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In the present study, arrayed nano-precipitated TiC in a ferrite matrix was obtained through thermal treatment. The size and shape of the arrayed interphase nano-precipitates were unaffected by fatigue deformation in this Ti-microalloyed steel. In addition, arrayed arrangement of the interphase nano-precipitates was not disrupted by fatigue and still remained only at low cycle numbers, which corresponded to the shortest fatigue life. The interphase nano-precipitated steel was strengthened through a bowing mechanism through which unpinning occurred, dislocation loops were formed, and the microhardness of ferrite was increased. The development of an incipient cell structure in the ferrite matrix was caused by wavy dislocation. After the onset of fatigue, the grain was preferentially rotated toward the {101}α orientation. Higher strain amplitude or lower strain rate led to a more favorable degree of misorientation in the low-angle grain boundaries.

M7C3 carbide [M: metallic element] is the primary reinforcement in white cast irons. Further improving its properties for maximized benefits is highly desired. However, no clear clues are available to guide the modification. Through atomic force microscopy in-situ analysis and first-principles calculations, we demonstrate that (Fe,Cr)7C3 carbide's strength is governed by its metallic bond component. We theoretically prove that M7C3 carbide is tailorable by partially replacing its metallic elements with substitutes selected based on their work function as an indicator. This study provides an insight into the electronic origin of carbide's mechanical behaviour, helping guide developing high-performance M7C3 carbides.

Continuous, uniform, and sub-10 nm Al2O3 high-K dielectrics upon two-dimensional exfoliated multilayer MoTe2 are realized by atomic layer deposition (ALD) based on a nucleation layer (NL) prepared by the ozone-based process, interfacial AlN, and low-temperature (low-T) physical adsorption. The NLs gives rise to significant reduction of the leakage current in the sub-10 nm Al2O3 high-K dielectrics as shown by conductive atomic force microscopy. For the ozone-based NL, X-ray photoelectron spectroscopy (XPS) reveals the oxidation of MoTe2 which is detrimental to the electrical properties of MoTe2. For the AlN NL, XPS reveals that Mo N bonds were formed without Mo O bonds and no chemical shift appeared in Te-3d XPS spectrum, indicating the AlN NL did not result in the MoTe2 oxidation but instead the formation of an MoN layer. For the low-T NL, the XPS spectra of MoTe2 are the same as those of the as-exfoliated MoTe2 flake, consistent with the absence of chemical reactions during low-T physical adsorption. The result demonstrates that the NLs prepared by the low-T physical adsorption and the interfacial AlN are effective and favorable for nucleating high-quality high-K gate dielectrics on MoTe2 transistors.

Using high-angle-annular-dark-field (HAADF) scanning-transmission-electron microscopy (STEM), we have investigated η-precipitates in the Al-Zn-Mg-Cu (AA7050) aluminium alloy. The HAADF STEM images taken along the zone axes of [10 1 ¯ 0]η, [1 2 ¯ 10]η, and [0001]η illustrated the projected atomic-scale configurations of η-MgZn2 crystal. The precipitates developed in layer-by-layer growth, supplied with precursors such as Zn, Cu, and Mg, which were solute atoms segregated around the η/Al interfaces due to the higher lattice strain energy. Stacking faults and defect layers composed of flattened hexagons were frequently observed along the zone axes of [1 2 ¯ 10]η and [10 1 ¯ 0]η, respectively, and their formation was elucidated, similarly taking into account the layer-by-layer growth. Occasional coalescence between two precipitates yielded a complicated boundary or a twin-like boundary. Based on the differences in orientation relationships between η-types and the Al matrix reported to date, two new types of η precipitates have been recognized and named η4' and η12.

This study investigated the changes in dielectric properties of the disc capacitor and base metal electrode (BME) multilayer ceramic capacitor (MLCC) fabricated from the host material (BaTiO3 + 1.0 mol% MgO + 0.05 mol% MnCO3 + 1.37 mol% BaSiO3) with different amounts of Sc2O3 addition. The results of transmission electron microscope (TEM) analysis verified that the addition of 0.6 mol% Sc2O3 led to the formation of a grain core–shell structure. The formation of the grain core–shell structure can improve the temperature stability of the dielectric properties. The presence of Sc2O3 can increase the room-temperature insulation resistance and reduce dielectric loss (tanδ); however, an excessive addition of Sc2O3 resulted in the decline of the insulating property of MLCCs at a high temperature. This study revealed that the temperature coefficient of the capacitance (TCC) curves can be flattened by adding 0.3–0.6 mol% Sc2O3 into the host material so that the developed BME MLCCs can comply with the requirements of the Electronic Industries Alliance (EIA) X8R specification. In particular, the MLCC with 0.3 mol% Sc2O3 addition had satisfactory TCC stability and insulating properties; the dielectric constant was 2084, tanδ was 1.23%, the TCC at −55 °C and 150 °C was −2.8% and −5.8%, respectively, and the insulation resistance at −55 °C and 150 °C was 3.6 × 1010 Ω and 2.6 × 107 Ω, respectively.

Compressive deformation tests were carried out during in-situ transmission electron microscopy to investigate the deformation behaviors of α-iron nanopillars with or without interphase-precipitated carbides. Three kinds of α-iron nanopillars were prepared from titanium-molybdenum-free and titanium-molybdenum-containing steels: one interphase-precipitated carbide-free and two interphase-precipitated carbide-bearing pillars with carbides of different sizes (4.7 nm and 3.0 nm after holding at 700 °C and 650 °C, respectively) and different number densities. In-situ observation provided direct evidence that interphase-precipitated nanometer-sized carbides confined the movement of the dislocations, resulting in smaller strain bursts with a significantly higher strength, in contrast to the behaviors of the pillars without carbides. The individual strain bursts in the in-situ stress-strain curves, which were associated with the corresponding dislocation substructures in the three kinds of pillars, are discussed.

We elucidate here the strain rate dependence on the deformation behavior and accompanying deformation mechanism in Fe-30Mn-0.3C TWIP steel via nanoscale deformation experiments and post-mortem microscopy of the deformed region. The nanoindentaion hardness increased with increased strain rate from 0.01 s-1 to 1 s-1, and exhibited a positive strain rate sensitivity of 0.095 with an activation volume of 18b3. At a low strain rate, dislocations dominated the deformation behavior with a high density of 2.7 × 1016 m-2. With increased strain rate, the dislocations decreased and the stacking faults and nanotwins gradually increased. However, nanotwins with secondary twins were the dominant deformation process at high strain rate of 1 s-1. The deformation behavior was significantly impacted by the interplay between strain rate, stacking fault energy and deformation mechanisms. A critical theoretical analysis suggested that the strain rate influenced the critical shear stress for twinning and dislocation slip, resulting in the change in deformation mechanism from dislocation slip to twinning.

In the present work, using high-angle-annular-dark-field scanning-transmission-electron microscopy, the twin boundary of η-MgZn2 precipitates has been investigated in the AA7050 aluminium alloy. The atomic-scale images, taken along [101¯0]η // [110]Al zone axis, indicated the configurations of twin boundaries nearly on the (002¯)Al planes. This twin boundary, composed of well-matched interface and defects, is ascribed to the coalescence of two developed η precipitates grown on the respective (1¯11)Al and (1¯11¯)Al habit planes, and possess a twin relationship with an angle of 109.5ο between the (002¯)Al plane.

Split Hopkinson pressure bar (SHPB) experiments were conducted to investigate the mechanical behavior and microstructure evolution of CoCrFeMnNi alloy (Cantor alloy) under three strain rates (3 × 103, 6 × 103, and 9 × 103 s−1) at room temperature. The corresponding deformed samples were studied mainly by transmission electron microscopy (TEM), with the focus on the defected structures. The results indicate that the flow stress and strain-hardening rate are both significantly dependent on strain rate. Nano-twinning occurred during deformation at 9 × 103 s−1, whereas dislocation-glide was dominant during deformation at 3 × 103 s−1. The transition of deformation modes under different dynamic compression rates is discussed.

The nanohardness values and modulus of the prior δ phase in both direct current (DC) and vibration-assisted (VIB) weld metal were higher than those of the base metal. The dislocation density in the δ phase matrix was the highest in the VIB weld metal, followed by the DC weld metal and the base metal. Therefore, the higher nanohardness value in the δ phase, especially for the VIB welding, was attributed to the existence of a higher dislocation density. By contrast, the nanohardness values of the γ phase in both weld metals were not affected by the welding process. The predicted nanohardness value of the composite phase with the rule-of-mixture was consistent with the result of nanoindentation measurements. The deformation induced a V-shaped double twin was revealed by the effect of nanoindentation on the austenite grain of the VIB weld metal.

In the present study, dielectric formulations compatible with the base-metal electrode process, using a host material (BaTiO3 + 0.05 mol% MnCO3 + 1.37 mol% BaSiO3) with 0.3–0.6 mol% Sc2O3 and 0–2.0 mol% MgO, were examined. The addition of Sc2O3 and MgO led to the formation of a grain core-shell structure and therefore gave rise to temperature-stable dielectric properties, while the TEM and EDS results revealed the chemical inhomogeneity of the Sc3+ ions across the grains. The Sc2O3 content played an important role in the reducing the dielectric constant (K), improving the electrical resistivity, and flattening the TCC curve of the ceramics by inhibiting grain growth. Meanwhile, the existence of MgO was proven to enhance the densification, affect the room-temperature electrical resistivity, reduce the tanδ value, and improve the TCC value at - 55 °C. The optimum formulation derived in the present study involved adding 0.45 mol% Sc2O3 and 1.0 mol% MgO to the host material. This yielded the following dielectric properties: K = 1,744, tanδ = 0.58%, TCC = - 3.9% (- 55 °C) and - 8.5% (150 °C), and electrical resistivity = 2.8 × 1012 Ω cm (25 °C) and 1.7 × 1011 Ω cm (150 °C). These dielectric properties satisfied the requirements of the EIA-X8R specification, thus validating the material's potential for application to commercial capacitors.

The deformation mechanisms operating in nanostructured bainite, leading to its excellent combination of strength and ductility, are far from being understood. Its nanocrystalline nature and its multiphase-evolving structure underlie the plastic flow and the strain-hardening behaviour. In this work, the microstructural and crystallographic bulk changes of a high-C nanostructured bainite under tensile testing have been evaluated. The influence of the mechanically-induced transformation of the C-enriched retained austenite into α martensite and other deformation mechanisms on the texture evolution has been analysed by electron backscatter diffraction (EBSD). Additionally, the undeformed and the deformed conditions have been examined by electron channelling contrast imaging (ECCI) and transmission electron microscopy (TEM). Results reveal the presence of plate martensite and suggest a strong variant selection during the transformation, mainly responsible for the texture observed. Mechanical twinning in austenite seems to be basically the mechanism of accommodation of the displacive bainitic transformation, while some direct interaction with the applied stress also appears., EBSD and ECCI experiments were accomplished at MPIE during a stay of LMR partially funded by the Spanish Ministry of Economy and competitiveness (MINECO), ref. BES-2011-044186 and ref. EEBB-I-14-08919. TEM experiments were accomplished at NTU during a stay of MBA partially funded by the Ministry and the National Science Council of Taiwan.

An efficient and accurate method is developed for microstructural quantification of complex phases in a low carbon Nb-Mo bearing steel, of which optical micrographs show that it consists of granular bainite and a small amount of ferrite. Our previous work, proposing a method to measure the misorientation angles via electron backscatter diffraction (EBSD) to differentiate granular bainite and ferrite, has been reported. That method is accurate for phase quantification but laborious for the characterization process. To resolve this difficulty, in this study, EBSD combined with a kernel average misorientation (KAM) map is used for phase characterization. Comparisons are made among KAM maps with different kernel sizes (300 to 600 nm) and various step sizes (100 to 600 nm). It is found that a kernel size close to the sub-structure size of granular bainite (500 nm) is optimal for phase identification, while varied step sizes produce relatively invariant results. Therefore, KAM maps can be used for fast and reliable phase quantification, provided that an appropriate kernel size and a large step size are used.

A unique batch tempering treatment for industrial scale direct-quenched steel coils has been studied using laboratory simulations. The tempering treatment was non-isothermal with slow heating to 570 °C and slow cooling to simulate the tempering of large steel coils. The paper presents the effect of finishing rolling temperature (FRT) relative to the non-recrystallization temperature (TNR) and the effect of long time tempering on the microstructure, dislocation density and mechanical properties of direct-quenched coiled strips. Conditioning austenite below the recrystallization stop temperature resulted in a finer effective grain size distribution, which correlated strongly with the impact toughness of the final product. Furthermore low finish rolling temperature resulted in partially ferritic microstructures while higher finishing rolling temperatures led to mixtures of bainite and martensite. Dislocation densities determined with TEM and XRD showed somewhat different trends regarding the effect of tempering: intra-lath dislocation density, as measured with TEM, showed a statistically significant drop in only one case, while XRD analysis indicated a drop in all cases. Furthermore, no significant correlation between finishing rolling temperature and dislocation density existed in XRD studies. The XRD results indicate that the decrease in dislocation density corresponds to about 100 MPa lower dislocation strengthening. However, precipitation hardening and potential internal micro stress relief compensates this as yield strength remains unchanged or even increases during tempering.

The fatigue behavior and dislocation structures of SAF 2507 alloy were investigated. Specifically, as-received superduplex stainless steel SAF 2507 (AS) alloy and its weld metals from direct current welding without and with electrodynamic vibration (VIB) were studied. Fatigue experiments were conducted at total strain amplitudes of 1.2% and 2.4% ( R = −1), respectively; the VIB samples exhibited higher strength but shorter fatigue lives. In the fatigued AS sample, ferrite exhibited characteristic dislocation structures, namely veins, walls, and cells, and austenite had persistent Luders bands (PLBs) and mechanical twins. In the fatigued weld metals, the dislocation structures in austenite exhibited only PLBs; however, in ferrite, two types of walls formed due to entanglement of dislocations and rearrangement of Cr 2 N dislocation bundles, indicating that the fatigue strains are accommodated mostly by ferrite. The shorter fatigue lives of the VIB samples are attributed to the increased amount of intragranular austenite, and increased boundaries between intragranular austenite and δ-ferrite result in faster dislocation entanglement. Consequently, the initial response stress was the highest and work hardening rate was higher, which eventually induced cracks in δ-ferrite.

Understanding of resistive switching mechanism through H2O2 sensing and improvement of switching characteristics by using TaOx-based material in W/Al2O3/TaOx/TiN structure have been reported for the first time. Existence of amorphous Al2O3/TaOx layer in the RRAM devices has been confirmed by transmission electron microscopy. By analyzing the oxidation states of Ta2+/Ta5+ for TaOx switching material and W0/W6+ for WOx layer at the W/TaOx interface through X-ray photoelectron spectroscopy and H2O2 sensing, the reduction-oxidation mechanism under Set/Reset occurs only in the TaOx layer for the W/Al2O3/TaOx/TiN structures. This leads to higher Schottky barrier height at the W/Al2O3 interface (0.54 eV vs. 0.46 eV), higher resistance ratio, and long program/erase endurance of >108 cycles with 100 ns pulse width at a low operation current of 30 μA. Stable retention of more than 104 s at 85 °C is also obtained. Using conduction mechanism and reduction-oxidation reaction, current-voltage characteristic has been simulated. Both TaOx and WOx membranes have high pH sensitivity values of 47.65 mV/pH and 49.25 mV/pH, respectively. Those membranes can also sense H2O2 with a low concentration of 1 nM in an electrolyte-insulator-semiconductor structure because of catalytic activity, while the Al2O3 membrane does not show sensing. The TaOx material in W/Al2O3/TaOx/TiN structure does not show only a path towards high dense, small size memory application with understanding of switching mechanism but also can be used for H2O2 sensors.

With austenite under 50% compression using a dilatometer, ferrite transformation in a low-carbon Ti-V-bearing steel was sequenced through the isothermal transformation at 650 °C for various holding times (10 s, 20 s, 30 s). Bimodal grain size distribution was observed in the prolonged holding conditions. Corresponding measurements of Vickers and nano-indentation hardness were carried out; the tiny ferrite grains possessed lower hardness (303 HV and 4.8 GPa) as compared to larger ferrite grains (336 HV and 5.6 GPa). Transmission electron microscopy provided strong evidence attributing this discrepancy to interphase precipitation taking place in the larger-grained ferrite, instead of in the tiny-grained ferrite.

Mn-free Fe27Co24Ni23Cr26 high-entropy alloy (cold-rolled 40% and then annealed at 800 °C) was subjected to ballistic deformation (strain rate > 106 s−1) at room temperature. The microstructural examinations of the bulletproof plate were focused on three regions (labeled I, II and III) starting from the no-deformation region to the heavy deformation region adjacent to the bullet hole. In region I, lamellar annealing nanotwins were found in the nearly recrystallized grains, and the boundaries of annealing nanotwins had accumulated dislocations, presumably created by prior cold-rolling. The intermediate deformation region was full of complex nanotwin structures and a high density of dislocations. The intersection block structures created by the boundaries of annealing and deformation nanotwins significantly confined the dislocation movement. Alternatively, in the heavy deformation region, besides the typical intersection structures of deformation nanotwins, the boundaries of annealing and deformation nanotwins were found to be significantly deviated from the ideal twin orientation, presumably due to successive deformation during the course of ballistic deformation. Moreover, high-resolution transmission electron microscopy (HRTEM) clearly revealed that bundles of stacking faults were created at the intersections of two bundles of deformation nanotwins.

High-resolution (HR) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has revealed the atomic lattice defects in different types of η precipitates in the Al–Zn–Mg–Cu aluminium alloy subjected to creep-age forming treatment (with a constant stress lower than its room-temperature yield strength during ageing). Along the zone axes of [110]Al//[2 1 ‾ 1 ‾ 0]η of η1 and η12, [112]Al//[2 1 ‾ 1 ‾ 0]η of η2 and [100]Al//[2 1 ‾ 1 ‾ 0]η of η13, atomic projections of (2 1 ‾ 1 ‾ 0)η have been investigated. In those types of η, elongated hexagonal lattice defects (labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings. Furthermore, in η12, elongated hexagonal lattice defects with a much higher aspect ratio (labelled as Type II defects) are uniquely observed. These atomic lattice defects are presumably pertinent to the lattice accommodation in the course of creep-age forming. Additionally, in η1 and η12, the features of a Penrose tiling defect connecting with Type I defects are observed, and these complex defects obviously affect the growth direction of the precipitate, resulting in a nearly spherical morphology. Alternatively, several entirely-passed faulted layers in a new type of precipitate, η14, consequently bring about a new orientation relationship: (51 3 ‾ )Al//(0001)η14 and [112]Al//[2 1 ‾ 1 ‾ 0]η14. Moreover, in an atomic STEM image of η14, the significant Z-contrast gradient adjacent to the transformation front of η14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {1 1 ‾ 1 ‾ }Al planes at the interface.

In this work, the effects of tempering at three high temperatures (660, 680 and 700 °C) for different holding times (5 min to 16 h) on the secondary hardening in Nb-bearing and Nb-Mo-bearing bainitic steel strips were investigated. It was found that with 0.3 wt% Mo addition in Nb-Mo-bearing bainitic steel strip, the peak hardening occurred with a significant increment in hardness (up to 40 HV) after approximately 10 min of tempering at all temperatures (660, 680 and 700 °C). In contrast, for the Nb-bearing steel strip, the corresponding increment in hardness was small (about 15 HV). The microstructural evolutions of Nb-bearing and Nb-Mo-bearing steel strips during tempering at 700 °C were investigated with scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. It was found that during tempering, bainitic ferrite platelets in Nb-Mo-bearing steel strips remained much more stable and had a higher dislocation density than those in Nb-bearing steel strips. High-resolution transmission electron microscopy provided strong evidence that the Mo addition had significant effects on suppressing the annihilation rate of dislocations and retarding the coarsening rate of nanometer-sized carbides at higher temperatures, leading to a remarkable resistance to softening after secondary hardening.

The objective of this paper is to study the influence of thermal cycles produced by the later welding passes on the properties of the sub-regions in reheated zone of multi-pass weld metal for a 550 MPa grade offshore engineering steel. A Gleeble-3500 simulator was applied to simulate microstructural evolution in sub-regions of the reheated zone and its effect on the properties. The results indicated that the reheating process changed the prior austenitic morphology from columnar structure to equiaxed structure and similar columnar structure with quasi-polygonal ferrite (QPF) or blocky M-A (martensite/austenite) constituent distributed on the grain boundaries while the matrix microstructure (acicular ferrite) changed slightly. Charpy impact results indicated that WM region (as-deposited) had the highest impact energy. However, the actual impact sample showed lower impact energy because the machined notch contained one or more brittle reheated zones. In these brittle reheated zones, the necklace-type M-A constituent was hard phase decorating prior columnar or equiaxed austenite grain boundaries, which yielded stress concentration significantly and was mainly responsible for lower toughness of the entire weld metal. Fortunately, this deterioration in toughness could be reduced as decomposition of necklace-type M-A constituent occurred due to later welding passes.

In this study, the effects of the Mg 2+ ions replaced by Ca 2+ ions on the microwave dielectric properties of newly developed MgZrTa 2 O 8 were investigated. Mg 1-x Ca x ZrTa 2 O 8 (x = 0–1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca 2+ significantly inhibited the densification of Mg 1-x Ca x ZrTa 2 O 8 , led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa 2 O 6 . The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg 2+ ions by the more polarizable Ca 2+ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τ f values was in accordance with that of relative permittivity. The best composition appeared to be Mg 0.9 Ca 0.1 ZrTa 2 O 8 , which showed excellent microwave dielectric properties of e r = 22.5, Q × f = 231,951 GHz, and τ f = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.

Nanostructured bainite has been developed to meet the stringent demands of ballistic impacts. In a previous report [Y.T. Tsai et al., Scripta Mater. 2016;115:46], the deformation behavior of nanostructured bainite under high strain rate (2×103 s-1) was studied using a split Hopkinson pressure bar. Deformation twinning was found in both blocky austenite and film austenite, and it was concluded that twinning occurred first in the blocky austenite, enhancing the initial work-hardening, followed by twinning in film austenite, promoting further straining. In the present work, the deformation structure has been further studied using a severely deformed sample subjected to a high strain rate (3×103 s-1). It is found that blocky austenite, due to the inherent size inhomogeneity, can produce various deformation twinning structures; i.e., one variant or two variants of dominating mechanical twins, including twin kinking. The dominating twins are so-called primary twins, the apparent twinning planes of which deflect from the ideal {111}γ, resulting from the fact that the apparent twinning planes are stepped, presumably due to dislocation locking during deformation. Small twins are also found to be dispersed in parent austenite matrix, and they are categorized as secondary twins. Strong evidence suggests that the formation of α’-martensite usually occurs in the intersection of two primary twins. For film austenite, the deformation structure is one-variant twinning, and the twinning planes are all ideal.

In this work, titanium-free and titanium carbide-strengthened dual-phase steels were studied. While both had the same base composition of Fe–0.1C–1.5Mn–0.5Si–0.6Cr (wt%), the latter included a 0.1 wt% Ti addition for interphase precipitation. Utilizing the Gleeble thermal simulations, both two dual-phase steels were produced with the same ferrite grain size and the same ferrite/martensite volume fractions of 0.7 and 0.3, respectively. Their corresponding microstructures (including deformed structures) and mechanical properties were examined. The results indicate that the titanium carbide-strengthened dual-phase steel possesses an excellent combination of the strengthened ferrite and the weakened martensite, which brings about increased yield and tensile strengths without sacrificing total elongation.

In-situ compression tests in a transmission electron microscope were carried out to investigate the deformation behavior of δ-ferrite nanopillars of a duplex stainless steel. The δ-ferrite with a spinodal nanostructure, which formed due to aging at 475 °C for 64 h, obviously suppressed the progress of serrated yielding (a series of short strain bursts) relative to that without the spinodal nanostructure. The results revealed that during compression deformation, the spinodal nanostructure confined the movement of dislocations (leading to a significant increase in dislocation density), causing a notable strengthening effect, and also kept the slip band morphology planar.

Cold-rolling Fe22Co22Ni20Cr22Mn14 high entropy alloy (with a deformation of 70%), and annealing it at two temperatures (1100 and 800 °C) respectively created a bimodal grain size distribution in large-grain (LG) samples having an average grain size of 45.9 ± 20.0 μm and in small-grain (SG) samples having an average grain size of 4.3 ± 3.0 μm. Under three cryogenic temperatures (−50, −100, and −150 °C), high-speed deformation (~9 × 103 s−1) was conducted on a split Hopkinson pressure bar (SHPB) system to investigate the microstructural evolution of deformation nanotwins in the bimodal-structured samples. Subjected to high-speed deformation at decreasing cryogenic temperatures, the mechanical behaviors of LG samples were superior to those of SG samples. Notably, under high-speed deformation at −150 °C, LG structures achieved excellent mechanical strength of ~3.3 GPa with good ductility of ~31.9%. Profuse lamellar annealing nanotwins, which pre-existed in the coarse grains of LG samples, promoted efficient refinement strengthening. High-resolution transmission electron microscopy (HR-TEM) clearly revealed that the deformation nanotwins induced by high-speed deformation further refined the pre-existing annealing nanotwins in the coarse grains of LG samples, presumably providing advanced mechanical sustainability for high-speed deformation at cryogenic temperatures. It is suggested that the micrometer-scaled and nanometer-scaled annealing twins appear first in the matrices of coarse grains, enhancing the initial work-hardening; subsequently the deformation nanotwins form in pre-existing annealing nanotwins and narrow strips of the matrix, effectively providing the dynamic grain refinement and the work hardening capacity.

We performed the modified-embedded-atom-method (MEAM) based molecular dynamics (MD) simulations to investigate the plastic deformation and phase transformation behaviors in the CoCrFeMnNi HEA under high compression at room temperature. Our MD simulations revealed that the stress-induced phase transformations in the CoCrFeMnNi HEA are strongly crystal orientation-dependent. The [0 0 1] uniaxial compression can induce the significant face-centered-cubic (fcc) -to-hexagonal-close-packed (hcp) phase transformation via successive emissions of partial dislocations from the extended stacking faults, twin boundaries and hcp-lamellas created during the early stage of deformation. As for the [1 1 0] and [1 1 1] uniaxial compressions, however, the transformed hcp atoms can simply form the intrinsic/extrinsic stacking faults. Although the [0 0 1] uniaxial compression produced a much lower dislocation density than the other two systems, it induced much more constituents transformed into the hcp atoms at the end of phase transformation. Our results clearly indicated that the deformation twin boundaries and extended hcp-lamellas play a critical role in facilitating the stress-induced fcc-to-hcp phase transformation in the CoCrFeMnNi HEA. Furthermore, it was found that the phase transformation in the CoCrFeMnNi HEA can be effectively facilitated by a large deviatoric compressive stress while it may tend to be significantly retarded by a hydrostatic compression. Our results also showed that the plastic deformation behaviors in Ni under high compression are very similar to those occurred in the CoCrFeMnNi HEA though nearly all the hcp atoms can simply constitute the intrinsic/extrinsic stacking faults without the formation of any bulk hcp phase in the fcc lattice. The main discrepancy in the phase transformation behaviors between the Ni and CoCrFeMnNi HEA can be largely attributed to the much lower stacking fault energy of the CoCrFeMnNi HEA than other fcc metals.

For study the high temperature application of beta titanium, all experimental creep test temperatures and the pre-aged temperature were lower than the predicted β-transus temperature (1067 K). The creep properties of the pre-aged material were superior to those of the as-received material. The activation energies of the pre-aged creep material under various test conditions were approximately twice those of the as-received material under dynamic-aging creep, even though the stress exponent of the pre-aged material was lower than that of the as-received material. Creep deformation was predominated by the diffusion mechanism. The decline in the hardness of the pre-aged creep material under an applied stress of 200 MPa was ascribed to the coarsening and degeneration of the acicular α (secondary α).

This paper reports the transient photoluminescence properties of a typical InGaN/GaN multiple quantum well light emitting diode structure. Two decay processes were found to contribute to the photoluminescence dynamics. Based on the exciton localization model, key factors, transient lifetimes of radiative/nonradiative recombination, were obtained respectively for two decay processes by numerically fitting and separating the mixed photoluminescence efficiencies and photoluminescence decay data, which provide guidance to trace the origins of exciton localization. The origins of slow PL process and fast PL process were reasonably assigned to local compositional fluctuations of indium and thickness variation of InGaN layers, respectively.

Nanometer-sized carbides that precipitated in a Ti–Mo bearing steel after interrupted continuous cooling in a temperature range of 620–700 °C with or without hot deformation were investigated by field-emission-gun transmission electron microscopy. The nanometer-sized carbides were identified as randomly homogeneous precipitation carbides and interphase precipitation carbides coexisting in the ferrite matrix. It is found that this dual precipitation morphology of carbides in the steel leads to the non-uniform mechanical properties of individual ferrite grains. Vickers hardness data mainly revealed that, in the specimens cooled at a rate of 0.5 °C/s without hot deformation, the range of Vickers hardness distribution was 230–340 HV 0.1 when cooling was interrupted at 680 °C, and 220–360 HV 0.1 when cooling was interrupted at 650 °C. For the specimens cooled at a rate of 0.5 °C/s with hot deformation, the range of Vickers hardness distribution was 290–360 HV 0.1 when cooling was interrupted at 680 °C, and 280–340 HV 0.1 when cooling was interrupted at 650 °C. Therefore, a narrower range of hardness distribution occurred in the specimens that underwent hot deformation and were then cooled with a lower interrupted cooling temperature. The uniform precipitation status in each ferrite grain can lead to ferrite grains with a narrower Vickers hardness distribution. On the other hand, interrupted cooling produced a maximum Vickers hardness of 320–330 HV 0.1 for the hot deformed specimens and 290–310 HV 0.1 for the non-deformed specimens with cooling interrupted in the temperature range of 660–670 °C. The maximum Vickers hardness obtained in such a temperature range can be ascribed to the full precipitation of the microalloying elements in the supersaturated ferrite matrix with a tiny size (~ 4–7 nm).

To explore the mechanical behavior and microstructure evolution of nanostructured bainite under deformation at high strain rates (> 10 3 s − 1 ), split Hopkinson pressure bar experiments were conducted. It was found that nanometer-sized twinning occurred in both blocky austenite and film austenite. In the former case, the intersections of different twin variants brought about strain-induced α’-martensite, while in the latter case, the lamellar twinned structures were occasionally accompanied by a small amount of e martensite. It is concluded that twinning occurs first in blocky austenite, enhancing the initial work-hardening; subsequent twinning takes place in film austenite, promoting further straining.

It is generally accepted that reducing the strength difference between soft ferrite and hard martensite in a dual-phase (DP) steel can delay fracture during plastic deformation. However, it is still not clear whether the better approach to attain the goal is to decrease the strength of martensite or to increase the ferrite strength. In the present paper, low carbon steel was microalloyed with Ti and Cu to alleviate the strength difference between the hard martensite and soft ferrite phases. The results showed that the Vickers hardness (HV0.1) differences between ferrite and martensite after tempering at 500 °C for 8 h were ∼9 HV0.1 for Ti-Cu DP steel and ∼83 HV0.1 for Ti DP steel. It is found that Cu particles that precipitate on the pre-existing titanium carbide can increase the strength of ferrite and martensite after long time aging.

After deformation, super duplex stainless steel (DSS) was annealed at 1100 °C for a two-phase solid–solution region to avoid σ formation. Preferred orientations in δ and γ phases changed to (111)δ, (100)δ, and (101)γ from the rough random orientation of as-received super DSS. The kernel average misorientation (KAM) angle was less than 5° implying the grain boundary misorientation strain shifted to a higher value after cold rolling, which was consistent with the X-ray diffraction (XRD) microstrain results. After annealing, the misoriented angle was restored to a low angle of approximately 0.5°, which implied elimination of grain boundary misorientation strain when the annealing time increased from 15 to 30 min. The calculated microstrain based on XRD peaks demonstrated that the γ phase deformed before the δ phase at 50% CR. The recrystallized δ grains with slow growth and high staking fault energy (SFE) mainly showed dynamic recovery and a wide misorientation distribution of high angle grain boundary (HAGB) in the range between 15° and 58°, whereas the recrystallized γ grains with low SFE exhibited rapid dynamic recrystallization; the angles of the most HAGB grains were greater than 50°, because of the dissipation of the stored strain energy. This recrystallized austenite adhered to the Kurdjumov–Sachs relationship with ferrite.

Thermal cycling induced the phase transformation originated from the physical simulation of power plant on/off operation. The effect of thermal cycling on duplex stainless steel was studied to understand its phase transformation comprehensively. Cyclic thermal processing is a nonisothermal heat treatment method known to accelerate the kinetics of phase transformation. In this study, when the number of thermal cycles increased, the ferrite grains gradually contracted from the grain boundaries and finally disappeared due to the eutectoid reaction. However, the austenite phase demonstrated a parabolic increase and the intermetallic sigma (σ) phase, formed at the grain boundaries, exhibited a similar trend. The KAM maps reveal the highest degree of misorientation (up to 2°) may have either been at the γ′ or σ phase at 40 and 80 cycles. In DSS, the misorientations (>15°) among grains also affect σ formation. A high crystallographic misorientation between the austenite and ferrite phases favored σ precipitation. The calculated thermal stress of 422 MPa assisted the σ formation by reducing the activation energy and accelerating the alloying element diffusion. The σ phase formation caused by thermal cycling increased the yield strength and ultimate strength but degraded ductility. The formation of tensile cracks was related to the σ phase at the interface or the σ matrix. Keywords: Super duplex stainless steel, EBSD, Thermal cycling, σ phase, Thermal stress

The formation of massive ferrite in an interstitial-free (IF) steel with three different austenitization conditions (at 1200, 1100 and 1000 °C respectively for 3 min) has been investigated. To conduct the rapid continuous-cooling transformations at a cooling rate of 500 °C/s, hollow rod specimens were employed in a dilatometer. Thermal arrests, which were associated with the phase transition, were detected in the cooling curves, and thereby the massive ferrite transformation temperatures were determined. The corresponding treated samples were studied by optical metallography and transmission electron microscopy. The features of massive ferrite and its transformation characterization are discussed.

Improved resistive switching phenomena such as device-to-device uniformity, lower formation voltage (2.8 V) and RESET current, >500 program/erase cycles, longer read endurance of >10 6 cycles with a program/erase pulse width of 1 μs, and data retention of >225 h under a low current compliance of 300 μA have been discussed by using Cu-Al alloy in Cu:AlO x /TaO x /TiN conductive bridging resistive random access memory (CBRAM) device for the first time. The switching mechanism is based on a thinner with dense Cu filament formation/dissolution through the defects in the Cu:AlO x /TaO x /TiN structure owing to enhance memory characteristics. These characteristics have been confirmed by measuring randomly picked 100 devices having via-hole size of 0.4 × 0.4 μm 2 . The Cu-Al alloy becomes Cu:AlO x buffer layer and Ta 2 O 5 becomes TaO x switching layer owing to Gibbs free energy dependency. All layers and elements are observed by high-resolution transmission electron microscope (HRTEM) image and energy dispersive X-ray spectroscopy (EDX). By developing a numerical equation in between RESET current and formation voltage, it is found that a higher rate of Cu migration is observed owing to both the defective switching layer and larger size, which results a lower formation voltage and RESET current of the Cu:AlO x /TaO x /TiN structure, as compared to Cu/Ta 2 O 5 /TiN under external positive bias on the Cu electrode. This simple Cu:AlO x /TaO x /TiN CBRAM device is useful for future nanoscale non-volatile memory application.

Examination of thin foils of specimens with various austenitizing conditions by transmission electron microscopy revealed randomly homogeneous precipitation in the ferrite for each experimental condition. Though no interphase precipitation was found in the present study, two types of random precipitation morphologies were identified in the ferrite matrix. One was randomly and homogeneously precipitated carbides of smaller size ( Vickers hardness data revealed that, in specimens austenitized at 1200 °C and deformed at 900 °C with strains of 10% and 30%, the ranges of hardness distribution were 250–360 HV 0.1 and 310–400 HV 0.1, respectively. For specimens austenitized at 1000 °C and deformed at 900 °C with strains of 10% and 30%, the ranges of hardness distribution were 220–250 HV 0.1 and 220–260 HV 0.1, respectively. Therefore, the average Vickers hardness increased with the austenitizing temperature and deformation strain. However, a wider range of hardness distribution occurred in specimens that underwent treatment at higher austenitizing temperatures. The wider Vickers hardness distribution reflects non-uniform precipitation in each ferrite grain.

Experimental evidence indicates that bainitic ferrite formed by transformation at low temperatures (200–350 °C) contains quantities of carbon in solid solution far beyond those expected from para-equilibrium. A change in the conventional symmetry of the bainitic ferrite lattice from cubic to tetragonal explains the abnormal solid solubility detected. This carbon supersaturation was measured by atom probe tomography, and the tetragonality of the bainitic ferrite, was characterized by means of X-ray diffraction analysis and high resolution transmission electron microscopy, support of the European Research Fund for Coal and Steel, the Spanish Ministry of Economy and Competitiveness and the Fondo Europeo de Desarrollo Regional (FEDER) for partially funding this research under the contracts RFSR-CT-2012-00017 and MAT2013-47460-C5-1-P respectively. LM-R also acknowledges this same Ministry for financial support in the form of a PhD research Grant (FPI). Atom probe tomography was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The work was also partly supported by the Australian Research Council, and the technical and scientific assistance of staff at the University of Sydney node of the Australian Microscopy & Microanaylsis Research Facility (ammrf.org.au) (Sydney Microscopy & Microanalysis) is gratefully appreciated

Li-rich layered cathode materials with composition of Li 1.2−x Mn 0.54 Ni 0.13 Co 0.13 O 2−x F x (x = 0, 0.02, 0.05, 0.08) are synthesized by co-precipitation method. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area analysis, high-resolution transition electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and galvanostatic charge–discharge measurements are employed to investigate the changes in crystal structure, chemical composition, particle morphology and electrochemical performance. Improved electrochemical performance is observed for fluorine doped materials, among which Li 1.15 Mn 0.54 Ni 0.13 Co 0.13 O 1.95 F 0.05 exhibits the best cyclic performance as well as rate capability at room temperature. The discharge voltage plateau is stabilized by F doping due to the retarded layered-to-spinel phase transition, thus a high energy density output of the cell could be maintained. The mechanisms for the improved electrochemical properties of fluorine doped Li-rich layered cathode are also discussed.