Search

Twin crystal structured Al-10 wt.% Mg alloys that were grown over a broad solidification velocity range were prepared and studied for the first time. The high thermal gradient (G) and growth velocity (V) of directional solidification resulted in the dominant solidification of twins: the twinned dendrite trunks at constant high Vs curved in the G direction with large angles in 7 mm diameter crucibles and invaded regular columnar grains because of a distinct kinetics growth advantage. Transitive deceleration experiments were designed to produce twin crystals that evolved with lower values of V (100, 10, and 0.5 μm/s) and had a structural coarsening trend. Twin cell growth in the absence of arms occurred at a growth velocity of 10 μm/s. A coherency loss was observed at a growth velocity of 0.5 μm/s with straight coherent twin boundaries turning into curved incoherent boundaries. Linear theoretical analyses were performed to understand the structural evolution of the twins. These results demonstrate the possibility of producing dense and controlled twin crystals in the Al-Mg system under most industrial production conditions; thus, this approach can be a new structural choice for designing Al-Mg-based alloys that have widespread commercial applications.

This work investigates the evolution of the crystal structure of the topologically closed packed orthorhombic P phase precipitate in a Re-containing Ni-based single crystal superalloy during thermal exposure. The P phase is formed with a thin needle morphology. The precipitate is formed from the matrix with an initial complex atomic arrangement which continues to evolve during the process of thermal exposure. Based on the experimental evidence of this study and the theoretically predicted structure reported in the literature, a mechanism of the structural transformation is proposed. The initial structure is composed of a parallelogram (P) atomic arrangement configuration, which gradually evolve into a rectangle (R) atomic arrangement configuration in the [100]P projection. In the [010] projection, the initial structure is composed of alternating rows of a larger parallelogram (P') and a larger rectangle (R') configurations, which gradually evolve into an intricate structure of P'-R' along the length [001]P direction and P'P'-R'R' along the transverse [100]P direction. The initial structure is formed for its structural similarities to the γ phase to minimize lattice mismatch. The final structure is evolved over time to conform to its thermodynamically more stable state. The structural evolution is achieved by collective atomic shuffling. These intricate atomic arrangements gives rise to the very faint and highly dense parallel striation lines along the transverse [010]P direction in [100]P view and along the length [010]P direction in [010]P view under transmission electron microscopy observation.

The deformation activities near crack tips of submicron-sized single-crystalline Mo were investigated using in situ transmission electron microscopy (TEM) with mixed mode I-II loading. Results show that dislocations in multiple slip systems were activated in front of crack tips. These dislocations glided on the uncommon slip planes of {123}, forming dislocation arrays. These dislocations moved at velocities of 3–5 nm/s with spacing of ∼10–34 nm in the zone of ∼50–300 nm away from crack tips. Dislocation velocity and spacing were influenced by the force from elastic crack stress field. Additionally, phase transformation from body-centered cubic to face-centered cubic was also activated in front of crack tips, and high densities of interface dislocations were observed at the semicoherent phase interfaces. Two kinds of phase transformation mechanisms were uncovered. One is the Pitsch mechanism, which is rarely accessed, while the other is the Nishiyama-Wasserman/Kurdjumov-Sachs mechanism.

Topologically close-packed (TCP) phases in Ni-based single crystal superalloys are known to exhibit different morphologies and orientation relationships within the matrix. This study investigated the influence of chemical composition on lattice parameters, which in turn determine the interface lattice misfit between TCP phase and matrix and thus shape and orientation of the TCP phase. In this study, TCP phase particles of different morphologies were formed by ageing at different temperatures. The level of TCP-phase element enrichment was found to increase with increasing growth time and ageing temperature, and thus leads to a same evolution behavior of the atomic volume.

Based on the simplest TEOS/P123/HCl(aq.) templating system, it is demonstrated in this work that different SBA-15 platelets can be controllably prepared via the partitioned cooperative self-assembly (PCSA) process without using any additives or complicated procedures. Experimental results show that the reduced amounts of TEOS added in the 2nd additions in the PCSA process play a determining role in transforming the SBA-15 platelet morphologies from the plane ones to a unique kind of UFO-shaped ones. The obtained SBA-15 platelets, plane or UFO-shaped ones, are composed of plugged mesochannels aligned in a 2D hexagonal symmetry, with neighboring channels connected by large tunnel holes (intrawall pores). Different platelet morphologies and their evolution were found to be dependent on partitioning conditions (2nd TEOS addition and interval time). A competitive process involving the aggregation of SBA-15 primary particles dictates the formation of SBA-15 particles with different morphologies, meanwhile the UFO-shaped SBA-15 platelets (U-SBA-15P) can be deemed as intermediate ones between rod-like particles and plane platelets. Moreover, upon the release of indometacin (IMC) in simulating intestinal fluid, UFO-shaped SBA-15 platelets show faster drug release performance compared with their plane counterparts or conventional SBA-15 fibers. The larger the protruded areas in UFO-shaped platelets, the higher the release rate will be. This work, as we believe, presents a new strategy to regulate drug releasing performance by simply tuning particle morphologies of SBA-15 platelets and can allow rational design of IMC/meso-silica formulations.

The microstructure, thermal and mechanical stability of Ni-based single crystalline superalloys depend strongly on the alloying elements and their concentrations. Alloying has been the main design strategy for stabilizing the compositions, microstructures and thermal-mechanical properties. This article presents a review on the effects of some common alloying elements on the microstructural and mechanical property stability control of Ni-based superalloys. The various alloying elements are divided into four categories according to their main effects on these properties, comprising base elements, mechanical strengthening elements, long term stability elements and the oxidation resistance elements. The mechanical strengthening elements can further be divided into precipitation, solid solution and grain boundary segregation elements. The precipitation elements strengthen the alloys by forming the L12 structured γʹ phase. The solid solution elements strengthen primarily the γ phase, by increasing the solidus temperatures and decreasing the stacking fault energy, which in turn influences the thermal stability of the phases and the resistance of dislocation movement. The grain boundary elements strengthen the alloys by the formation of carbides and borides along the grain boundaries during solidification, which help to prevent the formation of casting pores and hot tearing and to strengthen low angle boundaries. The long-term stability elements inhibit the precipitation of topologically closed-packed phases causing deterioration of the mechanical properties. The oxidation resistance element, mainly Al, promotes the formation of protective Al2O3 surface layer.

The precipitation and growth morphology of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy during thermal exposure were studied. The alloy was found to develop acicular-shaped coherent μ phase particles along γ directions of the matrix after exposure at 950 °C. P phase precipitates were found to form with increased time and at higher temperatures, and in different morphologies, including acicular shaped particles along γ and γ directions, and plate-like particles in {110}γ planes of the matrix. The different P phase precipitates were found to have different lattice constants and different lattice correspondences with the matrix. The μ phase precipitate was also found to have different lattice correspondences with those reported in the literature. The multiplicity of lattice correspondences for the same TCP phase is attributed to the different lattice parameters, which render the precipitates to have different coherency relationships with the matrix.

In this work, it is demonstrated that hierarchical mesoporous silica monoliths (HMSMs) with trimodal porosities can be facilely prepared using P123 as structure-directing agent and tetraethoxysilane as precursor. In such HMSMs, classic co-continuous macrostructures with interconnected macropores and smooth isotropic skeletons can be obtained in a tunable way, depending on the synthesis variables, including the amounts of P123 and 1,3,5-trimethylbenzene and acid concentrations. Skeletons feature randomly oriented SBA-15-type primary particles with 2D hexagonally ordered mesochannels largely accessible from bimodal macropores. Co-continuous macropore size of a typical HMSM (P1.2) is 19.6 μm, intra-skeleton macropore of 1.9 μm and mesopore size of 9.9 nm. The BET surface area by N2 sorption and total pore volume by Archimedes principle reach 590 m2 g-1 and 3.56 cm3 g-1, respectively. Based on systematic study on the relationship between synthetic compositions and porous structures, the ternary diagram and formation mechanism of these HMSMs were analyzed.

Previous molecular simulations and experiments suggest that grain boundaries (GBs) serve as dislocation sources in nanocrystalline metals. Although a large number of studies have been carried out, direct experimental evidence of dislocation nucleation from GBs has rarely been achieved. In this work, we performed in situ transmission electron microscopy (TEM) observations for a Pt nanocrystalline film with an average grain size of ~10 nm during tensile deformation. This study revealed direct evidence of dislocation nucleation at the GBs at the atomic scale. This is different from the common hypothesis predicted by molecular dynamic simulations that only partial dislocations are emitted from GBs in small-grained structures.

Selective oxidation is one of the most important oxidation phenomena in multi-phase or multi-component alloys. Complex multi-layered oxides with randomly distributed pores beneath the outer oxide deeply and comprehensively hamper conventional studies. Multi-dimensional structures and elemental distributions are reconstructed on the initial oxidation of typical second- and third-generation Ni-based single crystal superalloys by electron tomography. The initial oxidation of the second-generation alloy has a two-layered structure with an outer Ni-rich oxide layer and an inner Al-rich oxide layer. The γ phase in the third-generation alloy is easily oxidized to form a rough surface with extruded oxide particles.

Understanding the high-temperature initial oxidation mechanism and its dynamic processes is generally important because a small amount of initial oxidation can induce rapid corrosion and cause catastrophic failure. We reveal the mechanism of initial oxidation in a third-generation Ni-based superalloy by in situ visualizations and investigations of the nano- and atomic-scale dynamic processes from room temperature to 900 °C with a Cs-corrected environmental transmission electron microscope. The initial oxidation starts from the γ/γ′ interface at low temperatures. The high-temperature oxidation process with deficient oxygen prefers oxidation sites at the cross-junctions of the γ/γ′ interfaces. The growth rate of the oxide nanoparticles depends on the oxidation temperatures, with a low rate below 600 °C and a rapid-growth speed at temperatures higher than 700 °C. Mass transfer from the γ′ and γ phases to the γ/γ′ interface, particularly at the cross-junctions of these interfaces, is observed to cause oxide accumulation. This study provides a direct observation of the interphase, interface-junction-initiated outward oxidization including Al and Cr elements. These results shed light on the initial oxidation mechanisms of materials containing a second phase with interphase interfaces and the materials at deficient oxygen conditions as well as those with protection coating layers.

In this work, two components of poly(ethylene glycol) (PEG) are used as dual-templates for facile and cost-effective preparation of hierarchically mesoporous silica monoliths (HMSMs) with controllable mesostructures and co-continuous macrostructures from low-toxicity tetraethoxysilane. The influence of different preparative parameters on the porous structures was studied, including the proportions of PEGs and acidity. The typical HMSM (PEG-1.5/0.5) with skeleton mesopore size of 6.3 nm, skeleton thickness of 1.25 μm and macropores of 1.87 μm can be obtained. The total pore volume and high surface area reach 711 m2 g− 1 and 3.37 cm3 g− 1 (including mesopore volume of 0.93 cm3 g− 1), respectively. The PEG400 and PEG35k are proved to play cooperative roles as dual-templates in harnessing the mesoporous skeletons and co-continuous macrostructures. Interestingly, in such preparation system, the use of PEG400 is found to be play key role in inducing the formation of skeletons with relatively large mesopores, while the amount of PEG35k is a more sensitive factor in shaping the macroporous structures. This work is believed to open a new path to engineering the macroporous and mesoporous structures via the TEOS/PEG/HCl(aq.) synthesis system.

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B4C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B4C/C preform and the resultant RBBC composites is reported. The B4C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m1/2, respectively.

This work reports the controllable preparation of a unique kind of hierarchically mesoporous silica (HMS) fibers with bimodal porosities based on a simple TEOS/P123/HCl(aq.) templating system via the partitioned cooperative self-assembly (PCSA) process. Experimental results show that the formation of hierarchical mesostructures, especially the 2nd mode porous structure, depends on the interval time pertaining to the PCSA process. Synthetic conditions, including the interval time, temperature and stirring, are all found to be important in the evolution of fiber particle morphology. Moreover, such HMS fibers can be used as building units to prepare mesoporous silica monoliths with hierarchical trimodal pore systems via a simple gel-casting method, with additional (the 3rd modal) macropores originating from the packing of micron-sized HMS fibers. Such materials might further expand the application of both HMS fibers and HMS monoliths in various fields.

Spallation of thermal oxidation-induced cavities is the main failure mode in CoAl-based alloys, and the γ/γ′ phase structure in Co–Al–W–Ti–Ta alloys has a propensity to form oxide cavities. Multidimensional investigations were carried out herein on irregular oxide cavities by analytical microscopy, including electron tomography and energy dispersive X-ray spectroscopy techniques, to disclose the time-resolved three-dimensional morphological structure and elemental distributions. The alloy nanotip was exposed isothermally in air at 350 °C, and its oxidation evolution was characterized and analysed. The size and quantities of cavities in the γ phase were larger than those in the γ′ phase, which was attributed to the higher content of cobalt in the γ phase than in the γ′ phase. Different migration rates between cobalt outward and aluminium inward prompted the formation of cavities between the outer and inner layers.

In this study, we investigate the movement of dislocations in Ni-based superalloys under the influence of lattice misfit stresses between the ordered γ'-cuboids and the disordered γ-matrix during thermal exposure in the absence of an applied load. This study focuses on a different condition than the conventional creep testing, and thus offers a unique opportunity to study the intrinsic behavior of the alloy. The dislocation density increases substantially with time during thermal exposure, leading to the formation of various configurations of dislocation networks on the {100} γ/γ′ interfaces, including diamond-shaped, / mixed polygon-shaped and square-shaped networks. During thermal exposure, the b = 1 2 110 > native dislocations first move and evolve into 60° mixed dislocations along the directions on the {100} γ/γ′ interfaces, forming the diamond-shaped dislocation networks. In the case of longer thermal exposures, the dislocations further evolve into pure edge dislocations along the on the {100} γ/γ′ interface, leading to the evolution of the diamond-shaped dislocation networks into square-shaped networks, with the mixed / dislocation networks as an intermediate stage during the transition. These movements occur by sweep glide in the {111} planes and diagonal climb on the {100} planes for the edge and mixed dislocations and by cross-slip for the screw dislocations. The driving force for all of these movements is the interaction between the normal misfit stresses and the edge components of the Burgers vectors of the dislocations to relax the misfit stresses. An analysis based on the elastic strain energy considerations is presented to explain the driving forces for the dislocation movements.

Two distinct evolutionary processes of secondary γ′ precipitates in a Ni-based single crystal superalloy were revealed by means of Cs-corrected high-resolution transmission electron microscopy (HRTEM) and corresponding X-ray energy dispersive spectroscopy analyses. Upon creep testing at elevated temperatures, it was determined that large quantities of secondary γ′ precipitates were formed. The precipitates were found to select two distinctive forms and routes: one randomly dispersed in the γ matrix (2nd γ′ γ ), and the other formed at nodes of dislocation networks (2nd γ′ d ) in regular arrays. These two types of secondary γ′ precipitates exhibited different evolutionary morphologies and mechanisms. The 2nd γ′ γ precipitates progressively evolved in shape from spherical to cuboidal and then to butterfly-like shapes, and the mean size also increased gradually from 11 nm to 34 nm. The 2nd γ′ d precipitates exhibited stable cuboidal morphology and a mean size of ∼30 nm. These differences are attributable to the different element transport conditions for the two types of precipitates, i.e., an uphill diffusion for the 2nd γ′ γ precipitates and a dislocation network pipe transportation for the 2nd γ′ d precipitates. The 2nd γ′ γ precipitates show severe segregation of γ′-rich elements of Ni and Al and deficiency of γ′-poor elements of Cr compared with the 2nd γ′ d precipitates.

In this work, a facile and controllable multi-templating approach based on a solo nonionic surfactant P123 and peroxotitanic acid (PTA) was reported for the preparation of bimodal meso-mesoporous titania (BMM-TiO 2 ) with high surface area (150–243 m 2 g −1 ), high pore volume (0.3–0.6 cm 3 g −1 ), large mesopore sizes spanning from 8 to 16 nm (BJH pore size), and high & tunable crystallinity (Anatase or Anatase + Rutile). The BMM-TiO 2 was shown to be a homogeneous ‘mixture’ of two series of wormhole mesostructures with two modes of mesopore sizes that depend on both the contents of P123 and synthetic conditions. The formation of BMM structures was proposed to arise from both the formation of differentiated micellar structures of P123 (dual soft-templating) and their subsequent different mesostructural shrinkages driven by prolonged drying process ( i.e. , 100 °C for 12 h) in the presence of PTA. The low-temperature crystallization behaviours of PTA, unusual hard-templating effect of P123 and its carbon derivatives (3rd fold templating), large mesopores relative to the surrounding TiO 2 crystals are all believed to be responsible for the high thermal stability of the obtained BMM-TiO 2 . To our knowledge, it is for the first time reported that P123 plays such multiple templating roles (three folds in this work) in the preparation of meso-TiO 2 , not to mention that P123 was also confirmed to be a prerequisite for the formation bi-crystalline BMM-TiO 2 . In the photodegradation test of Rhodamine B in water by UV irradiation, other than the high surface area and synergistic effect between anatase and rutile phases, the BMM structures were also found to be advantageous to enhance the photocatalytic properties of mesoporous TiO 2 .

This work demonstrates that based on a simple improved ternary nonionic surfactant templating system, hierarchically meso-mesoporous (HMM) silicas with ultra-large pores and pore volumes can be facilely prepared by combining hydrothermal treatments (HTT) with the partitioned cooperative self-assembly (PCSA) process. The obtained clew-like HMM silica particles contain bimodal pore size distributions. The disordered 1st mode mesostructures (pore sizes of 7–20 nm) form the ‘yarns’, while the 3D-interconnected 2nd mode intra-particulate pores (33–118 nm in sizes) exist between the ‘yarns’. The ultra-large pore volume reaching 3.22 cm 3 g −1 can be achieved. The formation of such HMM silicas arises from the HTT induced fragmentation of the 1st mode mesostructures.

Tungsten oxide thin films were deposited at room temperature under different negative bias voltages (Vb, 0 to −500 V) by DC reactive magnetron sputtering, and then the as-deposited films were annealed at 500 °C in air atmosphere. The crystal structure, surface morphology, chemical composition and transmittance of the tungsten oxide thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrophotometer. The XRD analysis reveals that the tungsten oxide films deposited at different negative bias voltages present a partly crystallized amorphous structure. All the films transfer from amorphous to crystalline (monoclinic + hexagonal) after annealing 3 h at 500 °C. Furthermore, the crystallized tungsten oxide films show different preferred orientation. The morphology of the tungsten oxide films deposited at different negative bias voltages is consisted of fine nanoscale grains. The grains grow up and conjunct with each other after annealing. The tungsten oxide films deposited at higher negative bias voltages after annealing show non-uniform special morphology. Substoichiometric tungsten oxide films were formed as evidenced by XPS spectra of W4f and O1s. As a result, semi-transparent films were obtained in the visible range for all films deposited at different negative bias voltages.

In their comment (Scripta Mater. (2016)), Cormier et al. raised questions on the findings reported in our earlier publication (Acta Mater. (2016) of the formation of 2nd γ′d and 2nd γ′γ precipitates in a Ni-based single crystal superalloy during creep testing at elevated temperatures, and suggested that the 2nd γ′d particles were mere observations of surface protuberances of the primary γ′ rafts and that the 2nd γ′γ precipitates were possibly formed during cooling after the creep test. To clarify these, we conducted new experiments, and present the new evidence and our responses to the points raised in this short communication.

It is known that Ni-based superalloys experience microstructural rafting during high temperature straining. This paper presents a comparative study of the different rafting models by means of finite element modeling and experimentation investigation. It was found that uniaxial elastic loading does not alter the misfit or misfit isotropy of the structure, but causes repartitioning of the misfit into elastic strains in the γ and γ' phases. This causes an elastic strain energy anisotropy and hydrostatic stress anisotropy among the vertical and horizontal γ phase channels. Both anisotropies predict a raft structure that is contrary to the experimental observation. Through a carefully designed pre-treatment at 750 °C and under a uniaxial stress of 750 MPa, an anisotropic dislocation structure was created on the {001} γ'/γ interfaces without altering the cuboidal morphology of the γ' phase. This allows the evaluation of the influence of dislocations on the rafting behavior of the alloy. It was found that rafting occurred in the pre-treated samples containing anisotropic dislocation structures during thermal exposure without applied stress, instead of isotropic corsening, and that the raft structures conform to the expectations base don the anistropic dislocation strucutres. This demonstrates that an anisotropic dislocation structure on the {001} γ'/γ interfaces is a direct cause of rafting. This is attributted to the fact that dislocations on γ'/γ interfaces help to relax the local misfit strains, causing some to expand at the expense of others. At the same time, the formation of the anisotropic dislocation structure is a direct result of the anisotropy of elastic strains induced by the applied bia stess in during the pre-treatment. These explain the complex interplay of the elastic and plastic models reported the literature.

Modern in situ oxidation processes can be observed down to the nanoscale using environmental transmission electron microscope (TEM), and this technology has been applied to conventional corrosion science to provide more detailed intrinsic information on the oxidation properties of materials. The influence of chromium (0–10 at.%) addition on the microstructure and oxidation properties of a γ/γ′ two-phase Co82Al9W9-based superalloy was systematically investigated. The in situ oxidation behaviour of Cr-containing alloys was studied using environmental TEM, and the addition of moderate Cr amounts is suggested for the alloy design.

This study investigated a novel hypothesis on the influences of residual or pre-dislocation structures on creep life of Ni-based single crystal superalloys. Different types of anisotropic dislocation structures were created by pre-straining the samples at a relatively low temperature under different constant bias load conditions. After such pre-straining treatment, the samples were subjected to creep testing at higher temperatures under uniaxial tension. It was found that the pre-straining treatment was effective in creating different anisotropic dislocation structures without affecting the γ′ phase cuboid morphology and that the samples containing different anisotropic dislocation structures have all significantly reduced creep lives. The difference in the anisotropy of the dislocation structures has little influence on the reduction of the creep life or the rafting morphology of the γ′ phase cuboids. By using a Re-free alloy, the influence of the TCP phase precipitation on creep life is also clarified. These observations suggest that interruption to high temperature straining is the main contributor to the reduced creep lives. This finding raises a serious concern on the validity, or reliability, of creep life assessment based on continuous laboratory creep tests for real life applications, where the services are almost always interrupted into intermittent sessions. Keywords: Ni-based superalloys, Creep, Dislocation structures, Anisotropy, Pre-straining treatment

Ferrite thin-film RF integrated inductors using IC compatible processes are proposed and also demonstrated by experiments. Ferrites (Co-based, MnZn-based, NiZn-based and YFe garnet) for RF integrated inductors are investigated, and CoZrO ferrite shows the suitable high-frequency performance for RF applications. Various inductors with different kinds of ferrite thin-films have been fabricated using fully integrated processes. A typical inductor sample with COZrO thin-film is presented. The L and Q values of the sample are 2.05 nH and 20.5 at 2 GHz, respectively. The inductance is raised by 17% and the quality factor is raised by 41%, compared with the inductor, which doesn’t include ferrite thin-film. With the improvement of L and Q, the size of ferrite thin-film inductors can be reduced significantly.