle of the boundary increases. Finally, a first step towards the study of non-CSL (i.e. general) grain boundaries was taken by simulating three semi-general grain boundaries built by sticking two halves of CSL boundaries.

Experimental study of asymmetrical tilt boundaries in WC-Co alloys

The rotation angle distribution of grain boundaries with [2 -1 -1 0] misorientation axis in WC-Co alloys was investigated by electron backscatter diffraction for a range of cobalt contents. The effect of carbon content and sintering time was also studied. Preferred misorientations correspond to low-index boundary habit planes involving the basal or prismatic plane for at least one grain. The 26.2° [2 -1 -1 0] boundary was imaged by high resolution transmission electron microscopy. The joining of the mismatched planes at the boundary is accommodated by dislocations which are interpreted with reference to the 29.4° [2 -1 -1 0] boundary. The dislocation spacing and direction found experimentally are in agreement with calculated values obtained by the 02-lattice approach taking into account the misorientation and parametric misfit. The periodicity and characteristics of the dislocations indicate an optimization of the grain boundary structure and energy.

Three-dimensional in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium

Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel‑titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. Three-dimensional in situ reconstructions of the austenite microstructure are presented, including the structure and distribution of emergent grain boundaries. After 1 cycle, the subgrain structure is significantly refined, and heterogeneous Σ3 and Σ9 grain boundaries emerge. The low volume and uneven dispersion of the emergent Σ boundaries across the volume show why previous transmission electron microscopy investigations of Σ grain boundary formation were inconsistent.

Oxidation of stainless steel 316L – Oxide grains with pronounced inhomogeneous composition

The formation of oxide grains with pronounced inhomogeneous elemental distribution of Cr and Fe is reported for AISI 316 L stainless steel during the early stages of oxidation at 800 °C. Each oxide grain essentially consists of consecutive Cr-rich and Fe-rich layers parallel to the surface. Local elemental distribution, interplanar spacing and absence of grain boundaries or misorientation were characterized by state-of-the-art TEM. The oxide grains exhibit mainly corundum-type structure. Aspects of the oxide formation mechanism are discussed considering earlier findings, indicating a broader relevance of the new observations regarding the early stages of oxidation of austenitic stainless steel.

Enhancing stress corrosion cracking resistance of low Cu-containing Al-Zn-Mg-Cu alloys by slow quench rate

The corrosion behavior of Al-Zn-Mg-Cu alloy on different Cu content and quench rate has been investigated by inter-granular corrosion (IGC), exfoliation corrosion (EXCO), and stress corrosion cracking (SCC) test, and combined with scanning electron microscopy (SEM) and transmission electron microscopy-energy dispersive spectrometer (TEM-EDS) microstructural observation. The results showed that slow quench rate can effectively improve the SCC resistance of low Cu-containing alloys, especially decreasing crack propagation rate (the SCC propagation velocity (VII) is about an order of magnitude lower for slowly-quenched low Cu-containing alloys than fast quenched one), while it has less effect on high Cu-containing alloys. The IGC, EXCO, and SCC resistance with different Cu content and quench rate are analyzed by distribution, size and Cu content of grain boundary precipitates and width of precipitate-free zone. The corrosion mechanism has been studied in detail.

Low-Energy Grain Boundaries in WC-Co Cemented Carbides

The influencing factors, distribution rules and formation mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were studied. A number of characterizations were performed on the grain morphology and the features of low-energy grain boundaries in the cemented carbides, and the correlations of the grain shape aspect ratio, WC contiguity and distribution of the low-energy grain boundaries were analyzed. It was found that the ∑2 grain boundaries have a high fraction in the cemented carbide with stable complexions which lead to fine grain size and high WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbide with anisotropic surface energy by coalescence of WC platelets with a large shape aspect ratio. The present results suggest that the low-energy grain boundaries in the cemented carbides can be tailored by matching the grain growth inhibitor and the sintering temperature of the initial WC-Co composite powder.

INVESTIGATION OF SURFACES OF NOVEL IRON OXIDES WITH GRAIN AND GRAIN BOUNDARY

Hierarchical nano/microscale α-Fe2O3 iron oxide particle system was prepared by an improved and modified polyol method with the use of NaBH4 agent with high heat treatment at 900 °C in air. Here, α-Fe2O3 iron oxide particles with different shapes were analyzed. The morphologies of the surfaces of α-Fe2O3 iron oxide particles show the oxide structures with the different nano/microscale ranges of grain sizes. In this research, we have found that grain and grain boundary growth limits can be determined in α-Fe2O3 iron oxide structure. This leads to the possibility of producing new iron oxide structures with distribution of desirable size grain and grain boundary. With α-Fe2O3 structure obtained, the magnetic properties of the α-Fe2O3 iron oxide system are different from those of previously reported studies. in national and international studies.Keywords: Iron iron oxides, α-Fe2O3, chemical polyol methods, heat treatment.

Microstructure Evolution and the Resulted Influence on Localized Corrosion in Al-Zn-Mg-Cu Alloy during Non-Isothermal Ageing.

A non-isothermal ageing process was proposed for an Al-Zn-Mg-Cu alloy aiming to accommodate the slow heating/cooling procedure during the ageing of large components. The evolution of microstructure and microchemistry was analyzed by using transmission electron microscopy, high-angle annular dark field imaging, and energy dispersive spectrometry. The age-hardening of the alloy was examined to evaluate the strengthening behavior during the non-isothermal process. The corrosion behavior was investigated via observing the specimens immersed in EXCO solution (solution for Exfoliation Corrosion Susceptibility test in 2xxx and 7xxx series aluminum alloys, referring ASTM G34-01). Secondary precipitation was observed during the cooling stage, leading to increased precipitate number density. The distribution of grain boundary precipitates transits from discontinuous to continuous at the cooling stage, due to the secondary precipitation’s linking-up effect. The solutes’ enrichment on grain boundary precipitates and the depletion in precipitate-free zones develops during the heating procedure, but remains invariable during the cooling procedure. The corrosion in NIA (Non-isothermal Ageing) treated specimens initiates from pitting and then transits to intergranular corrosion and exfoliation corrosion. The transition from pitting to intergranular corrosion is very slow for specimens heated to 190 °C, but accelerates slightly as the cooling procedure proceeds. The transition to exfoliation corrosion is observed to be quite slow in all specimens in non-isothermal aged to over-aged condition, suggesting a corrosion resistance comparable to that of RRA condition.

Phase transition, impedance spectroscopy and conduction mechanism of Li0.5Na1.5WO4 material

In order to better characterize the tungsten-based samples, and to get better inside into this field, we synthesized the Li0.5Na1.5WO4 compound and identified its purity by X-ray diffraction which showed that it crystallizes in the orthorhombic system with the Pmmm space group. Accordingly we used Raman spectroscopy, differential scanning calorimetry and impedance spectroscopy as to distinguish its properties. All WO42− tetrahedral vibration modes appear in the Raman spectra. The calorimetric study displays two phase transitions at 498 K and 567 K. Impedance spectroscopy reveals the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries which allows us to use two cells mounted in series each one of them is composed of the combination in parallel of a resistance R and a fractal capacitance CPE. The variation σdc as a function of the inverse of temperature confirms the presence of three phases for each one of them the conductivity is ensured by a specific conduction mechanism. It has been pointed out that the first phase is described by the correlated barrier hopping model (CBH), the second one by the over loping polaron tunneling model (OLPT) and the third one by the non small polaron tunneling model (SPT). A comparison with the compound of Li1.5Na0.5WO4 confirms that the transport is dominated by the motion of the monovalent cations (Li+ and Na+) in the investigated material.

Phase transition and conduction mechanism study by overlapping large-polaron tunnelling model in Li2MoO4 ceramic compound

The Lithium-ionic conductor Li2MoO4 compound was prepared by the conventional solid-state reaction method. A Rietveld analysis of powder X-ray diffraction at room temperature has exhibited the title compound crystallizes in the trigonal system (space group) R3¯ with a phenacite-like structure. The calorimetric and Raman spectroscopy-studies revealed a single-phase transition at 635 K. The electrical conductivity was measured in the frequency range from 200 Hz to 1 MHz and temperatures between 578 and 723 K using impedance spectroscopy. Besides, the analysis of Nyquist plots revealed the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries. The electrical conductivity for the grain (σg) follows an Arrhenius behavior with two different activation energies Ea(I) = 1.038 eV for T < 635 K and Ea(II) = 0.481 eV for T > 635 K. The frequency dependence of the conductivity was interpreted in terms of Jonscher's law. Temperature dependence of the power law exponent s suggests that the overlapping large-polaron tunneling (OLPT) model is the dominant transport process in this material. The optimum hopping length of the polaron (1.8 <Rω < 3.4 Å), is large compared with the interatomic spacing (LiO = 1.964 Å; MoO = 1.8 Å).

Enhancing the Strength of Graphene by a Denser Grain Boundary

From a device application point of view, the extreme mechanical strength of graphene is highly desirable. However, the unavoidable polycrystalline nature of graphene films produced by chemical vapor deposition (CVD) leads to significant fluctuations in mechanical properties. Although the effects of atomic defects or grain boundaries (GBs) on mechanical strength have been widely studied and some modifications have been applied to enhance the stiffness of graphene, the problems of fragility as well as significantly reduced breaking strength arise. Here we report a systematic study on the effect of elastic modulus and breaking strength of CVD-derived graphene films with a controlled density and distribution of GBs. We find that graphene films become much stronger by hugely increasing the density of GBs without triple junctions (TJs) formed inside, in analogy to the two-dimensional (2D) plum pudding structures. The comprehensive performance with a 2D Young's modulus of 436 N/m (∼1.3 TPa) and 2D breaking strength of 43 N/m (∼128 GPa) can be achieved with the average grain size of 20 nm. Moreover, the existence of TJs will slightly reduce the strength in these GB structures. Due to defect types, the graphene films will show various tearing behaviors after indentation. All these mechanical studies of GBs provide a guideline to obtain the optimal performance of 2D materials through GB structure engineering.

Effect of Nd Doping on Dielectric and Impedance Properties of PZT Nanoceramics

Neodymium-doped lead zirconate tianate, i.e. Pb1−xNd x Zr0.52Ti0.48O3 (PNZT) ceramics, with x = 0–10 mol.% has been prepared by the sol–gel process. X-ray diffraction pattern at room temperature shows the pyrochlore free phase for all samples. The structural analysis suggests the coexistence of both rhombohedral (R3m space group) and tetragonal (P4 mm space group) crystal symmetries. Scanning electron micrographs of the samples show uniform distribution of grain and grain boundaries. Dielectric constant increases with the increase in neodymium concentration in the crystal lattice. Degree of diffuse phase transition increases with the increase in Nd3+ concentration in the sample. Nd3+ incorporation into the lead zirconatetitanate (PZT) lattice enhances the spreading factor. Interaction between neighbouring dipoles decreases with the increase of Nd3+ in PZT lattice. The conduction mechanism of the sample can be attributed to the overlapping large polar tunnelling. Second-order dielectric phase transition has been observed at the Curie temperature. The electrical properties of the sample can be explained by considering grain and grain boundaries contributions. All the samples show the poly-dispersive non-Debye type relaxation.

Microstructure and intergranular corrosion behavior of HAZ in DP-TIG welded DSS joints

In this study, 10.8-mm-thick workpieces of S32101 duplex stainless steel were welded using a keyhole deep penetration tungsten inert gas welding system. The misorientation angle distribution of grain boundaries in the heat affected zone was analyzed using the electron backscatter diffraction technique. Double-loop electrochemical potentiokinetic reactivation tests were performed to assess the intergranular corrosion resistance, based on the degree of sensitization. The volume fractions of ferrite and austenite and the grain boundary misorientation angle affect the intergranular corrosion of the welded joints. The relationship between the grain boundary misorientation angle of the austenite and the degree of sensitization was established. The volume fraction of austenite and the frequency of Σ3 coincidence site lattice boundary, increases with the increasing heat input. The misorientation angle distribution in the austenite, the frequency of Σ3 coincidence site lattice boundary, and the volume fraction of CrN influence the intergranular corrosion resistance of the heat affected zone.