Formation Of Nanocrystalline Grains Research Articles

Formation and Microstructural Evolution of Ferritic ODS Steel Powders during Mechanical Alloying.

Ferritic ODS steel elemental powder compositions with various Zr content (0.3-1.0 wt.%), ground in a Pulverisette 6 planetary ball mill, were extensively studied by X-ray diffraction line profile analysis, microscopic observations, microhardness testing and particle size measurements. A characteristic three-stage process of flattening the soft powders, formation of convoluted lamellae and, finally, formation of nanocrystalline grains was observed. In order to quantify the microstructural properties, expressed mainly in terms of crystallite size and dislocation density, a methodology for detailed and accurate microstructure analysis of nanosized and severely deformed materials was proposed by the Whole Powder Pattern Modelling (WPPM) approach. In the case of the proposed ODS alloy composition, the overlapping of Fe and Cr Bragg reflections makes the microstructure analysis certainly more complicated. The results showed that the microstructure of powders evolved towards the nanocrystalline state consisting of fine (diameter of ~15 nm) and narrowly dispersed domains, with extensive dislocation density exceeding 1016 m-2.

Enhanced Long Cycle Life Stability and High Storage Reversible Capacity Retention of a Sodium Vanadate Zinc Glass–Ceramic Network

This investigation records on a sodium vanadate oxide glass anode network mixed with zinc oxide nanocrystallites by heat treatment between Tg and Tc for different schedules. The glass anode sample is prepared with high-energy ball milling using the formula (37.5Na2O-62.5V2O5)(100–x):ZnOx (x = 0, 5, 10, 15, 20, and 25 mol %, labeled as Znx). Clear formation of nanocrystalline grains after heat treating the Zn20 glass anode for 15 h (Zn20-15h) supports the highest diffusion of Na+ ions. The specific discharge and charge capacities are recorded to be 380/370 (0 h), 394/381 (5 h), 427/420 (10 h), 444/439 (15 h), and 419/408 (20 h) mA h/g for different heat treating schedules. It is also worth noting that the initial charge and discharge capacities of the Zn20 glass–ceramic anode sample (after heat treatment) are higher than those of the corresponding glass sample (before heat treatment) but recorded to be the highest for the 15 h heat treating schedule compared with other schedules. The cycling behavior of the Zn20-15h glass–ceramic anode at 4000 mA/g displays longer cycle life stability and high rate capability along with Coulombic efficiency and relatively fast electronic and ionic conductivity even up to 3000 cycles.

Surface mechanical attrition treatment of additively manufactured 316L stainless steel yields gradient nanostructure with superior strength and ductility

Surface severe plastic deformation (S2PD) of additively manufactured/three-dimensional (3D) printed metallic parts is gaining increased attention as a post-manufacturing operation to enhance the material performance in a wide variety of applications. Surface mechanical attrition treatment (SMAT) is an S2PD technique that can yield a nanostructured surface layer induced by compressive stresses and work hardening. In the present study, SMAT was performed on 316L (austenitic) stainless steel (SS) processed by selective laser melting (SLM), and the consequent effects on mechanical response were investigated. The underlying mechanisms of microstructural evolution leading to the formation of nanocrystalline grains resulting from SMAT in SLM 316L SS are elucidated. The interactions between twins and deformation bands act as potential sites for impeding the movement of dislocations, which in turn leads to the formation of stacking faults, twinning, and occasionally transform to a different crystal structure. Twin-twin and/or twin-deformation band intersections sub-divide the matrix grains into smaller cells or low-angle disoriented blocks, which result in the formation of low-angle grain boundaries and finally in nanocrystallization at the surface. The size of nanocrystalline grains increases progressively with depth from the surface to micrometer size grains in bulk. The gradient nanostructure in the additively manufactured alloy after SMAT imparts an unusual combination of strength and ductility that markedly exceeds that of conventional, bulk nanostructured, or even high-performance 316L SS (containing nanoscale deformation twins embedded in submicron-sized austenitic grains obtained by dynamic plastic deformation processes). Analytical models revealed that strengthening results from a combination of grain boundaries and dislocations. The results of the present investigation pave the way for engineering high-performance SS for a variety of engineering applications.

Nucleation and growth dynamics of graphene grown by radio frequency plasma-enhanced chemical vapor deposition

We investigated the nucleation and grain growth of graphene grown on Cu through radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) at different temperatures. A reasonable shielding method for the placement of copper was employed to achieve graphene by RF-PECVD. The nucleation and growth of graphene grains during PECVD were strongly temperature dependent. A high growth temperature facilitated the growth of polycrystalline graphene grains with a large size (~ 2 μm), whereas low temperature induced the formation of nanocrystalline grains. At a moderate temperature (790 to 850 °C), both nanocrystalline and micron-scale polycrystalline graphene grew simultaneously on Cu within 60 s with 50 W RF plasma power. As the growth time increased, the large graphene grains preferentially nucleated and grew rapidly, followed by the nucleation and growth of nanograins. There was competition between the growth of the two grain sizes. In addition, a model of graphene nucleation and grain growth during PECVD at different temperatures was established.

Inhomogeneous Plastic Deformation Mechanisms of NiTiFe Shape Memory Alloy Subjected to Local Canning Compression

Mechanism for inhomogeneous plastic deformation of NiTiFe shape memory alloy (SMA) is revealed under local canning compression at room temperature. Microstructures of the compressed NiTiFe sample are very inhomogeneous due to the occurrence of the different plastic strains corresponding to the different deformation zones. B2 austenite, B19′ martensite, nanocrytalline grains, amorphous phase and the retained coarse grains are observed in the deformed NiTiFe sample. B19′ martensite phase is induced by means of mechanical loading and it is stabilized by large plastic strain. In addition, B19′ martensite phase is formed after plastic deformation for dislocation slip has occurred. The phenomenon demonstrates that at the initial stage of deformation, the critical resolved shear stress (CRSS) for dislocation slip of NiTiFe SMA is lower than the critical stress for inducing B19′ martensite. Formation of B19′ martensite is dependent on the higher plastic strain where the CRSS for dislocation slip of NiTiFe SMA is increased substantially enough to be higher than the critical stress for inducing B19′ martensite. With the progression of plastic deformation, a lot of dislocations are accumulated substantially and they provide the driving force for formation of nanocrystalline grains and amorphous phase.

Initial texture dependence of nanocrystalline omega phase formation during high pressure torsion of commercially pure titanium

The present investigation studied the influence of initial texture on large strain deformation behavior of hexagonal close packed (HCP) titanium in high pressure torsion (HPT). Discs of two distinct initial textures comprising of near basal and near prismatic-pyramidal orientations were subjected to HPT at room temperature. X-ray diffraction (XRD) revealed the formation of omega phase in HPT processed samples with the fraction of omega phase increasing with increase in strain. Transmission electron microscopy and precession electron diffraction showed the formation of nanocrystalline grains of alpha and omega phases in 5 turns samples for both the orientations. Although omega transformation initiated first in the prismatic-pyramidal textured sample, the basal textured sample showed higher omega volume fraction at higher strain. The mechanical response based on micro-hardness measurements is different for the aforementioned two different initial textures. This is because hardness depends not only on the omega phase fraction, but also on the grain size and distribution of the nanocrystalline omega phase. Further, the relative contributions of strengthening imparted by the omega phase and hardening caused by the accumulated strain component determines the overall hardness profile. Also, synchrotron XRD measurements were performed to estimate crystallite size, microstrain and dislocation densities. The latter analysis coupled with the microstructural analysis indicates that initial texture and crystallographic orientation relationships of omega transformation influence the distribution of omega phase and strain partitioning between the alpha and omega phases, resulting in a strong texture dependence of hardness response.

Effect of cyclic rapid thermal loadings on the microstructural evolution of a CrMnFeCoNi high-entropy alloy manufactured by selective laser melting

Metallic materials produced by additive manufacturing experience complex stress and thermal gyrations along the build direction. This has the potential to produce complicated heterogeneous microstructures that may exhibit a wide variety of mechanical properties. There remains a paucity of studies on the nature and the formation mechanisms of the microstructural heterogeneity and this limits our capability for microstructural design in additively manufactured metallic materials. Here, we present an electron microscopy-based investigation of a CrMnFeCoNi high-entropy alloy produced by selective laser melting. We have focussed on a systematic investigation of the microstructural evolution along the build direction. Our results reveal a remarkable hierarchy of microstructures, including the formation of nanocrystalline grains, elemental segregation and precipitation, cellular dislocation structures, deformation twinning, and deformation-induced phase transformation. Our research clarifies the relationships amongst different features, and provides guidance for future structural manipulation of materials produced by additive manufacturing.

On the Formation of Nanocrystalline Grains in Metallic Glasses by Means of In-Situ Nuclear Forward Scattering of Synchrotron Radiation.

Application of the so-called nuclear forward scattering (NFS) of synchrotron radiation is presented for the study of crystallization of metallic glasses. In this process, nanocrystalline alloys are formed. Using NFS, the transformation process can be directly observed during in-situ temperature experiments not only from the structural point of view, i.e., formation of nanocrystalline grains, but one can also observe evolution of the corresponding hyperfine interactions. In doing so, we have revealed the influence of external magnetic field on the crystallization process. The applied magnetic field is not only responsible for an increase of hyperfine magnetic fields within the newly formed nanograins but also the corresponding components in the NFS time spectra are better identified via occurrence of quantum beats with higher frequencies. In order to distinguish between these two effects, simulated and experimental NFS time spectra obtained during in-situ temperature measurements with and without external magnetic field are compared.

Electrodeposition of Nanocrystalline Fe-P Coatings: Influence of Bath Temperature and Glycine Concentration on Structure, Mechanical and Corrosion Behavior

A detailed electrochemical study and investigation of a Fe-P glycine bath as a function of the temperature and glycine concentrations and current density, and their resulting corrosion and mechanical behavior is presented. A low addition of glycine to the electrolyte led to a drastic increase of the P content. At low Fe-P deposition rates, heterogeneous rough deposits with morphological bumps and pores were observed. By increasing the Fe-P deposition rate, the number of pores were reduced drastically, resulting in smooth coatings. Increasing the P content led to the formation of nanocrystalline grains from an “amorphous-like” state. Coatings with higher P contents exhibited better corrosion resistance and hardening, most likely attributed to grain boundary strengthening.

Shock-induced reaction synthesis of cubic boron nitride

Here, we report ultra-fast (0.1–5 μs) shock-induced reactions in the 3B-TiN system, leading to the direct synthesis of cubic boron nitride, which is extremely rare in nature and is the second hardest material known. Composite powders were produced through high-energy ball milling to provide intimate mixing and subsequently shocked using an explosive charge. High-resolution transmission electron microscopy and X-ray diffraction confirm the formation of nanocrystalline grains of c-BN produced during the metathetical reaction between boron and titanium nitride. Our results illustrate the possibility of rapid reactions enabled by high-energy ball milling possibly occurring in the solid state on incredibly short timescales. This process may provide a route for the discovery and fabrication of advanced compounds.

Grinding-induced metallurgical alterations in the binder phase of WC-Co cemented carbides

The metallic binder phase dictates the toughening behavior of WC-Co cemented carbides (hardmetals), even though it occupies a relative small fraction of the composite. Studies on deformation and phase transformation of the binder constituent are scarce. Grinding represents a key manufacturing step in machining of hardmetal tools, and is well-recognized to induce surface integrity alterations. In this work, metallurgical alterations of the binder phase in ground WC-Co cemented carbides have been assessed by a combination of electron back scattered diffraction and transmission electron microscopy techniques. The Co-base binder experiences a martensitic phase transformation from fcc to hcp crystal structure, predominantly in the first 5μm below the surface. The hcp fraction decreases gradually along a depth of 10μm. Surface Co displays severe plastic deformation under the highest strain, resulting in formation of nanocrystalline grains in the first micrometer below the surface. Microstructural refinement within the binder phase is observed even at greater depth. Stacking faults were detected in most of the refined grains. The metallurgical alterations of the binder phase modify the local stress distribution during grinding, which affects the discerned subsurface microcracking. The resulting residual stress profile is the sum of multiple subsurface changes, such as phase transformation, severe plastic deformation and grain refinement, where it is discerned that the depth profile of the transformed hcp-Co fraction coincides with the grinding-induced residual stress profile.

Correlation of splat state with deposition characteristics of cold sprayed niobium coatings

The cold spray technique has a great potential to deposit refractory metals for a variety of potential applications. Cold spraying of different metals have been addressed comprehensively to understand the deposition characteristics of the coatings. Since there is no available data on the deposition characteristics of cold sprayed Niobium, impact behavior of splats at different deposition conditions were simulated and numerically analyzed using Finite Element Modeling (FEM) and correlated with the experimental observations that highlight the role of the velocity and temperature of the particle upon impact on the bonding features. The increase in temperature of the splat drastically reduces the flow stress at the interface leading to best inter-splat bonding state. The synergistic effect of the temperature and the velocity leads to the formation of very dense, defect free niobium coating associated with deformation localization including interface melting. Formation of nanocrystalline grains at the inter-splat boundary was confirmed through TEM and compared with the FEM findings. Finally, understanding the deformation and deposition behavior of refractory metal such as niobium will be helpful to engineer the coatings for potential applications.

Structural, morphological and magnetic properties of Eu-doped CoFe2O4 nano-ferrites

Europium (Eu) doped spinel cobalt ferrites having composition CoEuxFe2−xO4 where x=0.00, 0.03, 0.06, 0.09, 0.12 were fabricated by co-precipitation route. In order to observe the phase development of the ferrite samples, thermo-gravimetric analysis was carried out. The synthesized samples were subjected to X-ray diffraction analysis for structural investigation. All the samples were found to constitute face centered cubic (FCC) spinel structure belonging to Fd3m space group. Scanning electron microscopy revealed the formation of nanocrystalline grains with spherical shape. Energy dispersive X-ray spectra confirmed the presence of Co, Eu, Fe and O elements with no existence of any impurity. The magnetic hysteresis curves measured at room temperature exhibited ferrimagnetic behavior with maximum saturation magnetization (Ms) of 65emu/g and coercivity (Hc) of 966Oe. The origin of ferrimagnetism in Eu doped cobalt ferrites was discussed in detail with reverence to the allocation of Co2+ and Fe3+ ions within the spinel lattice. The overall coercivity was increased (944–966Oe) and magnetization was decreased (65–46emu/g) with the substitution of Eu3+. The enhancement of former is ascribed to the transition from multi domain to single domain state and reduction in lateral is attributed to the incorporation of nonmagnetic Eu ions for Fe, resulting in weak superexchange interactions.

Highly conductive cathode materials for Li-ion batteries prepared by thermal nanocrystallization of selected oxide glasses

Glassy analogs of two important cathode materials for Li-ion cells: V2O5 and phosphoolivine LiFePO4 were heat-treated in order to prepare nanocrystallized materials with high electronic conductivity of up to 7×10−2Scm−1 and ca 7×10−3Scm−1 at 25°C, respectively. There is a clear correlation between the crystallization phenomena and the increase in the electrical conductivity for both groups of glasses. Electrochemical tests of heat-treated glasses of the V2O5–P2O5 system, used as cathodes in lithium cells confirm their good gravimetric capacity and reversibility. Heat-treatment of glasses of the Li2O–FeO–V2O5–P2O5 system also leads to a high increase in the conductivity and to formation of nanocrystalline grains in the glassy matrix, evidenced by HR-TEM images. The temperature dependence of the conductivity of these materials follows the Arrhenius formula. The presented results indicate that the overall increase in conductivity in nanocrystallized materials is due to good charge transport properties of their interfacial regions.

Comparison of Microtexture Evolution During Cryo and Room Temperature Rolling of Ti–5Ta–2Nb Alloy

An α + β Ti–5Ta–2Nb alloy has been developed for applications under severe oxidizing conditions of boiling concentrated nitric acid in spent fuel reprocessing plant. The formation of nanocrystalline grains has been observed in the cryo-rolled sample accompanied by a significant improvement in yield strength whereas the grain refinement is not significant in case of room temperature rolled sample. Suppression of dynamic recovery plays a major role in the formation of nanostructured material. The evolution of micro-texture during room temperature rolling and cryo-rolling has been compared in this study. The rolling texture of room temperature and cryo-rolled samples are similar to the rolling texture predicted for hexagonal materials with c/a <1.6. XRD analysis indicates peaks shift to lower 2θ with increase in rolling strains indicating the presence of tensile residual strains. Peak broadening is also observed indicating an increase in defect density and decrease in grain size.

FEM-simulation of machining induced nanocrystalline surface layers in steel surfaces prepared for tribological applications

A formation of nanocrystalline grains due to dynamic recrystallization within the workpiece surface layer (AISI4140) resulting from machining has proven to be suitable for obtaining improved tribological and fatigue behaviour. In the work presented an optimization of the machining process is carried out with cutting simulations using a continuous remeshing method and describing surface layer generation. The simulations describe the influence of process parameters and tool geometry on evolution of grain size distributions within the subsurface and affected depth after this thermo-mechanical processing. The validation is performed by experimental analyses based on cutting technology and focused ion beam technique.

Influence of Oxygen Plasma on the Growth, Structure, Morphology, and Electro-Optical Properties of p-Type Transparent Conducting CuBr Thin Films

p-type transparent conducting CuBr thin films were grown by thermal evaporation of CuBr followed by oxygen plasma treatment. Efficient incorporation of oxygen into the CuBr films was revealed, and the influence of plasma exposure on the electrical, structural, optical, and electronic properties of CuBr films was investigated. X-ray diffraction (XRD) analysis indicated the Zincblende structure of the oxygen plasma treated CuBr (OCB) films with the formation of nanocrystalline grains preferentially oriented along the (111) direction. p-type conducting OCB films show >85% average transmittance along with hole concentration, conductivity and Hall mobility values of ∼1019 cm–3, ∼1.5 S cm–1 and ∼0.45 cm2 V–1 s–1, respectively. The X-ray photoelectron spectra of the OCB films demonstrated that the plasma exposure resulted in a significant increase in the O 1s signal at the surface of OCB films. On the basis of the experimental results from the Hall measurements and X-ray photoemission, a possible explanation comprising the formation of a surface CuO layer is also proposed to elucidate the increase in the p-type conductivity of the OCB films. Strong room temperature emission of OCB films at around 416 nm was also observed, the intensity of which decreased with the increase of oxygen plasma exposure time. The results present oxygen plasma exposure as a simple and promising technique for the production of CuBr-based p-type materials for future transparent electronics.

Nanocrystallization of Zr-Cu-Ni-Al-Au glassy alloys during severe plastic deformation

A study has been carried out into the formation of nanocrystalline grains during high-pressure torsion (HPT) deformation of Zr65Cu17Ni5Al10Au3 bulk alloys prepared using tilt casting. As a preliminary to this, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses were carried out on as-cast Zr65+xCu17-xNi5Al10Au3 (x=0~5 at.%) and Zr65Cu20Ni5Al10Au3 alloys, in order to determine the effect on the microstructure of the excess Zr content x and the presence of Au. From the XRD patterns, it was determined that all of the alloys had a metallic glassy nature. For Zr65Cu17Ni5Al10Au3, the DSC results indicated the presence of a wide supercooled liquid region between the glass transition temperature (Tg) of 644 K and the crystallization temperature of 763 K, where the stable body-centered tetragonal Zr2Cu phase was formed. In contrast, for the Zr65+xCu17-xNi5Al10Au3 alloys, precipitation of an icosahedral quasicrystalline phase (I-phase) was observed in the supercooled liquid region at about 715 K. HPT deformation of the Zr65Cu17Ni5Al10Au3 alloys was carried out under a high pressure of 5 GPa. Both as-cast specimens and those annealed at Tg-50 K for 90 min were used. Following a single HPT rotation (N=1), transmission electron microscopy identified the presence of face- centered cubic Zr2Ni precipitates in the as-cast alloy, with a size of about 50 nm. For the annealed alloy, a high density of I-phase precipitates with sizes of less than 10 nm was observed following HPT with N=10, indicating that the combination of severe plastic deformation and annealing is effective at producing extremely small grains.

Tensile behavior and strengthening mechanisms in a submicron B4C-reinforced Al trimodal composite

The physical and mechanical characteristics of cryomilled Al 5083-boron carbide (Al-B4C) metal matrix composites (MMCs) render them both scientifically interesting as well as technologically important. Trimodal architectures (containing three components, nano/ultrafine grains, coarse grains and ceramic reinforcements) allow for multiple degrees of freedom in the design of the microstructures that can be engineered, for example, to achieve combinations of strength, ductility and damage resistance that are not attainable with conventional materials. Various manufacturing strategies have been formulated to fabricate trimodal MMCs. Among these, cryomilling, which involves mechanical milling in a cryogenic liquid slurry (e.g., nitrogen), has been widely studied because in addition to minimizing recovery during the formation of nanocrystalline grains, it facilitates the incorporation of reinforcement particles in the metal matrix. In this study, novel trimodal composites reinforced with faceted, submicron (~500nm), commercially-available B4C with varying volume fractions of coarse grain phase (0–30%) were fabricated. Within this class of engineering materials ultimate tensile strengths of 620–760MPa and tensile elongations of 1.8–3.7% are observed. The microstructural features are investigated in depth, the strengthening mechanisms quantitatively evaluated and the mechanisms for enhancing strength and/or ductility are discussed.

Plasticity enhancement in 25Cr15Co hard magnetic alloy deformed in bridgman anvils

In the work, we studied the evolution of structural and mechanical properties of 25Cr15Co hard magnetic alloy under shear deformation in Bridgman anvils at different rotation angles. It is shown that at the initial stage, severe plastic strains in a highly coercive (α1 + α2) state are localized in shear bands, in which the α1 and α2 phases are dissolved and an oversaturated a solid solution is formed. As this takes place, there arises a mixed structure consisting of misoriented fragments of the (α1 + α2) phase surrounded by interlayers of the a solid solution. A further increase in strain degree results in a single-phase nanocrystalline structure with a grain size of about 50 nm. It is found that the dissolution of the α1 phases in the α2 matrix under severe plastic deformation causes an increase in the strength characteristics and plasticity of 25Cr15Co alloy at all strain degrees under study. Maximum plasticity is found in the alloy with a mixed structure consisting of submicrocrystalline and cellular sites, and formation of nanocrystalline grains causes the plasticity to decrease somewhat.