Effect of Effective Grain Size and Grain Boundary of Large Misorientation on Upper Shelf Energy in Pipeline Steels

X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very important for exploring even higher grade pipeline steels. We established the relationship between toughness and crystallographic parameters of high grade pipeline steels by studying the crystallographic parameters of X65, X70, and X80 using EBSD and analyzing Charpy CVN of X65, X70 and X80. The results show that the effective grain size, the frequency distribution of grain boundary misorientation and the ratio of high angle grain boundary to small angle grain boundary are important parameters. The finer the effective grain size, and the higher the frequency distribution of grain boundaries （〉 50~）, the more excellent toughness of high grade pipeline steels will be.

Strength criterion for crystalline rocks considering grain size effect and tensile-compressive strength ratio

The macroscopic mechanical properties of rocks are significantly influenced by their microstructure.As a material bonded by mineral grains,the grain morphology of crystalline rock is the primary factor influencing the strength.However,most strength criteria neglect the strength variations caused by different grain characteristics in rocks.Furthermore,the traditional linear criteria tend to overestimate tensile strength and exhibit apex singularity.To address these shortcomings,a piecewise strength criterion that considers the grain size effect has been proposed.A part of an ellipse was employed to construct the envelope of the tensive-shear region on the meridian plane,to accurately reproduce the low tensile-compressive strength ratio.Based on the analysis of experimental data,both linear and exponential modification functions that account for grain size effects were integrated into the proposed criterion.The corresponding finite element algorithm has been implemented.The accuracy and applicability of the proposed criterion were validated by comparing with the experimental data.

Effect of Cooling Rate on Microstructure and Effective Grain Size for a Ni–Cr–Mo–B High-Strength Steel

The effect of cooling rate on microstructure and effective grain size(EGS)of a Ni-Cr-Mo-B high-strength steel has been studied by dilatometer,field emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM)and electron backscattered diffraction(EBSD).The results show that the microstructure of the Ni-Cr-Mo-B steel is dependent on cooling rate in the following sequence:lath martensite(LM),mixed LM and lath bainite(LB),mixed LB and granular bainite(GB)and GB.The critical cooling rates for appearance of LB and GB are about 10℃/s and 0.5℃/s,respectively.The LM(>10℃/s)consists of few blocky regions with a width of several micros.Compared with the lath regions,the blocky regions in LM form at higher actual transformation temperatures during cooling.The blocky region area percentage in LM keeps almost constant about 8%at different cooling rates(>10℃/s)due to similar martensite transformation starting temperature(M_(s)).The LB percentage in mixed LM/LB increases gradually with decreasing cooling rate(10-0.5℃/s).The EBSD results show that different microstructures have different EGS.The mixed LM/LB exhibits the smallest EGS due to the separation of the prior austenite grains by the pre-formed LB and the refinement of the LM.Meanwhile,the mixed LM/LB at different cooling rates(10-0.5℃/s)exhibits almost the same EGS because the LB and LM in the mixed LM/LB have a similar high-angle grain boundary density and similar EGS.Because the blocky regions contain few high-angle grain boundaries and have similar area percentages in the LM,the LM at different cooling rates(>10℃/s)exhibits almost the same EGS.The ferrite in GB exhibits as a whole with few high-angle grain boundaries;thus,the mixed LB/GB exhibits the largest EGS.

Grain size effect on cyclic deformation behavior and springback prediction of Ni-based superalloy foil

In order to clarify the influence of grain size on cyclic deformation response of superalloy sheets and springback behavior,cyclic loading-unloading and shearing tests were performed on the superalloy foils with 0.2 mm in thickness and diverse grain sizes.The results show that,the decline ratio of elastic modulus is weakened with increasing grain size,and the Bauschinger effect becomes evident with decreasing grain size.Meanwhile,U-bending test results determine that the springback is diminished with increasing grain size.The Chaboche,Anisotropic Nonlinear Kinematic(ANK)and Yoshida-Uemori(Y-U)models were utilized to fit the shear stress-strain curves of specimens.It is found that Y-U model is sufficient of predicting the springback.However,the prediction accuracy is degraded with increasing grain size.

Microstructure evolution and mechanical properties of brazing joint for ultra-thin-walled Inconel 718 considering grain size effect and brazing temperature

The systematic investigation of the mechanical properties and microstructure evolution process of ultra-thin-walled Inconel 718 capillary brazing joints is of great significance because of the exceptionally high demands on its application.To achieve this objective,this study investigates the impact of three distinct brazing temperatures and five typical grain sizes on the brazed joints’mechanical properties and microstructure evolution process.Microstructural evolution analysis was conducted based on Electron Back Scatter Diffraction(EBSD),Scanning Electron Microscopy(SEM),X-Ray Diffraction(XRD),High-Resolution Transmission Electron Microscopy(HRTEM),and Focused Ion Beam(FIB).Besides,the mechanical properties and fracture behavior were studied based on the uniaxial tension tests and in-situ tension tests.The findings reveal that the brazing joint’s strength is higher for the fine-grain capillary than the coarse-grain one,primarily due to the formation of a dense branch structure composed of G-phase in the brazing seam.The effects of grain size,such as pinning and splitting,are amplified at higher brazing temperatures.Additionally,micro-cracks initiate around brittle intermetallic compounds and propagate through the eutectic zone,leading to a cleavage fracture mode.The fracture stress of fine-grain specimens is higher than that of coarse-grain due to the complex micro-crack path.Therefore,this study contributes significantly to the literature by highlighting the crucial impact of grain size on the brazing properties of ultra-thin-walled Inconel 718 structures.

Strengthening-softening transition and maximum strength in Schwarz nanocrystals

Recently,a Schwarz crystal structure with curved grain boundaries(GBs)constrained by twin-boundary(TB)networks was discovered in nanocrystalline Cu through experiments and atomistic simulations.Nanocrystalline Cu with nanosized Schwarz crystals exhibited high strength and excellent thermal stability.However,the grainsize effect and associated deformation mechanisms of Schwarz nanocrystals remain unknown.Here,we performed large-scale atomistic simulations to investigate the deformation behaviors and grain-size effect of nanocrystalline Cu with Schwarz crystals.Our simulations showed that similar to regular nanocrystals,Schwarz nanocrystals exhibit a strengthening-softening transition with decreasing grain size.The critical grain size in Schwarz nanocrystals is smaller than that in regular nanocrystals,leading to a maximum strength higher than that of regular nanocrystals.Our simulations revealed that the softening in Schwarz nanocrystals mainly originates from TB migration(or detwinning)and annihilation of GBs,rather than GB-mediated processes(including GB migration,sliding and diffusion)dominating the softening in regular nanocrystals.Quantitative analyses of simulation data further showed that compared with those in regular nanocrystals,the GB-mediated processes in Schwarz nanocrystals are suppressed,which is related to the low volume fraction of amorphous-like GBs and constraints of TB networks.The smaller critical grain size arises from the suppression of GB-mediated processes.

Microscopic origin and relevant grain size effect of discontinuous grain growth in BaTiO_(3)-based ferroelectric ceramics

Barium titanate[BaTiO_(3)(BT)]-based ceramics are typical ferroelectric materials.Here,the discontinuous grain growth(DGG)and relevant grain size effect are deeply studied.An obvious DGG phenomenon is observed in a paradigmatic Zr^(4+)-doped BT-based ceramic,with grains growing from∼2.2–6.6 to∼121.8–198.4μm discontinuously near 1320℃.It is found that fine grains can get together and grow into large ones with liquid phase surrounding them above eutectic temperature.Then the grain boundary density(D g)is quantitatively studied and shows a first-order reciprocal relationship with grain size,and the grain size effect is dependent on D g.Fine grains lead to high D g,and then cause fine domains and pseudocubic-like phase structure because of the interrupted long-range ferroelectric orders by grain boundary.High D g also causes the diffusion phase transition and low Curie dielectric peak due to the distribution of phase transition temperature induced by internal stress.Local domain switching experiments reveal that the polarization orientation is more difficult near the grain boundary,implying that the grain boundary inhibition dominates the process of polarization orientation in fine-grain ceramics,which leads to low polarization but a high coercive field.However,large-grain ceramics exhibit easy domain switching and high&similar ferroelectricity.This work reveals that the grain boundary effect dominates the grain size effect in fine-grain ceramics,and expands current knowledge on DGG and grain size effect in polycrystalline materials.

Microstructure Characteristics and Mechanical Properties of X80 Pipeline Steels

The relation between microstructure characteristics and mechanical properties of X80 pipeline steels was investigated using optical microscopy, scanning electron microscopy, etc. It is shown that the structure consists of polygonal ferrite （PF）, quasi-polygonal ferrite （QPF）, acicular ferrite （AF）, and granular bainitic ferrite （GF）. With increasing volume fraction of M-A islands （below 3%）, the yield strength increases. With increasing content of higher angle grain boundaries（HAGBs）, the yield strength, elongation, and DWTT properties at -15 ℃ increase, and the volume fraction of M-A islands reaches its highest point in the steel containing the most volume fraction of GF.

Effects of grain size on phase transition behavior of nanocrystalline shape memory alloys

We report recent advances in the experimental and theoretical study of grain size(GS)effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy(SMA).It is shown that when GS<60 nm,the superelastic stress-strain hysteresis loop area(H)of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers.At the same time,the temperature dependence of the transition stress also decreases with GS and eventually approaches zero,leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship.Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization.It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces(grain boundary and Austenite-Martensite(A-M)interface)in the polycrystal.In particular,with GS reduction,interfacial energy terms will gradually become dominant over the bulk energy of the crystallite,eventually bring fundamental changes in the phase transition responses of the material.Modelling strategy leading to the establishment of quantitative relationships among GS,grain boundary,A-M interfaces and the macroscopic responses of the material are outlined.

Effects of microrolling parameters on the microstructure and deformation behavior of pure copper

Microrolling experiments and uniaxial tensile tests of pure copper under different annealing conditions were carried out in this paper. The effects of grain size and reduction on non-uniform deformation, edge cracking, and microstructure were studied. The experimen- tal results showed that the side deformation became more non-uniform, resulting in substantial edge bulge, and the uneven spread increased with increasing grain size and reduction level. When the reduction level reached 80% and the grain size was 65 μm, slight edge cracks occurred. When the grain size was 200 μm, the edge cracks became wider and deeper. No edge cracks occurred when the grain size was 200 μm and the reduction level was less than 60%; edge cracks occurred when the reduction level was increased to 80%. As the reduction level increased, the grains were gradually elongated and appeared as a sheet-like structure along the rolling direction; a fine lamellar structure was obtained when the grain size was 20 lam and the reduction level was less than 60%.

Interfacial Characteristics of Nanocrystalline FeMoSiB Alloys

By using X-ray diffraction (XRD), transmission electronic microscopy (TEM) and transmission Mssbauer spectroseopy (TMES), the formation, structure and properties including microhardness and electrical resistivity of nanocrystalline FeMoSiB alloys have been investigated. By annealing the as-quenched FeMoSiB sample at 833-1023K for 1 h, nanocrystalline materials with grain sizes of 15 to 200 nm were obtained. Mssbauer spectroscopy results reveal a quasi-continuous distribution feature of P(H)-H curves for 15 nm-and 20 nm-grained samples. Also, it was found that resistivity and microhardness of nanocrystalline Fe-Mo-Si-B alloys exhibit strong grain size effect.

Effect of grain size on the mechanical properties of Mg foams

The grain size of Mg foams was innovatively refined without alteration of pore structure and relative density by subjecting multi-axial forging(MAF)process to Ti-Mg composite,an intermediary product of the fabrication process of Mg foams where the spherical Ti particles were utilized as the replication material.The feasibility of the MAF process and the grain size effect on the mechanical properties of Mg foams were discussed.The results showed that,with the appropriate strain of 0.24 applied in the MAF process,Ti-Mg composites returned to original physical appearance without generating microcracks.And complete recrystallization was achieved after heat treatment,with the grain size of the MAFprocessed Mg foams two to three orders of magnitude smaller than that of as-cast foam.The mechanical properties of Mg foams were enhanced extensively after grain refinement with the yield strength and the plastic collapse strength increased by 147%and 50.7%,respectively.A revised model integrated by the Hall-Petch law and Gibson-Ashby model was proposed,which gave a good estimation of the yield strength and the plastic collapse strength of Mg foams from the compressive behavior of the corresponding parent material,though a knockdown factor of 0.45 was introduced for the yield strength.

Mechanical behaviors of polycrystalline NiTi SMAs of various grain sizes under impact loading

This work presents mechanical properties of the NiTi polycrystalline superelastic shape memory alloys(SMA) of 5 different grain sizes under high-speed impacts. The amorphous, nanocrystalline(40, 80, 120 nm) and coarse grain(20 μm) sheets are manufactured with cold rolling and suitable heat treatments. A Hopkinson tensile bar is used to perform tests up to 45 m/s. Highspeed camera system and digital image correlation method are used to get the strain field and particle velocity field at a sampling frequency of 2×10~6 frames/s with a resolution of 924×768 pixels. Nominal stress-strain curves are obtained for all the sheets with a strain rate of about 1000 s~(-1) and they have a similar evolution to the quasi-static case but with much higher stress levels. The rate sensitivity is increased with the grain size and the stress level can reach up to a 70% growth for a coarse grain sheet but be totally insensitive for the amorphous sheet in the strain rate from 10~(-4) to 10~3 s~(-1). A single transformation front can be found under high-speed impact(45 m/s) at the early loading stage. The speed of the transformation front is calculated from strain time histories and the highest front speed of 811 m/s is observed which is never observed before. It also reveals that the front speed depends also on the grain size. With the same loading speed, the bigger the grain size is, the slower the transformation front speed is.

Triaxial tension-induced damage behavior of nanocrystalline NiTi alloy and its dependence on grain size

This study focused on the effect of grain size(GS)on dynamic damage performance of nano-crystalline nickel titanium(NC NiTi)alloy.Molecular dynamics simulations were conducted to triaxially expand it at a high strain rate(4×10~9 s^(-1)),while the temperature and initial pressure remained 300 K and 0 bar,respectively.It was discovered that the superelastic NiTi alloy exhibited the similar damage response as ductile metallic materials,which was vividly characterized by void nucleation,growth,and coalescence.The stress-strain curves demonstrated that the void nucleations always occurred near the start of the strain softening region at various grain sizes.Interestingly,it was discovered that the void evolution was characteristic of an almost double-linear behavior,and the piecewise linearity became more prominent for the void volume fraction increase at larger grain size.More importantly,the fracture behavior was found to be strongly dependent upon the grain size in the NC NiTi alloy.For small grain size,the existing voids propagated along the grain boundaries and in the grains,leading to intergranular and transgranular fracture.Contrarily,the intergranular-dominated fracture was responsible for the void propagation in the large grain.In addition,the starting time,ending time,and threshold of void nucleation were found to be weak sensitivity to GS,and a reverse effect was appropriate to the void growth.The results highlighted that as the GS increased,more complete stress relaxation and shorter duration time were produced,leading to larger void volume fraction and faster growth rate.

Microstructural Evolution and Toughness in the HAZ of Submerged Arc Welded Low Welding Crack Susceptibility Steel

Microstructural characteristics of different sub-regions of heat affected zone （HAZ） of low welding crack susceptibility steel weldment were investigated by using optical microscopy and scanning electron microscopy equipped with electron backscattered diffraction system. And the focus was put on the correlation between microstructural characteristics and HAZ toughness of the weldment. The results reveal that the toughness of fusion line zone （FLZ） specimens is much lower than that of fine grained HAZ （FGHAZ） specimens. The coarse inclusions in the weld metal and the large martensite-austenite constituents in the coarse grained HAZ （CGHAZ） have an obvious negative effect on the crack initiation energy of FLZ. Meanwhile, the coarse granular bainite with large effective grain decreases the crack propagation energy seriously. By contrast, fine crystallographic grains in the FGHAZ play a key role in increasing toughness, especially in improving crack propagation energy.

Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas

Biogenic elements and six phosphorus （P） fractions in surface sediments from the Changjiang Estuary and adjacent waters were determined to investigate the governing factors of these elements, and further to discuss their potential uses as paleo-environment proxies and risks of P release from sediment. Total organic carbon （TOC） and leachable organic P （Lea-OP） showed high concentrations in the estuary, Zhejiang coast and offshore upwelling area. They came from both the Changjiang River and marine biological input. Biogenic silicon （BSi） exhibited a high concentration band between 123 and 124°E. BSi mainly came from diatom production and its concentration in the inshore area was diluted by river sediment. Total nitrogen （TN） was primarily of marine biogenic origin. Seaward decreasing trends of Fe-bound P and Al-bound P revealed their terrestrial origins. Influenced by old Huanghe sediment delivered by the Jiangsu coastal current, the maximum concentration of detrital P （Det-P） was observed in the area north of the estuary. Similar high concentrations of carbonate fluorapatite （CFA-P） and CaCO3in the southern study area suggested marine calcium-organism sources of CFA-P. TOC, TN and non-apatite P were enriched in fine sediment, and Det-P partially exhibited coarse-grain enrichment, but BSi had no correlation with sediment grain size. Different sources and governing factors made biogenic elements and P species have distinct potential uses in indicating environmental conditions. Transferable P accounted for 14%-46% of total P. In an aerobic environment, there was low risk of P release from sediment, attributed to excess Fe oxides in sediments.

Unravelling the competitive effect of microstructural features on the fracture toughness and tensile properties of near beta titanium alloys

The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.

Study of high pressure structural stability of CeO_2 nanoparticles

In situ high pressure XRD diffraction and Raman spectroscopy have been performed on 12 nm CeO2 nanoparticles. Surprisingly, under quasihydrostatic conditions, 12 nm CeO2 nanoparticles maintain the fluorite- type structure in the whole pressure range （0-51 GPa） during the experiments, much more stable than the bulk counterpart （PT-31 GPa）. In contrast, they experienced phase transition at pressure as low as 26 GPa under non- hydrostatic conditions （adopting CsC1 as pressure medium）. Additionally, 32-36 nm CeO2 nanoparticles exhibit an onset pressure of phase transition at 35 GPa under quasihydrostatic conditions, and this onset pressure is much lower than our result. Further analysis shows both the experimental condition （i.e., quasihydrostatic or non-hydrostatic） and grain size effect have a significant impact on the high pressure behaviors of CeO2 nanomaterials.

High-temperature deformation behaviour of duplex stainless steel with hard boronised layer

In order to understand the high-temperature deformation behaviour of alloy having hard surface layer,thermo-mechanically treated duplex stainless steel(DSS)is boronised for 0.75-6 h at 1223 K and subsequently deformed under compression mode at the same temperature under strain rate condition of 1×10^(-3),2×10^(-4) and 6×10^(-5) s^(-1) until strain of 0.4.The substrate microstructure is almost isotropic with grain size after boronising with layer thickness between 1.61 and 2.74μm.X-ray diffraction results confirm the formation of boride on DSS surface.The surface hardness of DSS increases from 387 to 1000-2400 HV after boronising.Uniform boronised layer with thickness of 20-40μm is formed at DSS surface.Compression results show that the flow stress of the deformation increases with the strain rate and boronising time.For the boronised samples,the flow stress range is between 5 and 89 MPa.To determine the actual effect of the boronised layer on the flow stress,the results are also compared with those from un-boronised samples having similar microstructure.The results suggest that at a constant grain size,even with the hardest layer,the effect of hard surface layer on the flow stress almost could be negligible when the deformation rate is slow,but at faster deformation rate,even in the layer with the least hardness,the flow stress shows a significant increase.It is also observed that the hard boride surface disintegration could be avoided at a sufficiently low deformation flow stress that could be attributed to superplasticity.