Mosaic Microstructure Research Articles

On the microstructure and texture evolution in 17-4 PH stainless steel during laser powder bed fusion: Towards textural design

Additive Manufacturing (AM) of metals allows the production of parts with complex designs, offering advanced properties if the evolution of the texture can be controlled. 17-4 precipitation hardening (PH) stainless steel is a high strength, high corrosion resistance alloy used in a range of industries suitable for AM, such as aerospace and marine. Despite 17-4 PH being one of the most common steels for AM, there are still gaps in the understanding of its AM processing–structure relationships. These include the nature of the matrix phase, as well as the development of texture through AM builds under different processing conditions. We have investigated how changing the laser power and scanning strategy affects the microstructure of 17-4 PH during laser powder bed fusion. It is revealed that the matrix phase is δ-ferrite with a limited austenite presence, mainly in regions of the microstructure immediately below melt pools. Austenite fraction is independent of the printing pattern and laser power. However, reducing the time between adjacent laser passes during printing results in an increase in the austenite volume fraction. Another effect of the higher laser power, as well as additional remelting within the printing strategy, is an increase in the average grain size by epitaxial ferrite grain growth across multiple build layers and the development of a mosaic type microstructure. Changes to the scanning strategy have significant impacts on the textures observed along the build direction, while 〈100〉 texture along the scanning direction is observed consistently. Mechanisms for texture formation and the mosaic structure are proposed that presents a pathway to the design of texture via AM process control.

Rod‐Like SnO2/SnS Mosaics for Reversible Large‐Capacity Li‐Ion Storage

AbstractHindering the phase aggregation and improving the electrochemical reversibility is a primary task of Sn‐based anodes for lithium‐ion storage. Constructing a mosaic structure is an alternative solution to accelerate the reaction kinetics and relieve the aggregation of Sn. Herein, a novel rod‐like SnO2/SnS mosaic microstructure is proposed to obtain a reversible high‐capacity anode. The microstructure is composed of alternative ionic conductive SnS blocks into SnO2 nanodomains. Serving as an anode, the SnO2/SnS mosaic harvests a highly reversible capacity of 1182.5 mA h g−1 after 50 cycles at 0.1 A g−1. At a relatively high current density, it delivers a stable capacity of 1271.9 mA h g−1 after 600 cycles at 1 A g−1 and a capacity of 657 mA h g−1 after 300 cycles at 2 A g−1, which is superior to most reported Sn‐based anodes. The soaring electrochemical performance could be attributed to highly reversible layer‐structured SnS blocks with a fast channel for electron/Li+ transfer.

Effect of CrSi2 on the ablation resistance of a ZrSi2-Y2O3/SiC coating prepared by SAPS

To improve the ablation resistance of carbon/carbon (C/C) composites, a proportional amount of ZrSi2-CrSi2-Y2O3 mixed particles were deposited on the surface of SiC-coated C/C composites by supersonic air plasma spraying (SAPS) to form a ZrSi2-CrSi2-Y2O3/SiC coating. The microstructure and phase compositions of the coating were studied by SEM, EDS, XRD and its anti-ablation performance was tested by oxyacetylene torch. The experimental results showed that the ZrSi2-CrSi2-Y2O3 outer coating had a dense microstructure without obvious pores and microcracks, and the thickness reached approximately 150 μm. In the process of being eroded and scoured by the oxyacetylene flame, the coating exhibited excellent anti-ablation property, which was attributed to the mosaic microstructure formed by ZrO2 and a Si-O-Cr liquid film on the coating surface. After experiencing an ablation time of 80 s, the linear ablation rate and the mass ablation rate of the coating were -1.0 ± 0.03 μm s-1 and -0.16 ± 0.014 mg s-1, respectively.

Influence of trimethylaluminum predoses on the growth morphology, film-substrate interface, and microstructure of MOCVD-grown AlN on (1 1 1)Si

Aluminum nitride (AlN) was deposited on (111) silicon by metalorganic chemical vapor deposition after varied trimethylaluminum “predoses”. Growth morphologies, film-substrate interfaces, and film microstructures were examined using scanning electron microscopy, atomic force microscopy, X-ray diffraction, and transmission electron microscopy. In samples grown with predoses, lateral growth was observed over faceted “patches” formed during the predose. Three-dimensional growth was observed to seed from small islands on the surface of these patches and eventually overgrow them. The three-dimensional growth mode was similar to that observed when AlN was grown without a predose, resulting in similar morphologies in all films, regardless of predose, after the islands coalesced. The AlN-silicon interface was found to be predominantly amorphous when no predose was used. However, narrow regions were observed over which the film was in atomic registry with the substrate. This indicates AlN nucleates in epitaxy with the substrate and amorphous silicon nitride forms between nucleation sites due to ammonia exposure. Films grown with predoses had structurally abrupt interfaces, suggesting aluminum within the observed patch features inhibits the reaction between ammonia and silicon at the onset of growth. A structure distinct from both wurtzite AlN and diamond cubic silicon was observed at the substrate interface in films grown with a predose, consistent with either zinc blende AlN or a strained Si/Al alloy. A mosaic microstructure was observed in all films, grown with or without predoses, which consisted of sub-boundaries formed by clusters of threading dislocations. Threading dislocations, separated by hundreds of nanometers, were found to be tilted along common directions, providing evidence for a dislocation bending mechanism possibly enhanced by the predose.

The effect of substrate clamping on the paraelectric to antiferroelectric phase transition in Nd-doped BiFeO3 thin films

Thin films were deposited on Pt/Ti/SiO2/Si substrates using pulsed laser deposition from a target with a composition (Bi0.825Nd0.175Fe0.97Ti0.03O3) with 5mol% excess Bi2O3 within the antiferroelectric (AFE) region of the NdFeO3-BiFeO3 phase diagram. However, Raman spectroscopy and transmission electron microscopy (TEM) revealed that films consisted of a mosaic microstructure in which (AFE), ferroelectric (FE) and paraelectric (PE) phases coexisted. Variation in the spatial distribution of Nd is typically greater in bulk ceramics than in thin films and therefore, the absence of single phase AFE cannot be attributed to local changes in composition. Instead, it is proposed that clamping due to mismatch in thermal expansion coefficient with the substrate suppresses the large volume change associated with the PE-FE and PE-AFE transition in bulk and its absence in the thin film prevents an avalanche-like transition throughout grains, which in bulk sustains single phase AFE, irrespective of local deviations in the Nd concentration.

High‐Hardness Diamond Composite Consolidated by Spark Plasma Sintering

A high‐hardness diamond‐based composite was synthesized by spark plasma sintering (SPS) under 100 MPa, using SiC‐coated diamond powder prepared via chemical vapor deposition (CVD). SiC layers 20–40 nm were uniformly deposited on diamond powders by a rotary CVD technique. The SiC‐coated diamond powder was consolidated with SiO powder by SPS at the sintering temperature of 1873 K, resulting in the formation of fully compacted mosaic microstructure with the Vickers hardness of 36 GPa.

Primary dolomite in the Late Triassic Travenanzes Formation, Dolomites, Northern Italy: Facies control and possible bacterial influence

AbstractIn the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ13C values range between ca −6‰ and +4‰, with the most 13C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most 13C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ18O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ13C as low as −6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ13C values of ca +3‰. Major shifts of δ13C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.

High Hardness and Ductile Mosaic SiC/SiO2 Composite by Spark Plasma Sintering

SiC powder was coated with SiO2 layer by chemical vapor deposition, and the SiC(core)/SiO2(shell) composite powder was consolidated to a SiC/SiO2 composite with a mosaic microstructure by spark plasma sintering (SPS) at 1923 K for 1.8 ks. The SiC(core)/SiO2(shell) powder with a 80–100 nm thick SiO2 layer resulted in a SiC/SiO2 composite with a relative density of 97% and hardness and fracture toughness of 17.1 GPa and 8.4 MPa m1/2, respectively.

John Wesley Mitchell. 3 December 1913 — 12 July 2007

John (‘Jack’) Mitchell was a New Zealander who came to Oxford University on a scholarship of the Commission of the 1851 Exhibition. Mitchell’s association with Nevill (later Sir Nevill) Mott FRS during World War II, when they both worked at the Armament Research Department, afterwards led Mitchell to join Mott at Bristol University, where he began the research into the photographic process for which he is best known. His pursuit of an understanding of the mechanism through which a latent image forms led to the important discovery of the decoration by silver particles of individual dislocations in silver halide crystals and the mosaic microstructure of them. In turn the decoration technique provided the first clear experimental evidence of the link between plastic deformation and the creation and movement of dislocations. In 1960 Mitchell was appointed as professor in the Physics Department at the University of Virginia, where, apart from a brief spell in England as the Director of the National Chemical Laboratory, he worked happily for some 40 years. During this time his research group published many papers describing and explaining the mechanisms of plastic deformation in metallic alloys, devising and using state-of-the-art methods, thereby adding much to a wider understanding of mechanical properties and strength. All the time, working alone, Mitchell further developed and refined his photoaggregation theory of the photographic process, gaining worldwide recognition and honours for his effort.

Effect of Controlled Growth Dynamics on the Microstructure of Nonpolar a-Plane GaN Revealed by X-ray Diffraction

This paper reports the effect of controlled growth dynamics, as monitored by in situ optical reflectance, on the microstructure of nonpolar a-plane GaN films grown on r-plane sapphire. The mosaic microstructure of a-plane GaN and its anisotropy are evaluated by X-ray rocking curve (XRC) measurements. By inserting a pronounced islanding stage followed by an enhanced lateral growth, pit-free a-plane GaN has been achieved showing an XRC linewidth of ∼0.18 and ∼0.3° for on- and off-axes planes, respectively, with only minor anisotropy. The density of basal-plane stacking faults is reduced by ∼70% as determined by a modified Williamson–Hall X-ray analysis.

Atomic ordering in nano-layered FePt

Monte Carlo simulation of chemical ordering kinetics in nano-layered L1 0 AB binary intermetallics was performed. The study addressed FePt thin layers considered as a material for ultra-high-density magnetic storage media and revealed metastability of the L1 0 c-variant superstructure with monoatomic planes parallel to the surface and off-plane easy magnetization. The layers, originally perfectly ordered in a c-variant of the L1 0 superstructure, showed homogeneous disordering running in parallel with a spontaneous re-orientation of the monoatomic planes leading to a mosaic microstructure composed of a- and b-L1 0-variant domains. The domains nucleated heterogeneously on the surface of the layer and grew discontinuously inwards its volume. Finally, the domains relaxed towards an equilibrium microstructure of the system. Two “atomistic-scale” processes: (i) homogeneous disordering and (ii) nucleation of the a- and b-L1 0-variant domains showed characteristic time scales. The same was observed for the domain microstructure relaxation. The discontinuous domain growth showed no definite driving force and proceeded due to thermal fluctuations. The above complex structural evolution has recently been observed experimentally in epitaxially deposited thin films of FePt.

Nitrogen-polar GaN growth evolution on c-plane sapphire

This letter presents a study on N-polar GaN growth evolution on sapphire using a low-temperature GaN buffer, which is distinctly different from the two-step growth of Ga-polar GaN according to both in situ reflectance and ex situ microscopy. Annealed N-polar GaN buffer exhibits densely packed tiny grains, serving as a template for the subsequent high-temperature GaN growth, which starts in a quasi-two-dimensional mode without any roughening-recovery process. Atomically smooth N-polar GaN has been achieved with no stacking fault or inversion domain observed. The mosaic microstructure, electrical, and optical properties of N-polar GaN are compared with those of Ga-polar GaN.

Microstructure and magnetic properties of electrodeposited CoPtP alloys

CoPtP alloys have been grown by electrochemical process in an external magnetic field in order to enhance the perpendicular magnetic properties. The coercivity and squareness of the alloys were improved as the external magnetic field was increased. The alloys have a mosaic microstructure consisting of grains with size in the range of 10–15nm. The grains are separated by highly well defined 1–2nm grain boundaries. The alloys exhibit a columnar microstructure with a hcp [002] fiber texture. The films exhibit significant perpendicular magnetic anisotropy and permanent magnetic properties in the perpendicular direction, with coercivity of up to 6.1kOe and squareness of up to 0.9.

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al x Ga1−xN (0.1 < x < 0.8) films, 0.4 μm in thickness, were prepared on (0001) sapphire substrates at 500,∘C by alternate supply of Ga and Al alkyls and ammonia (NH3). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 μm in thickness, were also grown at 1000,∘C on the LT-Al x Ga1−xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al x Ga1−xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al0.43Ga0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al x Ga1−xN buffer layer induced mosaic microstructure and bonding defects in the films.

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al x Ga1−xN (0.1 < x < 0.8) films, 0.4 μm in thickness, were prepared on (0001) sapphire substrates at 500,∘C by alternate supply of Ga and Al alkyls and ammonia (NH3). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 μm in thickness, were also grown at 1000,∘C on the LT-Al x Ga1−xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al x Ga1−xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al0.43Ga0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al x Ga1−xN buffer layer induced mosaic microstructure and bonding defects in the films.

Microstructural and conductive properties of BaRuO 3 thin films

We have grown conducting BaRuO 3 films on (100) LaAlO 3 substrate using pulsed laser deposition technique over temperature range varying from 500 to 775 °C. The films are well textured and are c-axis oriented with an in-plane epitaxial relationship of 〈010〉〈100〉BaRuO 3∥〈110〉 LaAlO 3. Atomic force microscopy observation shows that they consist of a fine arranged network of grains and have a mosaic microstructure. Surfaces with smooth terraces have been observed by Scanning Tunneling Microscopy. The resistivity of the films has been found to be a strong function of substrate temperature during film deposition. Both metallic and semiconducting behaviour has been observed in these films. Temperature-dependence resistivity measurement shows that the film has a metallic curve if it is deposited at 700 °C or lower but it transfers to a semiconducting-metallic twofold curve if the deposition temperature is increased. This unique phenomenon, which is not observed in bulk, may provide new features useful in the fabrication of novel electronic devices.

Mullite single crystal fibres produced by the internal crystallization method (ICM)

Abstract Optically clear, inclusion-free mullite fibers up to 80 mm in length and 80×70 μm 2 in cross-section were grown from an alumino silicate melt by the internal crystallization method (ICM). Microprobe analysis reveals a high chemical homogeneity at 76.5±0.5 wt.% Al 2 O 3 and 23.5±0.5 wt.% SiO 2 , which is close to the 2/1-mullite composition. Areas of slightly decreased Al 2 O 3 content (≈74.5 wt.%) occur rarely, whereas areas of increased Al 2 O 3 content were not observed. Polarized infrared-reflection micro-spectroscopy using spot sizes of 60 μm diameter on sections cut parallel and perpendicular to the fiber axis show the single crystal character of the fibers. Optical microscopy shows that the fibers consist of a mosaic of single crystal areas up to about 5 mm in length. The c axis of the single crystal individuals are misalligned up to θ ≈±3° with respect to the fiber axis. These orientational misfits are believed to be the reason for the development of the mosaic type microstructure during the growth process.

Raman identification of materials used for jewellery and mosaics in Ifriqiya

AbstractBeads, rings and mosaic tessarae from Tunisia (Carthage and Utica, 1st centuries BC and AD, El Djem, 2nd–3rd centuries and Mahdia, 10th century) were analysed by Raman spectroscopy for non‐destructive differentiation of various types of glasses (most have Si‐ and Na/Ca‐rich compositions, some others are lead‐containing glass) and identification of synthetic or natural minerals (crystal or stone). The results are discussed in the light of chemical analyses. A green glass is coloured by a Cr‐based nanosized pigment and a red mosaic tesserae is obtained by a colloidal dispersion of gold (Cassius purple). Cassiterite (SnO2) is used as an opacifier or fluxing agent. The examination of glass mosaic and jewellery microstructure shows that some pieces are made of sintered glass, i.e. using a paste technology. Copyright © 2003 John Wiley &amp; Sons, Ltd.

SiC voids, mosaic microstructure and dislocations distribution in Si carbonized layers

The defect structure of SiC/Si layers obtained by carbonization of Si is reported by means of transmission electron microscopy in high-resolution (HREM) and conventional (CTEM) modes. 3C–SiC was obtained after a rapid thermal annealing treatment and good interfacial quality is reported in terms of small void dimensions and densities. Moreover, high misfit dislocation densities are observed close to the Si/SiC interface and inside the SiC layer without observable generation of threading dislocations. The mosaic grain structure is also evidenced, with low misorientation with respect to the substrate. These results are encouraging for further growth of III–N alloy heterostructures.

Hyalotekite, (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2(Si,B)2O28F, a tectosilicate related to scapolite: new structure refinement, phase transitions and a short-range ordered 3bsuperstructure

AbstractHyalotekite, a framework silicate of composition (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2(Si,B)28F, is found in relatively high-temperature (⩾ 500°C) Mn skarns at Långban, Sweden, and peralkaline pegmatites at Dara-i-Pioz, Tajikistan. A new paragenesis at Dara-i-Pioz is pegmatite consisting of the Ba borosilicates leucosphenite and tienshanite, as well as caesium kupletskite, aegirine, pyrochlore, microcline and quartz. Hyalotekite has been partially replaced by barylite and danburite. This hyalotekite contains 1.29–1.78 wt.% Y2O3, equivalent to 0.172–0.238 Y pfu or 8–11% Y on the Ca site; its Pb/(Pb+Ba) ratio ranges 0.36–0.44. Electron microprobe F contents of Långban and Dara-i-Pioz hyalotekite range 1.04–1.45 wt.%, consistent with full occupancy of the F site. A new refinement of the structure factor data used in the original structural determination of a Långban hyalotekite resulted in a structural formula, (Pbl.96Bal.86K0.18)Ca2(B1.76Be0.24)(Sil.56B0.44)Si8O28F, consistent with chemical data and all cations with positive-definite thermal parameters, although with a slight excess of positive charge (+57.14 as opposed to the ideal +57.00). An unusual feature of the hyalotekite framework is that 4 of 28 oxygens are non-bridging; by merging these 4 oxygens into two, the framework topology of scapolite is obtained. The triclinic symmetry of hyalotekite observed at room temperature is obtained from a hypothetical tetragonal parent structure via a sequence of displacive phase transitions. Some of these transitions are associated with cation ordering, either Pb–Ba ordering in the large cation sites, or B–Be and Si–B ordering on tetrahedral sites. Others are largely displacive but affect the coordination of the large cations (Pb, Ba, K, Ca). High-resolution electron microscopy suggests that the undulatory extinction characteristic of hyalotekite is due to a fine mosaic microstructure. This suggests that at least one of these transitions occurs in nature during cooling, and that it is first order with a large volume change. A diffuse superstructure observed by electron diffraction implies the existence of a further stage of short-range cation ordering which probably involves both (Pb,K)–Ba and (BeSi,BB)–BSi.