Submicron Grain Research Articles

Regulation of energy density and efficiency in transparent ceramics by grain refinement

Seeking for lead-free transparent ferroelectric ceramics with excellent recoverable energy storage density (Wrec) and high energy storage efficiency (η) is conducive to the development of transparent pulse capacitors for use in energy storage. In this work, we realized an effective grain size engineering via introducing a second component, namely, SrZrO3, into the K0.5Na0.5NbO3 (KNN) ceramics. Interestingly, the obtained submicron grains not only improve the transparency of the ceramics but also enlarge the breakdown field strength (Eb) and thus the Wrec. A good transparency of up to ~68% in the near-infrared region, an excellent Wrec of ~2.81 J·cm−3 and an extremely high η of ~80% were simultaneously achieved in the 0.91 K0.5Na0.5NbO3-0.09SrZrO3 (0.91KNN-0.09SZ) ceramics. The reduced grain and domain sizes, as well as the increased cation disorder, are believed to be responsible for the obtained narrow P-E loops and calculated large diffusion factor value (γ). This work provides a useful reference for developing KNN-based lead-free relaxor ferroelectric materials with both high energy storage properties and a good transparency.

Effects of dielectric thickness on energy storage properties of 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 multilayer ceramic capacitors

Multilayer Ceramic Capacitors (MLCCs) for energy storage applications require a large discharge energy density and high discharge/charge efficiency. Here, 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 (BTBZNT) powders were synthesized via solid-state reactions, and MLCCs with sub-micron grains were fabricated using a two-step sintering method. The BTBZNT MLCCs, which are well-defined relaxor ferroelectrics, possessed relatively high permittivity, dielectric sublinearity, and negligible hysteresis. An AC breakdown strength (BDS) enhancement from 511 to 1047 kV/cm was obtained as the dielectric thickness (D) decreased from 26 to 5 μm. The strong correlation between BDS and D, described asBDS∝D−0.368, agreed well with the general macroscopic theory of thermal breakdown in insulators. The enhanced BDS afforded the BTBZNT MLCCs (D∼9 μm) with excellent energy storage properties with a maximum discharge energy density of 10.5 J/cm3 and discharge/charge efficiency of 93.7% under a maximum electric field of 1000 kV/cm. The MLCCs also exhibited excellent thermal stability with discharge energy density variation <±5% over a wide temperature range of −50 to 175 °C under an electric field of 400 kV/cm. These remarkable performances make BTBZNT MLCCs promising for energy storage applications.

Mechanistic investigation of a low-alloy Mg–Ca-based extrusion alloy with high strength–ductility synergy

High strength–ductility synergy is difficult to achieve in Mg alloys. Although high strength has been achieved through considerable alloying addition and low-temperature extrusion, these techniques result in low ductility (2%–5%). In this work, a novel low-alloy Mg–Ca-based alloy that overcomes this strength–ductility trade-off is designed. The alloy has an excellent tensile yield strength (∼425 MPa) and exhibits a reasonably high elongation capacity (∼11%). A microstructure examination reveals that a high density of submicron grains and nano-precipitates provides the alloy high strength, and the leaner alloy additions and higher extrusion temperatures initially improve ductility. As a result, the density of residual dislocations is reduced, and the formation of low-angle grain boundaries (LAGBs) is enhanced. With fewer residue dislocations, it becomes less probable for the newly activated mobile dislocations to be impeded and transformed into an immobile type during the subsequent tensile test. The LAGBs function as potential sites to emit new dislocations, thus enhancing the dislocation–multiplication capability. More importantly, they can induce evident sub-grain refinement hardening and guarantee that the alloy achieves high strength. The findings lead to a controllable Mg alloy design strategy that can simultaneously afford high strength and ductility.

Ultrafine grain recrystallisation of a metastable-β Ti-alloy via conventional thermomechanical processing

Alloys with ultrafine grains (UFG) offer high strength potentially combined with ductility. Until now, producing ultrafine grains in ingot alloys has required either severe plastic deformation techniques or flash annealing, neither of which are scalable to bulk alloy production. In this work, we formed submicronic grains in the metastable β titanium alloy Ti-20Nb-6Zr (at%), using conventional cold rolling and annealing at 823K in a conventional furnace. The cold rolling (298K) transformed the β structure mostly to α” martensite, but if the rolling temperature was raised to 453K, martensite formation was supressed, and no grain refinement occurred during the subsequent similar annealing treatment. Therefore, we attribute the formation of ultrafine grains to a mechanism involving stress-induced martensite and its reverse transformation.

Prediction of Heterogeneous Microstructural Evolution in Cold-Sprayed Copper Coatings Using Local Zener-Hollomon Parameter and Strain

Cold spray processing is a solid-state coating technique and an emerging method for additive manufacturing, in which metal powder particles are bonded through high-velocity impact-induced deformation. However, the severe plastic deformation of powder particles at extremely high strain rates, high strain gradients, and localized elevated temperatures yields rather complex and heterogeneous microstructures in materials produced as coatings or bulk forms. A good understanding, and even prediction, of such heterogeneous microstructures is essential for determining the post-processing conditions as well as the final properties of cold-sprayed products. In this study, we employ a cold spray system to deposit copper coatings over a large temperature range from 373 K to 873 K and we systematically investigate the microstructural evolutions of the coatings using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Diverse microstructures have been observed, including recrystallized grains, annealing twins, shear bands, submicron grains, deformation twins, and nanometer-sized grains. To understand the formation of such complex microstructures, we obtain local strains, strain rates and temperatures of the cold-sprayed powder particles using the finite element method (FEM). Based on our experimental and simulation results, we created the first deformation mechanism map for cold sprayed coatings to interpret and predict the heterogeneous microstructural evolutions in copper using the local Zener-Hollomon (Z) parameter and plastic strain (strain-Z-microstructure map). Such a map can be used to predict and design the microstructures of cold-sprayed copper samples based on processing parameters and can also be extended to other severe plastic deformation (SPD) processes, such as cutting, extrusion, and solid-phase welding.

Strain glass transition of cobalt phase in a cemented carbide

Cemented carbides are hard materials used for the fabrication of cutting tools. They consist of hard micron or sub-micron carbide grains held together in a matrix of a tough metallic binder such as cobalt. The understanding and control of cobalt characteristics are of crucial importance to improve the mechanical properties of WC-Co cemented carbide materials. Indeed, cobalt controls the toughness of the material and its properties seem to condition the durability of cemented carbides. At low temperature, pure cobalt is normally found in hcp structure, while above 426 °C (700 K) it changes to fcc crystalline structure. Remarkably, in cemented carbides, cobalt appears even at low temperature as fcc, otherwise as a mixture of fcc and hcp. It is unsure if these crystal structures are due to the presence of internal stresses or due to W and C soluted atoms. However, this paper demonstrates that cobalt in WC-10wt.%Co cemented carbide may show some characteristics of a glass transition at around 669 °C (942 K) in the 1 Hz frequency range, inducing a glassy state to cobalt, which is of great interest from a fundamental point of view, however challenging to control in materials technology.

One-step direct fabrication of phase-pure Cu2O films via the spin-spray technique using a mixed alkaline solution

The spin-spray technique was used to fabricate phase-pure Cu2O films at 70 °C with a high deposition rate of 0.3 μm/min. The source solution, containing a mixture of CuSO4･5H2O and l-ascorbic acid, and the reaction solution, containing NH3 and NaOH, were sprayed on the rotating substrates. It was possible to control the grain size, the crystalline orientation of the films, the surface morphology, and the optical band gap by adjusting the concentrations of NH3 and NaOH in the reaction solution. The films fabricated with a low NH3 aq. concentration were composed of nanosized grains and highly [111] oriented. On the other hand, films fabricated with a high NH3 aq. concentration were composed of submicron grains and highly [100] oriented. The surface morphologies of the films varied according to the concentration of NaOH. The surface of the films fabricated with a low NaOH concentration were almost flat and had a {100} orientation on their surface. On the other hand, films fabricated with a high NaOH concentration were composed of pyramidal grains. A smaller grain size in the films corresponded with a higher optical band gap.

Selective laser melting and heat treatment of precipitation hardening stainless steel with a refined microstructure and excellent mechanical properties

In this study, we concentrated on the microstructural characteristics and mechanical properties of precipitation hardening (PH) stainless steel 15-5PH via Selective laser melting (SLM). A large amount of retained austenite was observed in the as-built samples which contribute to excellent ductility. The amount of retained austenite reduced greatly after heat treatment and the remaining minor retained austenite completely transformed to martensite during tensile process. A finer grain structure and a concentration of dislocations around the inclusions and at the submicron grain boundaries following heat treatment promoted SLM 15-5PH to demonstrate a higher strength than those of wrought samples.

Fabrication of Er:Y2O3 transparent ceramics for 2.7 μm mid-infrared solid-state lasers

Laser grade 7 at.% Er:Y2O3 transparent ceramics with submicron grain size were fabricated by using one-step vacuum sintering followed by hot isostatic pressing (HIPing) technique. Through studying the sintering trajectory of Er:Y2O3 ceramics, the sintering temperature zone where sufficient relative density (>96%), no pore-boundary separation, and sub-micron grain size (<1 μm) ceramic samples could be identified. The samples pre-sintered in this zone were readily densified by HIPing. To maximum the densification and achieve high transparency, it is critical to suppress the final-stage grain growth. After HIPing at 1520 °C, the Er:Y2O3 ceramics were fully densified without further grain growth, and exhibited in-line transmission of about 81.6% at 2000 nm. Continuous wave (CW) room temperature laser operation of the Er:Y2O3 transparent ceramic at 2.7 μm was demonstrated.

Spatially resolved spectroscopy of the debris disk HD 32297

Context. Spectro-photometry of debris disks in total intensity and polarimetry can provide new insight into the properties of the dust grains therein (size distribution and optical properties). Aims. We aim to constrain the morphology of the highly inclined debris disk HD 32297. We also intend to obtain spectroscopic and polarimetric measurements to retrieve information on the particle size distribution within the disk for certain grain compositions. Methods. We observed HD 32297 with SPHERE in Y, J, and H bands in total intensity and in J band in polarimetry. The observations are compared to synthetic models of debris disks and we developed methods to extract the photometry in total intensity overcoming the data-reduction artifacts, namely the self-subtraction. The spectro-photometric measurements averaged along the disk mid-plane are then compared to model spectra of various grain compositions. Results. These new images reveal the very inner part of the system as close as 0.15″. The disk image is mostly dominated by the forward scattering making one side (half-ellipse) of the disk more visible, but observations in total intensity are deep enough to also detect the back side for the very first time. The images as well as the surface brightness profiles of the disk rule out the presence of a gap as previously proposed. We do not detect any significant asymmetry between the northeast and southwest sides of the disk. The spectral reflectance features a “gray to blue” color which is interpreted as the presence of grains far below the blowout size. Conclusions. The presence of sub-micron grains in the disk is suspected to be the result of gas drag and/or “avalanche mechanisms”. The blue color of the disk could be further investigated with additional total intensity and polarimetric observations in K and H bands respectively to confirm the spectral slope and the fraction of polarization.

Influence of milling direction in the machinability of Inconel 718 with submicron grain cemented carbide tools

The nickel-based alloys have a growing demand in many fields due to their outstanding properties at high temperatures. These properties lead to relatively low machinability, one of the main obstacles to its more extensive use. Improvements in cutting tool quality are one of the key points to overcome the challenges. The decrease to a submicron scale of the grains of cemented carbide tools is one of the alternatives to improve the machinability of the nickel-based superalloys. In this paper, two different grades of submicron grains of uncoated cemented carbide tools, TMG30 (10% Co, S30-40) and CTS18D (9% Co, S20-40), were evaluated in the end milling process of Inconel 718, through a 24 factorial design of experiments having as parameters the cutting speed, feed rate, machining direction (up and down milling), and tool grade. The tool life, machining power, and surface roughness were used as machinability evaluators. It was found that the machining direction, cutting speed, and feed rate had a significant influence on the machinability output variables, with the machining direction being the most significant one. The differences in the two tool grades were too small to be statistically significant. Simulations using the finite element method of the effective plastic strain, validated by the measurement of experimental machining power, showed that the up milling presented around 14% more plastic deformation than the down milling, which combined with the work-hardenability of the Inconel 718 explains the shorter tool life of this condition.

Macroscopic to nanoscopic in situ investigation on yielding mechanisms in ultrafine grained medium Mn steels: Role of the austenite-ferrite interface

Ultrafine austenite-ferrite duplex medium Mn steels often show a discontinuous yielding phenomenon, which is not commonly observed in other composite-like multiphase materials. The underlying dislocation-based mechanisms are not understood. Here we show that medium Mn steels with an austenite matrix (austenite fraction ∼65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. We observe that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for discontinuous yielding in such steels. We simultaneously study the Lüders banding behavior and the local deformation-induced martensite forming inside the Lüders bands. We find that grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Lüders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening.

Ionic and electronic transport in calcium-substituted LaAlO3 perovskites prepared via mechanochemical route

The present work explores mechanosynthesis of lanthanum aluminate-based perovskite ceramics and corresponding effects on ionic-electronic transport properties. La1-xCaxAlO3-δ (x = 0.05–0.20) nanopowders were prepared via one-step high-energy mechanochemical processing. Sintering at 1450 °C yielded dense ceramics with submicron grains. As-prepared powders and sintered ceramics were characterized by XRPD, XPS and SEM. Electrochemical studies showed that partial oxygen-ionic conductivity in prepared La1-xCaxAlO3-δ increases with calcium content up to 10 at.% in the lanthanum sublattice and then levels off at ∼6 × 10−3 S/cm at 900 °C. La1-xCaxAlO3-δ ceramics are mixed conductors under oxidizing conditions and ionic conductors with negligible contribution of electronic transport in reducing atmospheres. Oxygen-ionic contribution to the total conductivity is 20–68% at 900 °C in air and increases with Ca content, with temperature and with reducing p(O2). Impedance spectroscopy results showed however that electrical properties of mechanosynthesized La1-xCaxAlO3-δ ceramics below ∼800 °C are determined by prevailing grain boundary contribution to the total resistivity.

Effect of cryogenic cooling on texture and microstructural evolution of ZK60 magnesium alloy during machining

Producing ultra-fine grains on a material’s surface is highly pronounced nowadays, which has more influence on the surface integrity properties of the finished component. In this report, severe plastic deformation (SPD) was employed through cryogenic machining to form fine grains on the machined surface of the ZK60 magnesium alloy. Using liquid nitrogen (LN2) as a coolant during machining of ZK60 alloy induced deformation twins in the grains located on the deformation bands due to severe plastic deformation. Nucleation for the fine grain formation occurred at the deformation shear bands. Evolution of strong basal texture with deformation twins during cryogenic machining increased the fraction of submicron grains from <8% to ≈36%. The average diameter of fine grains was measured as ≈1.2 μm on the machined surface. Moreover, high lattice strain ensured the slip system that necessary for the occurrence of low temperature dynamic recrystallization (DRX). Pole figures resulted from EBSD analysis showed that the refined grains were oriented on the basal plane. The transformation of low angle grain boundaries (LAGB) to high angle grain boundaries (HAGB) about ≈61% under cryogenic machining which was ≈29% in as-received material confirmed the occurrence of grain refinement. Higher magnitude of compressive residual stress was also noticed below the machined surface under cryogenic machining.

Novel continuous forging extrusion in a one-step extrusion process for bulk ultrafine magnesium alloy

Highly-uniform ultrafine Mg–3Al–1Zn alloy was successfully fabricated by a simple conventional extrusion method called continuous forging extrusion (CFE). After CFE, the average grain size of Mg–3Al–1Zn alloy was refined to 0.6 μm. The ultra-fine grain (UFG) AZ31 alloy exhibited an excellent combination of yield strength of 307 MPa and ductility of 26.3%. The improved mechanical properties were mainly attributed to grain boundary strengthening and texture weakening. The formation of dynamic recrystallized submicron grain in the CFE was also discussed.

In-situ Ti-6Al-4V/TiC composites synthesized by reactive spark plasma sintering: processing, microstructure, and dry sliding wear behaviour

Titanium carbide (TiC) reinforced Titanium Matrix Composites (TMCs) have been synthesized via an in-situ reactive spark plasma sintering (SPS) process using commercial Ti-6Al-4V spherical powders pre-coated with 1 wt% carbon nanoparticles by low-energy ball milling. Graphite flakes are used as carbon source, which aids powder flow during mixing as lubricant. Graphite transforms to nano-crystallite carbon during mixing which is favourable for the rapid formation of TiC second phase in the following SPS process. The composites exhibited a novel honeycomb-like cellular microstructure with the formation of 5–6 vol% fine TiC submicron grains interconnected in the titanium α/β matrix. In addition, the reinforcement of the TiC phase with a nano-hardness of 12.4 GPa, improves the wear resistance of the parent alloy matrix (5.1 GPa), with a reduction of 26–28% in wear rate during dry reciprocating sliding tests against Si3N4 balls. During sliding, the wear debris (predominantly anatase TiO2) builds up on the raised TiC hard phase forming a barrier layer of adhered oxide that can protect the alloy matrix underneath from abrasion and oxidation, leading to a reduced wear rate.

Multi-Step Topochemical Pathway to Metastable Mo2AlB2 and Related Two-Dimensional Nanosheet Heterostructures.

The rational synthesis of metastable inorganic solids, which is a grand challenge in solid-state chemistry, requires the development of kinetically controlled reaction pathways. Topotactic strategies can achieve this goal by chemically modifying reactive components of a parent structure under mild conditions to produce a closely related analogue that has otherwise inaccessible structures and/or compositions. Refractory materials, such as transition metal borides, are difficult to structurally manipulate at low temperatures because they generally are chemically inert and held together by strong covalent bonds. Here, we report a multistep low-temperature topotactic pathway to bulk-scale Mo2AlB2, which is a metastable phase that has been predicted to be the precursor needed to access a synthetically elusive family of 2-D metal boride (MBene) nanosheets. Room-temperature chemical deintercalation of Al from the stable compound MoAlB (synthesized as a bulk powder at 1400 °C) formed highly strained and destabilized MoAl1-xB, which was size-selectively precipitated to isolate the most reactive submicron grains and then annealed at 600 °C to deintercalate additional Al and crystallize Mo2AlB2. Further heating resulted in topotactic decomposition into bulk-scale Mo2AlB2-AlOx nanolaminates that contain Mo2AlB2 nanosheets with thickness of 1-3 nm interleaved by 1-3 nm of amorphous aluminum oxide. The combination of chemical destabilization, size-selective precipitation, and low-temperature annealing provides a potentially generalizable kinetic pathway to metastable variants of refractory compounds, including bulk Mo2AlB2 and Mo2AlB2-AlOx nanosheet heterostructures, and opens the door to other previously elusive 2-D materials such as 2-D MoB (MBene).

Is there more than meets the eye? Presence and role of sub-micron grains in debris discs

Context. The presence of sub-micron grains has been inferred in several debris discs, usually because of a blue colour of the spectrum in scattered light or a pronounced silicate band around 10 μm, even though these particles should be blown out by stellar radiation pressure on very short timescales. So far, no fully satisfying explanation has been found for this apparent paradox. Aims. We investigate the possibility that the observed abundances of sub-micron grains could be naturally produced in bright debris discs, where the high collisional activity produces them at a rate high enough to partially compensate for their rapid removal. We also investigate to what extent this potential presence of small grains can affect our understanding of some debris disc characteristics. Methods. We used a numerical collisional code to follow the collisional evolution of a debris disc down to sub-micron grains far below the limiting blow-out size sblow. We considered compact astrosilicates and explored different configurations: A and G stars, cold and warm discs, bright and very bright systems. We then produced synthetic spectra and spectral energy distributions, where we identified and quantified the signature of unbound sub-micron grains. Results. We find that in bright discs (fractional luminosity ≳10−3) around A stars, the number of sub-micron grains is always high enough to leave detectable signatures in scattered light where the disc colour becomes blue, and also in the mid-IR (10 ≲ λ ≲ 20 μm), where they boost the disc luminosity by at least a factor of 2 and induce a pronounced silicate solid-state band around 10 μm. We also show that with this additional contribution of sub-micron grains, the spectral energy distribution can mimic that of two debris belts separated by a factor of ~2 in radial distance. For G stars, the effect of s ≤ sblow grains remains limited in the spectra although they dominate the geometrical cross section of the system. We also find that for all considered cases, the halo of small (bound and unbound) grains that extends far beyond the main disc contributes to ~50% of the flux up to λ ~ 50 μm wavelengths.

Strength enhancement of an aluminum alloy through high pressure torsion

High-pressure torsion (HPT) was used to investigate the potential of grain refinement and strengthening achievable for aluminum alloy 2139-T8. Discs of 20 mm diameter and approximately 0.9 mm thickness were HPT processed at room temperature under an applied pressure of 5 GPa after 1, 2, 4 and 8 revolutions. Microstructural characterization of the HPT-processed discs showed the formation of a sub-micron grain structure after one revolution. The HPT processing led to an exceptional increase of the hardness and tensile strength after only one revolution as well as a relatively homogenous microstructure containing refined grains. Some grain growth occurred with increasing number of revolutions. X-ray diffraction line profile analysis suggested dislocation arrangements consistent with the microstructural evolution and strengthening behavior. Overall, this study confirmed that there is a potential for using HPT for manufacturing Al 2139 with enhanced hardness and tensile strength.

Superior Strength with Enhanced Fracture Resistance of Al-Mg-Sc Alloy Through Two-Step Cryo Cross Rolling

The evolution of microstructure and the preferred orientation during the two-step cross rolling at room (RT) and cryogenic temperatures (CTs) on Al-Mg-Sc alloy were investigated and compared with the unidirectional rolled Al-Mg-Sc alloy in this study. In addition, the effects of two-step cross rolling on tensile, forming and void coalescence behavior were analyzed. After the solution heat treatment, the two-step cross rolling was executed with an initial 25 pct reduction in the unidirectional path and the final 25 pct reduction in the transverse direction. The two-step cross-rolled (TSCR) Al-Mg-Sc alloy at CT showed a higher fraction of sub-micron grains. The TSCR Al-Mg-Sc alloy exhibited brass and copper and a strong S texture. The texture indices and in-plane anisotropy values indicated the highly anisotropic nature of the TSCR Al-Mg-Sc alloy. During tensile deformation, the TSCR Al-Mg-Sc alloy at CT exhibited a strength value of 423 MPa, whereas the TSCR Al-Mg-Sc alloy at RT revealed only 378 MPa. The TSCR Al-Mg-Sc alloy at CT exhibited inferior formability compared with the TSCR Al-Mg-Sc alloy at RT and the solution heat-treated base alloy. The formability of the TSCR Al-Mg-Sc alloy was evaluated through the combined forming and fracture limit diagram (CFFLD). The presence of higher Goss-oriented grains enhanced the fracture resistance of the TSCR Al-Mg-Sc alloy at CT. Furthermore, consistency was found between the evaluated void coalescence parameters and the CFFLD.