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The atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel were investigated. Atom probe tomography and first-principles calculations were applied to evaluate and analyze the atomic interactions at the ferrite/cementite interface. In the initial stage of eutectoid transformation, enrichment of Si and Mn occurred at the ferrite and cementite sides, respectively, of the interface. This interfacial segregation phenomenon gradually diminished as the transformation proceeded. Calculations of the cohesive energy and formation energy revealed a clear enhancement in the chemical bonding and stability of the pearlite system when the Si atom was moved from the ferrite layer to the cementite layer and the Mn atom was moved in the opposite direction. The interfacial segregation of the Mn and Si atoms was mainly responsible for the insufficient diffusion and high hybridization degree of Fe, Mn, Si, and C atoms. Furthermore, the partitioning ratio of Mn in high-Si steel was greater than that in low-Si steel, leading to greater partitioning of Mn into the cementite phase. Calculations of the electronic structure revealed that the enrichment of Si in the ferrite phase promoted the partitioning of Mn into the cementite phase owing to the strong repulsive force between Mn and Si at the pearlitic interface. [ABSTRACT FROM AUTHOR]

Fine carbide particles are desirable for balancing the hardness-toughness tradeoff of high-speed steels (HSSs). The conventional processing method, i.e., carbide decomposition and spheroidization followed by heavy deformation, usually requires high heating temperature and large plastic strain for the carbide refinement. The present work has proposed a novel thermal–mechanical processing route, which employs a high-temperature pre-deformation step instead of the post-deformation. The results show that pre-deformation is both thermodynamically and kinetically favorable for the following carbide decomposition and spheroidization owing to the introduction of dislocations and stacking faults into primary carbides. Compared with the conventional method, the new route produces smaller carbides and causes higher hardness and wear resistance. This finding may provide an alternate pathway for the reduction of heating temperature and plastic strain while guaranteeing the microstructural homogeneity of HSSs. [ABSTRACT FROM AUTHOR]

In this study, the microstructural evolution and the interaction between the clad and the core alloys that occurs during the brazing process of two-layer Al sheets with equiaxed grains were examined. The study was carried out using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and glow discharge optical emission spectrometry (GDOES). The effects of microstructure on the brazing performances of two-layer sheets were clarified. Although the grains were fine and equiaxed before brazing, three typical microstructural evolutions happened during brazing, corresponding to three kinds of interactions between the clad and core alloys of the aluminum brazing sheets. In the alloys, which had either relatively uniform grain growth or no grain growth, the interaction between the clad alloy and the core alloy was weak; accordingly, they showed a smooth surface, an even microstructure, faint element mutual diffusion, and eventually good brazeability. Meanwhile, in the alloy with obvious abnormal grain growth (AGG), strain-induced liquid-film migration (SILFM) occurred when the energy was too low to cause strain-induced boundary migration (SIBM). This led to rough and uneven surface morphology, significant mutual diffusion, and surface segregation of elements; eventually, this produced the worst brazeability. [ABSTRACT FROM AUTHOR]

The physical properties of molybdenum semi-carbide (Mo2C) that usually serves as reinforced phases of engineering alloys are crucial for mechanical properties of bulk materials. In this work, multi-element doping is employed to modify physical properties of Mo2C. The thermodynamic stability, mechanical and electronic properties of doped L′3- and ζ-(Mo, M)2C (M = V, Cr, Fe and W) were examined by first-principles calculations and experiments. The results show that W stabilizes both L′3- and ζ-(Mo, M)2C, whereas Cr and Fe destabilize them. Among the four dopants, W greatly enhances the elastic anisotropy and Cr increases the ductility while retaining high hardness. Compared with L′3-(Mo, M)2C, ζ-(Mo, M)2C exhibits higher thermodynamic stability and stronger elastic anisotropy. Multi-element doping alters the relative fractions of covalent, ionic and metallic bonds in the compounds, accounting for the correlation between various dopants and physical properties. Further experiments confirm the structural and compositional dependence of thermal stability, which agrees well with the theoretical predictions. [ABSTRACT FROM AUTHOR]

Incoloy825 alloy is a Ni–Fe–Cr corrosion resistant alloy, which is widely used in oil and gas pipelines, heat exchangers, chemical plants and boiler pipelines. In the application environment of high-temperature and high-pressure sulfur-containing and chlorine containing steam pipelines, the pipeline surface is easy to deposit salt film, resulting in high-temperature hot corrosion, which requires the alloy to have excellent sulfur and chlorine corrosion resistance. The hot corrosion behavior of Incoloy825 alloy was investigated by using Na2SO4 + NaCl molten salt at 950 °C. The results show that the hot corrosion products mainly include (Cr,Fe)2O3, NiO, NiCr2O4, FeCr2O4, NiFe2O4 and (Fe,Ni)S2. Sulfur and chlorine elements in molten salt act as catalysts in vulcanization and chlorination reactions. The hot corrosion path penetrates along the random high-angle grain boundary, and the special grain boundary can block the penetration path of hot corrosion. Chromium mainly precipitates along the random high-angle grain boundary. The special grain boundary can inhibit the precipitation of chromium, so as to reduce the combination probability of sulfur and chlorine with chromium. Σ11 boundary, Σ5 boundary and incoherent Σ3 twin boundary do not block the corrosion path, and coherent Σ3 twin boundary plays a key role in blocking the corrosion path. For alloys with low stacking fault energy, taking advantage of the characteristics that their microstructure contains a large number of annealing twins, increasing the proportion of special grain boundaries is an effective means to improve their hot corrosion resistance. [ABSTRACT FROM AUTHOR]

The influence of iterative thermo-mechanical processing on the special grain boundary fraction and degree of sensitization (DOS) was investigated in austenitic steel TP321. The results show that the special grain boundary fraction fluctuates with the number of iterative processing, which is related to the fluctuation of residual strain stored energy. The special grain boundary fraction is closely related to grain size. From the fitting relationship of special grain boundary fraction with grain size, the special grain boundary fraction decreases slowly with the increase in grain size. The large grain size is not conducive to the increase in special grain boundary fraction. The decrease in special grain boundary fraction with the number of iterative processing is related to the degree of recrystallization and abnormal grain growth. The abnormal growth of grain is attributed to the uneven distribution of strain and grain size. The special grain boundary fraction and grain size both have a strong influence on the DOS, and the samples with higher special grain boundary fraction and smaller grain size have lower DOS value. [ABSTRACT FROM AUTHOR]

The hot deformation behavior of Incoloy825 alloy is investigated by isothermal compression test at the temperature range of 950–1150 °C with strain rates of 0.001–10 s−1. The dynamic recrystallization (DRX) mechanism in the transition zone is subdivided and clearly revealed. The results show that the instability domain is mainly located in the low temperature with high strain rate region (950–1050 °C, 1–10 s−1). The feasible domain can be divided into two parts: the first is located in the temperature range of 1000–1150 °C with a strain rate of 0.001 s−1, the second is located in strain rate of 0.01–1 s−1 at 1150 °C, and the optimum processing parameters is the temperature of 1150 °C with a strain rate of 1 s−1. A transition zone of the DRX mechanism appears during hot deformation, in which three DRX mechanisms occur, discontinuous dynamic recrystallization (DDRX) mechanism, continuous dynamic recrystallization mechanism (CDRX), and twin dynamic recrystallization (TDRX) mechanism. The formation of a DRX grain is the comprehensive effect of multiple mechanisms, which can promote and transform each other. Each mechanism contributes a different role. [ABSTRACT FROM AUTHOR]

Tuning eutectoid carbides through tailoring austenitization parameters offers an alternative pathway for the ductility enhancement of high-speed steels. This study probes the origin of austenitization-dependent eutectoid reaction and revisits the role of primary carbides in the annealing process. The increase of austenitization temperature and/or duration produces more rod-shaped eutectoid carbides while suppressing their granular counterparts, corresponding to a transition of eutectoid transformation from a divorced mode to a lamellar manner. Primary carbides can serve as chemical conditioners of the austenite as well as nucleation substrates of eutectoid carbides. Austenitization conditions determine the dissolution of primary carbides and concurrently affect the two essential factors of eutectoid reaction modes, i.e., the inter-particle spacing and the undercooling, thereby causing the austenitization dependence of eutectoid carbides in high-speed steels. [ABSTRACT FROM AUTHOR]

High‐speed steel usually uses very slow cooling rates for complete annealing to guarantee good ductility at the expense of sacrificing the production efficiency. Herein, the aim is to probe the full potential of elevating cooling rates in simultaneous enhancement of ductility and efficiency. The results show that higher cooling rates produce finer ferrite grains and larger‐fractioned eutectoid carbides, which evolve from granular M6C to rod‐shaped M23C6. The precipitate evolution originates from a transition of eutectoid transformation mechanism from a divorced mode to a lamellar manner due to upgraded undercoolings. Granular carbides display an excellent strain‐hardening ability compared with the rod‐shaped counterparts. Highly dispersive granular precipitates and fine ferrite grains formed under appropriate cooling conditions may contribute to larger work hardening rates and higher ductility. For the investigated M42 steel, cooling rates in the range of 2–5 °C min−1 can enhance the annealing efficiency while promising good ductility. [ABSTRACT FROM AUTHOR]

The local electrochemical measurement and corrosion mechanism of the multi-layer aluminium sheets (AA4343/AA3xxx/AA4343 and AA7072/AA3xxx/AA4343) after brazing are determined by a combination of mechanical polishing and work function calculation based on density functional theory. The work functions of (100), (110), and (111) crystal planes of Si and α-Al(Fe,Mn)Si particles which are common in multi-layer aluminium sheets are calculated. The results demonstrate Si and α-Al(Fe,Mn)Si particles are considered to be cathodic with respect to the Al matrix, thus promoting cathodic reactions. The potential differences between the Si and α-Al(Fe,Mn)Si particles with respect to the Al matrix may lead to localized dissolution of the Al matrix. Besides, the microstructure and composition of multi-layered aluminium sheets are studied by optical microscopy, scanning electron microscopy, and glow discharge optical emission spectrometry and compared with those obtained for conventional multi-layer aluminium sheets (AA4343/AA3xxx/AA4343). The presence of the AA7072 layer reduces the outward diffusion of copper. In addition, the AA7072 sacrificial layer exhibits a large potential difference against the core material for a long time, thereby improving corrosion protection. [ABSTRACT FROM AUTHOR]

Limited cold formability has been a longstanding challenge for processing high-speed steels. In this study, we probe the relationship between the ductility and eutectoid carbides. The results show that larger-fractioned eutectoid carbides are formed and gradually evolve from granular M6C to rod-shaped M23C6 with increasing annealing duration. Rod-shaped M23C6 carbides have limited shear resistance and are readily sheared by dislocations at large strains, which deteriorates work-hardening abilities and the ductility. Highly dispersive M6C eutectoid carbides are desirable for the ductility enhancement of annealed high-speed steels. [ABSTRACT FROM AUTHOR]

One key challenge for tailoring microstructure of high-speed steel (HSS) is to obtain small-sized carbides with a homogeneous distribution. Although diffusion annealing followed by post-deformation offers an opportunity to refine carbides, it requires high annealing temperatures and large strains. In this study, we employ a pre-compression step and examine its role in following carbide decomposition and spheroidization. The result shows that pre-deformation may introduce dislocations into M2C carbides and hence triggers a significant enhancement of M2C decomposition into M6C and MC. Structural defects in M2C act as potential nucleation sites of new precipitates and activate an alternative decomposition pathway via nucleation along dislocations or intersections of stacking faults and dislocations. The decomposition enhancement effect exhibits a dependence on strain and temperature, which may originate from a temperature-mediated balance between dislocation densities and element diffusion rates in M2C carbides. Compared with post-deformation, pre-deformation facilitates carbide spheroidization and enables smaller carbides as well as higher tempered hardness. This may offer an alternate pathway for carbide refinement in HSS. [ABSTRACT FROM AUTHOR]

Deformation-induced carbide transformation (DICT) is first observed in M2 high-speed steel (HSS) wherein deformation enables a lower transformation temperature and enhanced transformation rate of M2C carbide into M6C and MC. Dislocations introduced into M2C by pre-deformation act as nucleation sites of new precipitates and enable an evolution of the transformation pathway from a common interface-nucleation mode to a dislocation-nucleation manner. DICT offers an opportunity to circumvent the carbide-related trade-off between the wear resistance and toughness of M2 HSS. [ABSTRACT FROM AUTHOR]

The aim of this study is to evaluate the effect of secondary carbide precipitation on strain hardening and tensile ductility of high-speed steels and to develop a novel pathway for ductility enhancement by engineering the annealing microstructure. The results demonstrate a strong correlation between secondary precipitation and austenitization. With decreasing austenitization temperature, secondary carbides exhibit a transformation from a rod-shaped Cr-rich M23C6 type to a granular Mo-rich M6C one, corresponding to a transition of the underlying eutectoid decomposition mechanism from a cooperative growth mode to a divorced eutectoid manner. Highly dispersed rod-shaped M23C6 precipitates contribute to an enhanced tensile strength but lead to a degraded work hardening rate in the late deformation stage and, therefore, a lower total elongation. In contrast, granular M6C precipitates exhibit an excellent capacity of accumulating dislocations and enhancing the work hardening rate especially at a high strain, which enables a significant increase of ductility. It is suggested that granular M6C precipitates embedded in fine ferritic grains with lean dislocations is a desirable annealing microstructure to produce a more ductile high-speed steel. [ABSTRACT FROM AUTHOR]

The effects of Si on the aging behavior in severe cold-drawn pearlitic steel were investigated. Under the same conditions, partitioning ratio, κ , increased from 1.14 to 1.92 in 0.22 pct-Si steel as pearlitic transformation temperature rose from 793 K to 873 K (520 °C to 600 °C); however, κ of 0.95 pct-Si steel only increased from 1.87 to 1.94. Cementite lamellae spheroidization in high Si content steel wire was remarkably delayed during aging at 698 K (425 °C) by the combined effect of diffusion retardation of Si and C atom and cementite phase stabilization by Mn alloying. In addition, age-hardening duration is prolonged, thereby extending age-hardening duration of the as-drawn 0.95 pct-Si-873 K (600 °C) steel wire by approximately 10 times that of 0.22 pct-Si-793 K (520 °C) steel wire. [ABSTRACT FROM AUTHOR]

The site preference of Mn in alloyed cementite is investigated by using X-ray diffraction (XRD) and first-principles calculations. First-principles calculations of the cohesive energies and formation energies of Mn-alloyed cementite indicate that Mn atoms prefer to substitute the 8d sites of cementite. The calculation results of density of states (DOS) and charge-density difference show that the more stable outermost electron orbital of Mn(3d54s2) may be the main reason for the site preference of Mn at 8d sites in Mn-alloyed cementite. In addition, the experimental XRD spectrum of 25 at.% Mn-alloyed cementite, which is prepared by using mechanical alloying combined with spark plasma sintering, highly coincides with the calculated XRD spectrum of Mn 8d-substituted cementite, verifying that Mn atoms prefer to substitute the 8d sites of alloyed cementite. [ABSTRACT FROM AUTHOR]

The effects of Si and Mn interactions on the spheroidization and coarsening behavior of cementite during annealing in severe cold-drawn pearlitic steel were studied. Spheroidization of cementite particles annealed at 873 K (600 °C) in heavily cold-drawn pearlitic steel was obviously retarded by higher Si contents. The coarsening kinetics of cementite particles in heavily cold-drawn and annealed pearlite obeyed a power-law relationship. The values of ECD increase steadily according to a temporal dependence of n (0.21 ± 0.02) for S87B and n (0.23 ± 0.06) for S87BM. The n values lie between 0.20 and 0.25, indicating that cementite particles coarsening occurs through a combination of dislocation diffusion ( n = 0.20) controlled and grain boundary diffusion ( n = 0.25) controlled coarsening mechanisms. Higher Si content in S87BM led to more pronounced partitioning of Mn than that in S87B, resulting in more significant suppression of alloying elements and lower coarsening rate constant K on the cementite particles coarsening behavior. [ABSTRACT FROM AUTHOR]

In the current study, the effect of Sc addition on the interaction of concurrent precipitation and recrystallization in commercial AA3003 aluminum alloy was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. In case of AA3003 alloy, which was cold rolled to a true strain of 2.20 and heated at a heating rate of 150 K/s, the onset of precipitation and ending of recrystallization are signified by the critical temperature, T ~740 K (467 °C). There is a change in the shape of the recrystallized grains from pancake-like to equiaxed shape, as the annealing temperature increases greater than T. In case of AA3003 alloy microalloyed with 0.4 wt pct of Sc, the high no. density precipitation of coherent AlSc precipitates always occurs before recrystallization because of the small nucleation barrier and high rate of decomposition. This leads to extremely coarse pancake-like recrystallization grains with high fraction of low-angle grain boundaries in the entire annealing temperature range, even at a high brazing temperature of 883 K (610 °C). [ABSTRACT FROM AUTHOR]

The effect of microstructure on clad/core interactions during the brazing of 4343/3005/4343 multi-layer aluminum brazing sheet was investigated employing differential scanning calorimetry (DSC) and electron back-scattering diffraction (EBSD). The thickness of the melted clad layer gradually decreased during the brazing operation. It could be completely removed isothermally as a result of diffusional solidification at the brazing temperature. During the brazing cycle, the rate of loss of the melt in the brazing sheet, with small equiaxed grains' core layer, was higher than that with the core layer consisting of elongated large grains. The difference in microstructure affected the amount of liquid formed during brazing. [ABSTRACT FROM AUTHOR]

An equitable partition of a graph G is a partition of the vertex set of G such that the sizes of any two parts differ by at most one. The strong equitable vertexk-arboricity of G, denoted by v a k ≡ (G) , is the smallest integer t such that G can be equitably partitioned into t ′ induced forests for every t ′ ≥ t , where the maximum degree of each induced forest is at most k. In this paper, we provide a general upper bound for v a 2 ≡ (K n , n) . Exact values are obtained in some special cases. [ABSTRACT FROM AUTHOR]

The pitting corrosion resistance of S32750 super duplex stainless steel, annealing treated at temperatures of 950–1200 °C for 20–60 min, was investigated using potentiodynamic polarization tests. The results show that the volume fractions of ferrite in the S32750 duplex stainless steel increased from 48.9% to 68.4% as annealing temperatures increased from 950 to 1200 °C. The pitting potential of the sample increased first and then decreased from an annealing temperature of 950 to 1050 °C, and the highest pitting potential was observed after annealing at 1050 °C for 35 min. The pitting corrosion resistance of S32750 stainless steel is due to the combination of pitting resistance equivalent number (PREN) value, phase fraction and grain boundary area fraction, and the imbalance of corrosion potential. [ABSTRACT FROM AUTHOR]

Atom-probe tomography (APT) and first-principles calculations are employed to investigate the partitioning of Mo in the γ(f.c.c.)-and γ′(L12)-phases in a model Ni-6.5Al-9.9Mo at. % superalloy. Mo is experimentally observed to partition preferentially to the γ(f.c.c.)-matrix, which is consistent with the smaller value of the γ(f.c.c.)-matrix substitutional formation-energy, with a driving force of 0.707 eV for partitioning as determined by first-principles calculations. APT measurements of the γ′(L12)-precipitate-phase composition and Al-, Mo-centered partial radial distribution functions indicate that Mo occupies the Al sublattice sites of the Ni3Al(L12) phase. The preferential site-substitution of Mo at Al sublattice sites is confirmed by first-principles calculations. [ABSTRACT FROM AUTHOR]