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Rare earth (RE) addition to steels to produce RE steels has been widely applied when aiming to improve steel properties. However, RE steels have exhibited extremely variable mechanical performances, which has become a bottleneck in the past few decades for their production, utilization and related study. Here in this work, we discovered that the property variation of RE steels stems from the presence of oxygen-based inclusions. We proposed a dual low-oxygen technology, and keeping low levels of oxygen content in steel melts and particularly in the raw RE materials, which have long been ignored, to achieve impressively stable and favourable RE effects. The fatigue life is greatly improved by only parts-per-million-level RE addition, with a 40-fold improvement for the tension-compression fatigue life and a 40% enhancement of the rolling contact fatigue life. We find that RE appears to act by lowering the carbon diffusion rate and by retarding ferrite nucleation at the austenite grain boundaries. Our study reveals that only under very low-oxygen conditions can RE perform a vital role in purifying, modifying and micro-alloying steels, to improve the performance of RE steels.

In recent years, nanoscale thermal cloaks have attracted the interest of researchers. Several different types of nanoscale thermal cloak have been developed with their performance been experimental...

The search for new two-dimensional monolayers with diverse electronic properties has attracted growing interest in recent years. Here, we present an approach to construct MA2Z4 monolayers with a septuple-atomic-layer structure, that is, intercalating a MoS2-type monolayer MZ2 into an InSe-type monolayer A2Z2. We illustrate this unique strategy by means of first-principles calculations, which not only reproduce the structures of MoSi2N4 and MnBi2Te4 that were already experimentally synthesized, but also predict 72 compounds that are thermodynamically and dynamically stable. Such an intercalated architecture significantly reconstructs the band structures of the constituents MZ2 and A2Z2, leading to diverse electronic properties for MA2Z4, which can be classified according to the total number of valence electrons. The systems with 32 and 34 valence electrons are mostly semiconductors. Whereas, those with 33 valence electrons can be nonmagnetic metals or ferromagnetic semiconductors. In particular, we find that, among the predicted compounds, (Ca,Sr)Ga2Te4 are topologically nontrivial by both the standard density functional theory and hybrid functional calculations. While VSi2P4 is a ferromagnetic semiconductor and TaSi2N4 is a type-I Ising superconductor. Moreover, WSi2P4 is a direct gap semiconductor with peculiar spin-valley properties, which are robust against interlayer interactions. Our study thus provides an effective way of designing septuple-atomic-layer MA2Z4 with unusual electronic properties to draw immediate experimental interest., The discovery of a new two-dimensional van der Waals layered MoSi2N4 material inspires many attentions. Here, the authors report intercalation strategies to explore a much wider range of MA2Z4 family and predict amount of materials accessible to experimental verifications with emergent topological, magnetic or Ising superconductivity properties.

Conformal structured TiO2-reduced graphene oxide composite nanosheets (TiO2@rGO CNS) have been synthesized by a one-step hydrothermal method. The prepared TiO2 composite features a two-dimensional (2D) structure with an ultra-large specific surface area of 269 m2 g–1 and homogeneous TiO2 nanoparticles directly grown on the surface of reduced graphene oxide (rGO) sheets. With this special architecture of 2D sheet, it gives rise to improve light-harvesting and photo-to-electron conversion efficiency. When the TiO2@rGO CNS incorporated into the photoanode, it exhibits a greatly enhanced absorption in the visible range and low charge transfer resistance of 10.16 Ω compared with that of bare TiO2-based electrode (15.22 Ω). When the rGO content in TiO2@rGO CNS is 0.8 wt%, the fabricated solar cell obtains a remarkably enhanced power conversion efficiency (up to 7.05%), compared with that of the bare TiO2-based solar cell (4.36%), where the short-circuit current density, open-circuit voltage, and fill factor of the device are 13.63 mA cm– 2, 0.712 V, and 0.69, respectively. With the above-mentioned merits, this work supplies a feasible approach for the functionalization of 2D materials decorated with controlled metal oxide nanoparticles, which is expected as a promising material in high-performance photovoltaic devices for practical applications.

The title compound, a major animal feed supplement, abbreviated as HMTBA and alternatively called DL-methionine hydroxy analogue, C5H10O3S, (I), was isolated in pure anhydrous monomeric form. The melting point is 302.5 K and the compound crystallizes in the monoclinic space group P21/c, with two conformationally non-equivalent molecules [(I A ) and (I B )] in the asymmetric unit. The crystal structure is formed by alternating polar and non-polar layers running along the bc plane and features an extensive hydrogen-bonding network within the polar layers. The Hirshfeld surface analysis revealed a significant contribution of non-polar H...H and H...S interactions to the packing forces for both molecules.

Two-dimensional layered molybdenum disulfide (MoS2) exhibits great potential for applications in optoelectronic devices, especially photodetectors. However, most of the reported MoS2-based photodetectors only show limited absorption in visible range, resulting in narrow spectral response with poor sensitivity. Integrating plasmonic nanoparticles with MoS2 provides a promising approach to enhance light-matter interactions and detector responsibility. We here report a plasmon-enhanced photodetector based on Au/MoS2 heterostructure with Au nanoparticles (NPs) deposited on the surface of few-layer MoS2 by magnetron sputtering. By coupling with Au NPs, the interactions between ultra-thin MoS2 and incident lights are greatly enhanced via localized surface plasmon resonance. With this new architecture, the spectral response of few-layer MoS2 based photodetector is extended to near-infrared region (700–1600 nm). Further, measurements show that significant improvement of photocurrent is observed with the highest value reaching up to 64 mAW−1 at incident wavelength of 980 nm. With the demonstrated superb performance the proposed detector represents a viable path towards implementing ultra-sensitive near-infrared detection.

In crystalline solids, to computationally determine atomic migration energy barrier is a highly time consuming challenge within the framework of Density Functional Theory (DFT). Through first-principle calculations, here we have proposed a simple, high-throughput formula to fast, effectively calculate atomic migration energy barrier for fcc metals through three basic parameters of materials, the equilibrium volume ( V 0 ), the bulk modulus ( B 0 ) and the Poisson's ratio (ν). This formula is useful not only for the ideal strain-free lattices but also for the uniaxially strained lattices. It has been further validated by a series of fcc metals when compared with both available experimental or theoretical data and DFT-derived data obtained by Nudged Elastic Band (NEB) method. Moreover, we have investigated the effect of uniaxial deformation on the diffusion behavior of vacancy in fcc metals. Our calculations revealed that in fcc metals under uniaxial tensile deformation, vacancy prefers to diffuse along the direction that is perpendicular to the uniaxial tensile deformation.

The effects of different double ageing processes on the mechanical properties and microstructural evolution in the 1460 alloy were investigated. The corresponding microstructure and mechanical properties of the heat-treated specimens were studied by means of transmission electron microscopy, differential scanning calorimetry, hardness testing and tensile testing. The results show that the elongation of two-step aged (130 °C/84 h + 160 °C/24 h) samples can be increased to 8 % with a minor decrease in tensile strength (450 MPa). Throughout the two-step ageing process, the precipitation behaviour of the alloy exhibited the following characteristics: the δ′ phase formed at lower temperature dissolved into the matrix instead of coarsening and becoming over-aged at the beginning of the second ageing step; finely distributed θ′(Al2Cu) and δ′(Al3Li) at a steady state were obtained, resulting in a significant improvement in tensile strength; and the peak ductility occurred with the precipitation of T1(Al2CuLi) and consumption of δ′(Al3Li), as small δ′ particles caused a co-planar slip, resulting in lower ductility.

To understand the effects of nitrogen on precipitation and tensile behaviors at elevated temperatures, 25Cr 20Ni austenitic stainless steels with different nitrogen contents (0.065 wt %, 0.13 wt % and 0.17 wt %) were tensioned from room temperature to 1000 °C. Results show that the elongation and area reduction change differently with temperature increasing from 800 °C to 1000 °C, which are caused by different fracture modes. The necking concentrates in a small zone in the tensile samples of 0.17 wt % N steel, while the necking deformation occurs in all gauge length in specimens of other two steels. The local necking in 0.17 wt % N steel is induced by the enhanced recrystallization softening effect during tensile deformation. As the dislocation recovery are retarded by more solute nitrogen atoms and nitrides in 0.17 wt % N steel, the dislocation density difference between deformed grains and recrystallized grains turns larger, which results in a higher recrystallization rate and enhanced softening effect in further deformation. Additionally, the content of M23C6 precipitates unexpectedly increases with the increasing nitrogen after tensile test at 800 °C. This is because the increasing nitrogen promotes the formation of NbCrN in higher nitrogen steels during solution heat treatment, instead of Nb(C, N) in lower nitrogen steels. The precipitation of NbCrN releases carbon into austenitic matrix, leading to the acceleration of M23C6 precipitation during tensile test at 800 °C.

The deformation behaviors of 310s stainless steel weld metals (WMs) with different Nb contents are investigated under uniaxial tension at temperatures ranging from ambient temperature to 1000 °C. The results show that Nb effectively improves the strength of the WMs in the whole test temperatures range. The addition of Nb causes a change in the fractures of the WMs from intergranular brittle fractures to intragranular ductile fractures at elevated temperature. Hence, elemental Nb enhances the elevated temperature plasticity of the WMs. However, as the eutectic Nb(C, N) promotes the nucleation and propagation of the cracks, Nb decreases the elongation of the WMs at temperature below 700 °C. With the increase in the deformation temperature, the strength of the WMs decreases monotonously. However, the elongation of the WMs shows a nonmonotonic relationship with the temperatures. The deformation twins can occur at room temperature and improve the strength and elongation of the WMs. Meanwhile, the cracks nucleate around the eutectic Nb(C, N) at 1000 °C, leading to the minimum elongation of the Nb-bearing WMs in the whole test temperature range. The differential scanning calorimetry result suggests that the melting of the eutectic Nb(C, N) may be the dominant reason for the nucleation of the cracks and result in the reduction in the elongation of the Nb-bearing WM at 1000 °C.

The as-quenched microstructure of 2.25Cr-1Mo-0.25 V steel heavy forgings is granular bainite, which is composed of bainitic ferrite and blocky islands of martensite and retained austenite (RA). In this study, the characteristics of RA decomposition and its effects on the mechanical properties of the steel are investigated. The results show that RA decomposes into a cluster of coarse M23C6 carbides and ferrite during standard tempering at 700 °C. These coarse carbides decorate the boundary of the cluster, thus deteriorating the impact toughness of the steel. Accordingly, the size and distribution of these carbides are tentatively modified by introducing pre-tempering at different temperatures ranging from 180° to 650°C before the standard tempering at 700 °C. This is because during pre-tempering, RA first decomposes into various transitional microstructures such as martensite, bainite or pearlite, which further transform into M23C6 carbide clusters during the subsequent 700 °C tempering. The experimental results show that 455 °C is the optimal pre-tempering temperature to improve the impact toughness of the steel after the 700 °C tempering. Microstructural observations reveal that during the 455 °C pre-tempering step, the RA completely decomposes into bainite consisting of fine bainitic packets and a high density of M3C carbides, which provide additional nucleation sites for M23C6 carbides inside the carbide clusters during the subsequent 700 °C tempering, and thus avoid the formation of coarse M23C6 distributed along carbide cluster boundaries.

Based on first-principles calculations, we predict that the superconductor ${\mathrm{MgB}}_{2}$ with an ${\mathrm{AlB}}_{2}$-type centrosymmetric lattice hosts the so-called phononic topological Weyl nodal lines (PTWNLs) in its bulk phonon spectrum. These PTWNLs can be viewed as countless Weyl points (WPs) closely aligned along the straight lines in the $\ensuremath{-}H--K--H$ direction within the three-dimensional Brillouin zone (BZ) and are always paired with opposite Berry phase. Their topologically nontrivial natures are confirmed by the calculated Berry curvature distributions on the planes perpendicular to these lines. These lines are unique, because they are located exactly at the high-symmetry boundary of the BZ protected by the mirror symmetry and, simultaneously, are straightly transverse to the whole BZ, differently from known classifications, including nodal rings, nodal chains or nets, and nodal loops. On the $(10\overline{1}0)$ crystal surface, the PTWNL-induced drumhead-like nontrivial surface states appear within the rectangular area confined by the projected lines of the PTWNLs. Moreover, when the mirror symmetry is broken, the double-degenerate PTWNLs are further lifted to form a pair of WPs with opposite chirality. Our results pave the way for future experimental study of topological phonons on ${\mathrm{MgB}}_{2}$ and highlight similar results in a series of isostructural ${\mathrm{AlB}}_{2}$-type metallic diborides.

The high-temperature tensile behaviors of two 25Cr-20Ni austenitic stainless steels with different V concentration (0 wt% V and 0.3 wt% V, respectively), have been studied at temperature between 200 ℃ and 900 ℃ . The ultimate tensile strength of both steels is strong temperature dependent, which decreases slowly first at 200–300 ℃ , keeps platform then at 300–500 ℃ and decreases rapidly afterwards from 600 ℃ to 900 ℃ . It is caused by the decreasing strain hardening ability, dynamic strain aging and dynamic recovery together with dynamic recrystallization at different temperatures. At higher than 800 ℃ , the elongation of both steels increases markedly due to the dynamic recovery and dynamic recrystallization. However, because of the deteriorated effects of M23C6 precipitates at grain boundary, the elongation of both steels at 700 ℃ does not increase despite decreasing strength. Additionally, the addition of 0.3 wt% V decreases the ductility of the material in the temperature range of 800 ℃ to 900 ℃ , which is induced by the impeding effects of solute vanadium on dynamic recovery and recrystallization.

Flexible regulation of fluxes means a lot due to its enormous potential for efficient utilization of various energy sources. Benefitting from the emerging transformation optics-based technology and metamaterials, regulation structures manipulating different energy fluxes have aroused wide attention, relevant research have progressed rapidly in recent years. However, many studies are suffering from over-complicated problem and confined to a single function or a single available occasion. Hence, design of flux regulation device requires improvement to ease the processing burden, and functional diversity as well as application scope need in-depth development to meet requirements for more sophisticated practical settings. Furthermore, appropriate evaluation index for comprehensive assessment of energy transportation procedure should never be ignored. Here, we design an intelligent flux transfer regulator that can actively switch between concentration and rotation states based on temperature change by allocating thermal and DC electric fields. Improved geometric and conduction parameters are selected to avoid non-uniform distribution problem. Counting on the diffusivity of the implicated thermal field, the local entropy generation rate as a thermodynamic indicator is introduced to evaluate energy quality during state-switch process in transient mode. Influences of different design variables on control effect as well as energy fluctuation are also studied. Combing discretization method, effective medium theory, spiral bimetal sheet and other control setups, we experimentally confirm the effectiveness of the proposed device. Our work enriches the practicability and application of regulation device for energy control and utilization, opens an avenue for investigating energy transportation phenomena from the perspective of different disciplines.

By means of the first-principles calculations, the lattice parameters, electronic structures, phonon dispersions, and mechanical properties of the rare earth metal borocarbide YB 2 C have been theoretically investigated. The dynamically stability of the layered tetragonal YB 2 C has been evidenced based on the frozen phonon method. We have found that the covalent bonding between B-2 p , C-2 p and Y-5 d orbitals are responsible for the strong interlayer interactions based on the calculated electronic structures and ELF images. The estimated hardness of P 4 2 / mbc -YB 2 C is around 23.46 GPa which is comparable with the well-known ultra-incompressible oP 6-OsB 2 . Additionally, the analysis of the ideal shear and tensile strength of YB 2 C reveals the importance of covalent bonds between Y and B/C layer which help to enhance the resistance under deformation.

Stress relaxation cracking failure is widely reported in the materials used for thermal power plants, which seriously affected the safety of thermal power units under long-term service. In our study, the influence of intergranular precipitation behavior on the stress relaxation cracking susceptibility of Super304H austenitic stainless steel weld metal was investigated based on the pre-compressed CT technique. The 45° misorientation grain boundary has the lowest critical interfacial nucleation energy, followed by the grain boundaries away from the 45° misorientation. As a result, the coarse intergranular M23C6 carbides were more prone to precipitate at 45° misorientation grain boundary, which led to the lowest resistance to stress relaxation cracking susceptibility at 35 to 45° grain boundary. Further analysis indicated that intergranular cavities were easily initiated at the non-coherent M23C6/γ interface. Since the non-coherent interface with a low Cr concentration zone and high specific interfacial energy had a lower strength, the lattice distortion at the non-coherent interface could lead to higher strain concentration.

The evolutions of microstructure and mechanical properties during tempering at 700 °C, of normalized and oil-quenched 2.25Cr-1Mo-0.25V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated. It is found that the normalized sample has a granular bainite microstructure and the oil-quenched sample has a lath bainite microstructure. After 0.5 h of tempering, the normalized sample has a higher strength and ductile-to-brittle transition temperature (DBTT) than the oil-quenched sample because of the strengthening effect of the undecomposed martensite-austenite (M-A) constituents and the presence of coherent tiny VC type precipitates in granular bainite. However, when the tempering time is increased from 0.5 to 128 h, the strength as well as the DBTT of the normalized sample decreases more pronounced than that of the oil-quenched sample. This is attributed to the synergistic effect of the decomposition of M-A constituents, growth of VC type precipitate in the normalized sample, and the increase in the effective grain size in the oil-quenched sample.

The heat and mass transfer and the morphological evolution of the various weld solidification structures for the entire weld were simulated through the macro-micro modeling which combined the welding process and the solidification structures in the weld pool. The weld profiles were calculated for different welding conditions, and the effects of the welding parameters on the solidification conditions were analyzed along the trailing pool boundary. The formation mechanism of the axial columnar structure and the equiaxed grains in the weld were studied by the developed model. The results indicate that the temperature gradient G, solidification rate Rw, and the ratio G/Rw along the trailing pool boundary are related with the location in the weld and the welding parameters. If there does not exist heterogeneous nuclei in the weld pool, the axial columnar structure forms in the central region of the weld at low welding speeds owing to the anisotropic kinetics of the dendritic structure. When the heterogeneous nucleation occurs in the weld pool, the weld structure is directly determined by the competitive growth between the columnar and new nucleated grains. At low welding speeds, the preferential orientations of the survival grains get closer to the welding direction with the decrease of the distance to the weld centerline. With the increase of the welding speed and welding current, more grains form in the weld pool, and the new nucleated grains also become more competitive. The columnar grains from the fusion line are blocked by the equiaxed grains. The continuous equiaxed grain zone forms in the weld, and the columnar to equiaxed transition (CET) occurs.

Beryllium was recently discovered to harbor a Dirac nodal line (DNL) in its bulk phase and the DNL-induced non-trivial drumhead-like surface states (DNSSs) on its (0001) surface, rationalizing several already-existing historic puzzles [Phys. Rev. Lett., \textbf{117}, 096401 (2016)]. However, to date the underlying mechanism, as to why its (0001) surface exhibits an anomalously large electron-phonon coupling effect ($\lambda_{e-ph}^s$ $\approx$ 1.0), remains unresolved. Here, by means of first-principles calculations we have evidenced that the coupling of the DNSSs with the phononic states mainly contributes to its novel surface \emph{e-ph} enhancement. Besides that the experimentally observed $\lambda_{e-ph}^s$ and the main Eliashberg coupling function (ECF) peaks have been reproduced well, we have decomposed the ECF, $\alpha^{2}$$F$(\emph{k},\textbf{\emph{q}};\emph{v}), and the \emph{e-ph} coupling strength $\lambda(\emph{k},\textbf{\emph{q}};\emph{v})$ as a function of each electron momentum (\emph{k}), each phonon momentum (\textbf{\emph{q}}) and each phonon mode ($v$), evidencing the robust connection between the DNSSs and both $\alpha^{2}$$F$(\emph{k},\textbf{\emph{q}};\emph{v}) and $\lambda(\emph{k},\textbf{\emph{q}};\emph{v})$. The results reveal the strong \emph{e-ph} coupling between the DNSSs and the phonon modes, which contributes over 80$\%$ of the $\lambda_{e-ph}^s$ coefficient on the Be (0001) surface. It highlights that the anomalously large \emph{e-ph} coefficient on the Be (0001) surface can be attributed to the presence of its DNL-induced DNSSs, clarifying the long-term debated mechanism., Comment: 6 pages, 3 figures

The competitive growth of microstructures in the entire weld pool for both the Al–Cu alloy and the pure aluminum was simulated by the cellular automata method to comparatively investigate the micro-mechanisms for the morphological evolution of the axial structure and the curved columnar grain in the weld. The competitive mechanism of grains during the epitaxial growth and the morphological evolution of the grain structure in the weld with various welding speeds were studied. The results indicate that both the thermal conditions and the solidification characteristic of the weld metal exert an important influence on the grain competition and the resulting structure in the weld. For the Al–Cu alloy, the dendritic structure with a large S/L interface curvature appears during the epitaxial growth. The preferential orientation affects the competition result obviously. Owing to the anisotropic growth kinetics, the straight axial structure forms at low welding speeds. With the increase of the welding speed, the width of the axial region decreases and eventually disappears. For the pure aluminum, the S/L interface during the epitaxial growth is planar, and the grain competition is controlled by the thermal conditions completely. The columnar grains curve gradually to follow the highest temperature gradient direction at low welding speeds and become straight at high welding speeds.

In analogy to various fermions of electrons in topological semimetals, topological mechanical states with two types of bosons, Dirac and Weyl bosons, were reported in some macroscopic systems of kHz frequency, and those with a type of doubly-Weyl phonons in atomic vibrational framework of THz frequency of solid crystals were recently predicted. Here, through first-principles calculations, we have reported that the phonon spectra of the WC-type TiS, ZrSe, and HfTe commonly host the unique triply degenerate nodal points (TDNPs) and single two-component Weyl points (WPs) in THz frequency. Quasiparticle excitations near TDNPs of phonons are three-component bosons, beyond the conventional and known classifications of Dirac, Weyl, and doubly-Weyl phonons. Moreover, we have found that both TiS and ZrSe have five pairs of type-I Weyl phonons and a pair of type-II Weyl phonons, whereas HfTe only has four pairs of type-I Weyl phonons. They carry nonzero topological charges. On the ($10\overline{1}0$) crystal surfaces, we observe topological protected surface arc states connecting two WPs with opposite charges, which host modes that propagate nearly in one direction on the surface.

The structure, interdendritic, and intergranular segregation, and precipitates of an Alloy 690 electro-slag remelting (ESR) ingot in commercial scale (3t) were investigated by the optical microscopy, electroprobe microanalysis, scanning electron microscopy, and transmission electron microscopy (TEM) techniques. The results indicate that the central longitudinal section of the ESR ingot comprised the ramp-up, steady-state, and hot-top regions, which could be easily distinguished from each other through the macrostructures of them. In the interdendritic area, Cr and Ti were enriched, while Ni and Fe were depleted, and the nominal segregation indexes (ζ i = C 0 i /C interdendritic i ) of Ti, Cr, and Ni were 0.40, 0.91, and 1.04, respectively, in the hot-top region where suffered the severest segregation. Nitrides, principally precipitated between dendrites, were identified as TiN by TEM and EDS. The morphology, size distribution, and volume fraction of them were determined as well. In terms of the intergranular area, Cr and C coexisted, while Ni and Fe were depleted. And the dendrite-like carbides continuously distributed on the interface between grains, which were identified as M23C6 by the selected area diffraction pattern.

Deformation-induced phase transformation (DIPT) of metastable austenite in Fe–13% Cr–4% Ni steel occurs during the macroscopic elastic stage and is accompanied by the yielding of metastable austenite at microscopic level. The DIPT rate is accelerated by yielding the martensite matrix, which increases the available nucleation sites for DIPT by enhancing the plastic deformation in austenite.

The effect of Cr-enriched M23C6 carbides distributed at lath boundaries on the oxidation behavior of a high-silicon 12% Cr ferritic/martensitic (HSiF/M) steel has been investigated. The M23C6 carbides were oxidized during the exposure of HSiF/M in air at 823 K. The oxidation of M23C6 carbides promotes the external oxidation of Si at lath boundaries due to the third-element effect, which induces inhomogeneous distribution of elements at the microscale in the oxidation products.

The effects of boron on the microstructure and the pitting corrosion behavior, especially the metastable pitting, were investigated in Super304H austenitic stainless steels with 0-130 ppm boron solution treated at 1100 degrees C. The microstructures were analyzed using scanning electron microscopy, transmission electron microscopy with energy dispersive spectroscopy and secondary ion Mass spectroscopy. Meanwhile, the pitting corrosion behavior was evaluated by potentiodynamic and potentiostatic polarization tests in 3.5% NaCl solution at 25 degrees C. Microstructural characterizations showed that boron addition favored the precipitation of Cr-rich boride (M2B), which resulted in an asymmetric Cr-depleted zone around M2B and reduced the chromium content in the austenite matrix. The Cr-depleted zones could act as extra sites for pit initiation, along with the interfaces between the austenite matrix and non-metallic inclusions. The potentiodynamic polarization test results indicated that boron addition decreased the pitting potential. The potentiostatic test results revealed that boron addition promoted the initiation frequency and growth rate of metastable pits, and increased the probability of stable pitting. (C) 2015 The Electrochemical Society.

In order to guide the design of 9–12% Cr ferritic/martensitic (F/M) heat resistant steels appropriate for use in accelerator-driven system, the impact toughness and behavior of precipitation of a 12% Cr F/M heat resistant steel containing high C and Si concentrations have been investigated. Particular focus has been given to the interaction of carbon and carbonitride-forming element. Because of the presence of primary NbC and a large amount of M 23 C 6 , the impact toughness of the investigated steel was much lower than that of the commercial 9–12% Cr F/M heat resistant steels. The primary NbC crystallizes directly from liquid metal when the Nb concentration is higher than a critical value. The critical value of Nb decreases with increase in the carbon concentration. In addition, the higher content and faster precipitation kinetics of M 23 C 6 in the investigated steel during the tempering heat treatment also lowers the impact toughness.

Through first-principles calculations, we systematically investigated the hydrogen interactions with the oxygen-vacancy (O:V) pairs complex in bcc Fe matrix (mimic oxygen-enriched nanoclusters (NCs) of ODS steels) in comparison with the vacancy-alone defects. The results uncovered that the presence of the (O:V) pairs in oxygen-enriched NCs play a crucial role in prohibiting the growth and swelling of the hydrogen cluster but strongly trap a few hydrogen atoms around each cluster of vacancies. As accompanied with a high density of dispersed NCs in ODS steels, this fact significantly elevates the tolerance of the critical hydrogen concentration of ODS steels as compared with traditional steels. The underlying mechanism to pin the growth of hydrogen cluster has been elucidated to be strongly correlated with the viable charges transfer from the nearby Fe atoms around vacancies. This is the key to determine the trapped concentration and the distribution of hydrogen atoms in ODS steels.

The effects of M 23C6 (M = Cr, Fe) on the high-temperature performance of the NiCrFe-7 welding rods and weld metals were studied by high-temperature tensile tests and microstructure analysis. M 23C6 at the grain boundaries (GBs) has a cube-on-cube coherence with one grain in the NiCrFe-7 weld metals, and the adjacent M 23C6 has the coherence relationship with the same grain. The grain with a coherent M 23C6 has a Cr-depletion region. The number and size of M 23C6 particles can be adjusted by heat treatment and alloying. There are two temperatures [T E1: 923 K to 1083 K (650 °C to 810 °C) and T E2: 1143 K to 1203 K (870 °C to 930 °C)] at which the GBs and grains of the NiCrFe-7 welding rod have equal strength during the high-temperature tensile test. When the temperatures are between T E1 and T E2, the strength of the GBs is lower than that of the grains, and the tensile fractures are intergranular. When the temperatures are below T E1 or over T E2, the strength of the GBs is higher than that of the grains, and the tensile fractures are dimples. M 23C6 precipitates at the GBs, which deteriorates the ductility of the welding rods at temperature between T E1 and T E2. M 23C6 aggravates ductility-dip-cracking (DDC) in the weld metals. The addition of Nb and Ti can form MX (M = Ti, Nb, X = C, N), fix C in grain, decrease the initial precipitation temperature of M 23C6, and mitigate the precipitation of M 23C6, which is helpful for minimizing DDC in the weld.

Interaction between hydrogen (H) and metals is central to many materials problems of scientific and technological importance. H segregation or trapping at lattice defects plays a crucial role in determining the properties of these materials. Through first-principles simulations, we propose a unified mechanism involving charge transfer associated strain destabilization to understand H segregation behavior at vacancies. We discover that H prefers to occupy interstitials with high pre-existing charge densities and the availability of such interstitials sets the limit on H trapping capacity at a vacancy. Once the maximum H capacity is reached, the dominant charge donors switch from the nearest-neighbor (NN) to the next-nearest-neighbor (NNN) metal atoms. Accompanying with this long-range charge transfer, the sharply increased reorganization energy would occur, leading to the instability of the H-vacancy complex. The physical picture unveiled here appears universal across the BCC series and is believed to be relevant to other metals/defects as well.

The mechanical stability of reversed austenite at room temperature in two 13%Cr–4%Ni low carbon martensitic stainless steel samples after different heat treatments has been investigated. The uniaxial tensile tests indicate that the reversed austenite resulting from the one-stage and two-stage intercritical tempering heat treatment have different mechanical stability, which induces distinct strength-ductility balance of the material. Experiments and crystal plasticity finite method simulations reveal that the special grain orientation relationship between the reversed austenite and martensite matrix, besides chemical composition, plays important roles on the mechanical stability of the reversed austenite. It is found that in both samples the Nishiyama–Wassermann or Kurdjumov–Sachs relationship between the reversed austenite and the martensite matrix provides an easy way for the active slip systems in austenite to penetrate the phase boundary to the adjacent martensite. This results in a high mechanical stability of the reversed austenite. In addition, the larger austenite grains in the second-stage tempering sample have higher mechanical stability in the martensitic steel due to the favored austenite stabilizing elements distribution behavior, and the favored stress distribution originating from the intrinsic strengths of the austenite and martensite matrix.

The effects of the minor elements Ti and Nb on the microstructure and mechanical properties for multi-pass weldments from the alloy ER NiCrFe-7 were studied using an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), electron probe micro analysis (EPMA), as well as tensile and bend tests. The results show that grain size does not change significantly by increasing the Ti content from 0.28 wt% to 0.87 wt% in the weld metal (WM), whereas the grain boundaries become tortuous. The intragranular precipitate in the WM with Ti are AlO and Ti(C, N), whereas the intragranular precipitate in the WM with Nb are AlO and MX (M=Nb, Ti, X=C, N). As the Ti and Nb content increased in the WM, more MX was produced. Furthermore, the majority of C was fixed in the grain, not segregated to the grain boundaries; and less M 23 C 6 (M=Cr, Fe) formed at the grain boundaries. Fewer ductility-dip-cracking (DDC) was observed for WM with higher levels of Ti and Nb. The tensile strength and elongation simultaneously increased with an increase in Ti and Nb in WM. The number and length of the cracks in the bend specimens decreased upon adding Ti and Nb.

Beryllium is a simple alkali earth metal, but has been the target of intensive studies for decades because of its unusual electron behaviors at surfaces. Puzzling aspects include (i) severe deviations from the description of the nearly free electron picture, (ii) anomalously large electron-phonon coupling effect, and (iii) giant Friedal oscillations. The underlying origins for such anomalous surface electron behaviors have been under active debate, but with no consensus. Here, by means of first-principle calculations, we discover that this pure metal system, surprisingly, harbors the Dirac node line (DNL) that in turn helps to rationalize many of the existing puzzles. The DNL is featured by a closed line consisting of linear band crossings and its induced topological surface band agrees well with previous photoemission spectroscopy observation on Be (0001) surface. We further reveal that each of the elemental alakali earth metals of Mg, Ca, and Sr also harbors the DNL, and speculate that the fascinating topological property of DNL might naturally exist in other elemental metals as well., Comment: 5 pages, 4 figures

By means of first-principles calculations in combination with universal evolutionary structure search, we identified the crystalline structure of long-term argued ${M}_{3}{\mathrm{Bi}}_{2}$ ($M=\text{Sr}$, Ca, and Ba), which crystallizes in a tubelike structure stacked by buckled graphenelike layers. The analyses of electronic structures revealed that this type of ${M}_{3}{\mathrm{Bi}}_{2}$ is a native wide-gap three-dimensional topological insulator with the inverted band order induced mainly by crystal field effect. The spin-orbit coupling effect was found to open the band gap and further enhance the band inversion. Among them, ${\mathrm{Sr}}_{3}{\mathrm{Bi}}_{2}$ is most attractive due to its largest fundamental gap of about 0.3 eV and the directly inverted band gap of 0.81 eV at $\mathrm{\ensuremath{\Gamma}}$ obtained within the framework of Green functionals $(GW)$. Moreover, the computation also evidences that their tubelike structure is suitable for further treatment via magnetic dopants, which prefer to occupy $1b$ site. Interestingly, the ferromagnetic insulating state has been achieved for V- and Mn-doped cases. This may provide a further opportunity to observe the quantized anomalous Hall effect in its thin films.

The influence of tempering temperature on the reversed austenite formation and tensile properties are investigated in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steel in the temperature range of 550-950 degrees C. It is found that at the temperatures below 680 degrees C, the reversed austenite formation occurs by diffusion. Amount of the reversed austenite is determined I:IN the tempering temperature and the holding time. The segregation of Ni is the main reason for the stability of the reversed austenite. When the temperatures are above 680 degrees C, the reversed austenite formation proceeds by diffusionless. The reversed austenite will transform back to martensite after cooled to room temperature. The tensile properties are most strongly influenced by the amount of the reversed austenite obtained at room temperature. The excellent combination of good strength and ductility is at 610 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

The effect of the active element oxygen and the welding parameters (welding speed, welding current and electrode gap) on the liquid pool convections and the weld shape variations under helium gas shielding is systematically investigated using a mathematical model of the welding arc and weld pool during a moving GTAW of SUS304 stainless steel. Different welding parameters will change the temperature distribution on the pool surface, and affect the strength of the Marangoni convection and the weld shape. The inward convection induced by the electromagnetic force is contributive to the increase of the weld depth. A higher welding speed, lower welding current or larger electrode gap will make the weld D/W ratio decrease when the oxygen content in the weld pool is high. Under low oxygen content, a lower welding speed, welding current or shorter electrode gap will lead to a higher weld D/W ratio. The predicted weld D/W ratio by simulation agrees well with the experimental results. (C) 2010 Elsevier Ltd. All rights reserved.

Single dendrite and multi-dendrite growth for Al-2 mol pct Si alloy during isothermal solidification are simulated by phase field method. In the case of single equiaxed dendrite growth, the secondary and the necking phenomenon can be observed. For multi-dendrite growth, there exists the competitive growth among the dendrites during solidification. As solidification proceeds, growing and coarsening of the primary arms occurs, together with the branching and coarsening of the secondary arms. When the diffusion fields of dendrite tips come into contact with those of the branches growing from the neighboring dendrites, the dendrites stop growing and being to ripen and thicken.

A numerical modeling of the welding arc and weld pool is studied for moving GTA welding to investigate the effect of the surface active element oxygen and the plasma drag force on the weld shape. Based on the 2D axisymmetric numerical modeling of the argon arc, the heat flux, current density and plasma drag force are obtained under different welding currents. Numerical calculations to the weld pool development are carried out for moving GTA welding on SUS304 stainless steel with different oxygen contents 30 ppm and 220 ppm, respectively. The results show that the plasma drag force is another dominating driving force affecting the liquid pool flow pattern, except for the Marangoni force. The different welding currents will change the temperature distribution and plasma drag force on the pool surface, and affect the strength of Marangoni convection and the weld shape. The weld D/W ratio initially increases, followed by a constant value around 0.5 with the increasing welding current under high oxygen content. The weld D/W ratio under the low oxygen content slightly decreases with the increasing welding current. The predicted weld shape by simulation agrees well with experimental results.

The precipitation mechanism of cellular η phase at grain boundaries and its effect on the high temperature tensile properties in an Fe–Ni–Cr alloy were investigated by using TEM, SEM, EBSD and tensile testing. It was revealed that η phase nucleates at high energy grain boundaries and has a coherent orientation relationship with the austenite matrix: {0 0 1}η//{1 1 1}γ, η// γ. The cellular austenite in the vicinity of the η phase has the same orientation as the grain from which the η phase growth starts. The formation of cellular austenite is suggested to proceed by grain boundary migrating with precipitated η phase into the neighboring grain which is incoherent with the η phase. The precipitation of cellular η phase at grain boundary improves the high temperature ductility of the alloy dramatically.

Two-dimensional cellular automaton modeling has been performed to investigate the mechanism of ferrite refinement during the dynamic strain-induced transformation (DSIT) from austenite (gamma) to ferrite (alpha) in a low-carbon steel. The simulated results indicate that the refinement of ferrite grains derived from the DSIT could be interpreted in context of "un-saturated" nucleation and limited growth. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-temperature deformation-induced ferrite grains supersaturated with carbon offer superior properties such as higher elastic modulus and microhardness compared with proeutectoid ferrite because of they contain nanometer martensite flakes. After annealing at 700 °C, fine carbides precipitate due to the decomposition of these nanometer martensite flakes.

By means of first-principles calculations, we identified the structural phase transition of Na$_3$Bi from hexagonal ground state to cubic $cF$16 phase above 0.8 GPa, in agreement with the experimental findings. Upon the releasing of pressure, \emph{cF}16 phase of Na$_3$Bi is mechanically stable at ambient condition. The calculations revealed that the $cF$16 phase is topological semimetal, in similarity to well-known HgTe and it even exhibits an unusually low $C^\prime$ modulus (only about 1.9 GPa) and a huge anisotropy, $A^u$ of as high as 11, the third highest value among all known cubic crystals in their elastic behaviors. These facts render \emph{cF}16-type Na$_3$Bi very soft with a liquid-like elastic deformation in the (110)$$ slip system. Importantly, as accompanied with this deformation, Na$_3$Bi shows a topological phase transition from a topological semimetal state at its strain-free cubic phase to a topological insulating state at its distorted phase. Because the $C^\prime$ elastic deformation almost costs no energy in a reversible and liquid-like soft manner, \emph{cF}16-type Na$_3$Bi would potentially provide a fast on/off switching way between topological insulator and topological semimetal, which would be beneficial to the quantum electronic devices for practical applications., Comment: 5 pages, 4 figures

First-principles determination of the vacancy formation enthalpies has been long-term believed to be highly successful for metals. However, a widely known fact is that the various conventional density functional theory (DFT) calculations with the typical semilocal approximations show apparent failures to yield accurate enthalpies of Ag and Au. Recently, the previously commonly assumed linear Arrhenius extrapolation to determine the vacancy formation enthalpies at $T=0$ K from the high-temperature measured concentration of thermally created vacancies has been demonstrated to have to be replaced by the non-Arrhenius local Gr\"uneisen theory (LGT) [A. Glensk, B. Grabowski, T. Hickel, and J. Neugebauer, Phys. Rev. X 4, 011018 (2014)]. The large discrepancies between the conventional DFT-PBE data and the unrevised experimental vacancy formation enthalpies disappear for Cu and Al. Even by following the same LGT revisions for Ag, the large discrepancies still remain substantial at $T=0$ K. Here, we show that the hybrid functional (HSE), by including nonlocal exchange interactions to extend the conventional DFT method, can further correct these substantial failures. Upon a comparison of the experimental valence-band spectra for Cu, Ag, and Au, we have determined the HSE exchange-correlated mixing parameters $\ensuremath{\alpha}$ of 0.1, 0.25, and 0.4, and further derived the HSE enthalpies of vacancy formation of 1.09, 0.94, and 0.72 eV, respectively; in nice agreement with available LGT-revised experimental data. Our HSE results shed light on how to improve the theoretical predictions to accurately determine the defect formation energies and related thermodynamical properties.

A q-state Potts model-based Monte Carlo method is developed to model the austenite-ferrite transformation, which occurs during the isothermal austenite decomposition with a non-equilibrium austenite-ferrite interface in an Fe-C binary system on a mesoscale. In this simulation, the carbon diffusion in the ferrite is allowed and the carbon diffusion simulation is independent of the numerical solution of the diffusion equation. During the isothermal austenite decomposition, the non-equilibrium austenite-ferrite interface is mixed diffusion/interface controlled. It is found that the characteristic of the transformation varies as the transformation proceeds. The effects of the bulk carbon concentration and the soaking temperature on the characteristics of the isothermal austenite-ferrite transformation are investigated. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The shape-memory characteristics in the Ni41.3Ti38.7Nb20 alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni41.3Ti38.7Nb20 alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.

This part is the verification and application of the local and through models as described in previous part 1 [7]. In-house software system ROLLAN (Rolling Analysis) is developed in our laboratory to simulate the process history such as temperature evolution, strain and rolling force etc. Metallurgical phenomena include recrystallization in roughing and finishing, phase transformation in laminar cooling and final mechanical properties. Each local model is verified by laboratory and hot strip mill experiments, and a large amount of experimental data from the mill is collected for computational use. The mechanical properties of samples from the strip tail are compared with results predicted by the models. They agree very well for C-Mn steels. In addition, two typical strip coils are opened, cut in the room temperature and divided into multi-sections. The calculated and the measured microstructure and mechanical properties along the length direction are compared. The results show that both yield strength and tensile strength are in good agreement. Though elongation reveals a little deviation, reasonable agreement with the predicted results is achieved. However, it is also found that the properties vary in the length direction of the strip. Analysis indicates that this is caused by the inhomogeneity of the microstructure, while the latter is caused by process parameters.

A metallurgical through-process model is presented which describes the microstructural evolution and predicts the final mechanical properties of low carbon steel during hot strip rolling. Process models concern the thermal and deformation phenomena, which take into account the strain, strain rate and temperature distribution along the length of the strip. And the metallurgical models cover five modules, which are (i) austenitization of cast slab in reheating furnace, (ii) recrystallization of austenite in hot rolling, (iii) phase transformation of austenite-ferrite in laminar cooling on the run-out-table, (iv) grain growth after coiling, and (v) final structure-mechanical properties of products. Temperature is the main parameter and has dominant influence on the microstrutural evolution and the mechanical properties. The related temperature variation in hot strip rolling concerns air cooling, scaling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. These complex factors are incorporated into the thermal models to simulate the temperature distribution along the length of the strip from the reheating furnace exit to the down-coiler. A self-learning algorithm is employed to improve the calculation accuracy and the computational temperatures are compared with the measured ones at typical locations. In the structure-property relationships, two key process parameters (e.g., finishing exit temperature (FT7) and coiling temperature (CT)) are introduced in the model to consider the influence of morphology of microstructure on mechanical properties.

Hot strip rolling process includes four main stages, which are reheating process, roughing and finishing process, laminar-cooling process, and coiling process respectively. Temperature is the most sensitive parameter and has direct effect on the microstructural evolution and further the mechanical properties, and the accurate control of temperature guarantees the quality of products and homogeneity of properties along the strip length. However, for the conventional hot strip rolling process, thermal history along the strip length is very complex, the related temperature variation concerns air cooling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. Based on the actual hot strip mill, the thermal models are established in this paper to simulate the temperature distribution along the whole strip length from the reheating furnace exit to the down coiler. Different interface heat transmission coefficients are selected for the scale breaking and spray water-cooling process, and a self-learning algorithm is thus employed to improve the calculation accuracy. This model is characterized as simple and fast, and convenient for on-line/off-line prediction of temperature. Finally the simulated results are verified by the on-line temperature detection at typical points such as roughing exit (RT2), finishing exit (FT7) and coiling position (CT).