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.

Dependence of Effective Anisotropy on Grain Size in Nanocrystalline Nd_2Fe_(14)B Hard Magnetic Material

Taking nanocrystalline Nd_2Fe_(14)B as a typical sample, based on Herzer′s random anisotropy theory and the cubic grain model, the partial exchange-coupling interaction model was established and the dependence of effective anisotropy constant K_(eff) on grain size was investigated. Calculation results reveal that the exchange-coupling interaction enhances and the effective anisotropy of material K_(eff) decreases with the reduction of grain size. The variation of K_(eff) is basically the same as that of coercivity. The decrease of effective anisotropy is the main reason of the reduction of coercivity for nanocrystalline Nd_2Fe_(14)B permanent magnetic material.

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.

Effect of indentation size and grain/sub-grain size on microhardness of high purity tungsten

Hardness of materials depends significantly on the indentation size and grain/sub-grain size via microindentation and nanoindentation tests of high-purity tungsten with different structures.The grain boundary effect and indentation size effect were explored.The indentation hardness was fitted using the Nix-Gao model by considering the scaling factor.The results show that the scaling factor is barely correlated with the grain/sub-grain size.The interaction between the plastically deformed zone（PDZ） boundary and the grain/sub-grain boundary is believed to be the reason that leads to an increase of the measured hardness at the specific depths.Results also indicate that the area of the PDZ is barely correlated with the grain/sub-grain size,and the indentation hardness starts to stabilize once the PDZ expands to the dimension of an individual grain/sub-grain.

Natural and human-induced effects on grain size of surface sediments along the Lianyungang muddy coast,China

To examine spatial variations of grain-size characteristics caused by both natural and human-induced processes along the Lianyungang muddy coast, China, 129 samples were collected and measured using standard sieving and sedimentation techniques. Results show that sediment diameter tends to increase with increasing water depth from nearshore to offshore. Size-frequency distributions indicate a gradual mixing process of coarse and fine diameter material. Grain size trend analysis indicates that a man-made structure, the West has resulted in severe siltation in Haizhou Bay and Breakwater, along with Liandao Island itself, the Lianyungang port area, where sediment quality is also poor. Results demonstrate that grain size can be used as a natural tracer to infer how sediments respond to the effects caused by both natural and human-induced processes.

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.

INFLUENCE OF GRAIN SIZE AND ORDERING DEGREE OF PARENT PHASE ON SME IN A CuZnAl ALLOY CONTAINING BORON

The influence of grain size and ordering degree of the parent phase on the shape memory re- covery in a Cu-25.62Zn-3.97Al-0.0018B(wt-%)memory alloy is investigated.A mathematical relationship is set up between the recovery ratio and ordering degree,probabili- ty of atoms at their ordered sites,grain size,the thickness of the grain boundary affected re- gions,the stress during deformation,as well as the critical shear stress.Shape memory effect reaches a maximum with varying grain size and increases linearly with increasing ordering parameter,which agrees well with experimental results.

淬火温度对气瓶用Fe-Cr-Ni-Mo钢组织和力学性能的影响

为了优化一种气瓶用Fe-Cr-Ni-Mo钢的淬火工艺,采用SEM,TEM,EBSD和拉伸、冲击等观察和检测手段,研究了淬火温度对一种气瓶用Fe-Cr-Ni-Mo钢微观组织和力学性能的影响。结果表明,不同淬火温度处理后的合金钢,均呈现为板条马氏体组织,其碳化物析出和板条亚结构基本保持不变,而原始奥氏体晶粒则随淬火温度的提高而出现明显的粗化,由800℃的4.3µm长大到930,1200℃的29.6,371.1µm,同时有效晶粒尺寸(EGS)也逐渐增加,800,930,1200℃的EGS分别为0.60,1.20,3.22µm。淬火温度对合金钢的室温抗拉、屈服强度和断后伸长率影响较小,而随着淬火温度的提高,冲击吸收能量则出现了显著的下降,由800℃的119 J,下降到930,1200℃的68,38 J。EGS增大导致冲击断裂时出现解理断裂,是较高淬火温度合金钢冲击吸收能量下降的主要原因。淬火温度为800~860℃时,Fe-Cr-Ni-Mo钢具有良好的强韧性匹配,该研究结果对Fe-Cr-Ni-Mo钢制气瓶的工业化生产具有指导意义。

回火时间对BS960E钢快速加热淬火后组织与性能的影响

采用光学显微镜、扫描电镜、电子背散射衍射、能谱仪和X射线衍射等研究了回火时间对BS960E钢快速加热淬火后组织与性能的影响。结果表明:BS960E钢淬火后组织为板条状马氏体,其原始奥氏体平均晶粒尺寸为5.69μm,位错密度为4.02281×10^(15)cm^(-2)。随着回火时间的延长,马氏体板条结构逐渐分解;回火2 min时仍存在部分板条组织,有效晶粒尺寸(马氏体板条块)为2.47μm,位错密度急剧下降至9.48079×10^(14)cm^(-2);回火15和30 min时马氏体板条开始粗化,小角度晶界密度占比降低,此时有效晶粒尺寸分别为2.57和2.59μm,位错密度分别为7.80957×10^(14)和6.75406×10^(14)cm^(-2);回火60 min时,马氏体板条块合并明显,大角度晶界密度及占比下降,有效晶粒尺寸粗化达到2.99μm,位错密度降低至5.19655×10^(14)cm^(-2)。碳化物的析出位置可分为原始奥氏体晶界析出、马氏体板条块界析出和晶内析出3类。随着回火时间的延长,BS960E钢的抗拉强度和屈服强度逐渐减小,伸长率增加。回火时间达到60 min时由于析出的碳化物粗大导致伸长率降低。综合考虑BS960E钢经过快速加热淬火后,回火时间在2 min时综合力学性能最佳。

晶粒尺寸对竹节晶铜箔微轧制变形行为影响的模拟

高性能铜箔已经广泛应用于新能源、微电子等领域,为降低其制备成本和调控组织性能,对拉拔−压缩−剪切复合成形条件下的竹节晶铜箔轧制微观变形行为进行研究。建立微观尺度的晶体塑性有限元模型,分析竹节晶微轧制变形的晶粒尺寸效应。该微观模型将每个离散单元只是作为晶粒的组成部分,同时在积分点上采用单晶体本构模型建立本构方程。结果表明:微观层次复合轧制竹节晶铜箔的变形特征表现为高度的异向性和局部化;竹节晶铜箔轧制力变化符合类似Hall-Petch关系,即轧制力随晶粒尺寸的减小而增大;随着晶粒尺寸减小,复合轧制竹节晶铜箔的滑移均匀性和晶界滑移协调能力均增强;动态剪切带和变形带随着晶粒尺寸的减小向晶界偏移。为保证轧制竹节晶铜箔变形的稳定性和进一步减薄,晶粒尺寸小于100μm为宜。

Dosage effect genes modulate grain development in synthesized Triticum durum-Haynaldia villosa allohexaploid

Polyploidization in plants often leads to increased cell size and grain size,which may be affected by the increased genome dosage and transcription abundance.The synthesized Triticum durum(AABB)-Hay-naldia villosa(WM)amphiploid(AABBM)has significantly increased grain size,especially grain length,than the tetraploid and diploid parents.To investigate how polyploidization affects grain development at the transcriptional level,we perform transcriptome analysis using the immature seeds of T.durum,H.villosa,and the amphiploid.The dosage effect genes are contributed more by differentially expressed genes from genome V of H.villosa.The dosage effect genes overrepresent grain development-related genes.Inter-estingly,the vernalization gene TaVRN1 is among the positive dosage effect genes in the T.durum-H.villosa and T.turgidum-Ae.tauschii amphiploids.The expression levels of TaVRN1 homologs are positively correlated with the grain size and weight.The TaVRN1-B1 or TaVRN1-D1 mutation shows delayed florescence,decreased cell size,grain size,and grain yield.These data indicate that dosage effect genes could be one of the important explanations for increased grain size by regulating grain development.The identification and functional validation of dosage effect genes may facilitate the finding of valuable genes for improvingwheat yield.

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.

Toward tunable shape memory effect of NiTi alloy by grain size engineering:A phase field study

The inelastic deformations of shape memory alloys(SMAs)always show poor controllability due to the avalanche-like martensite transformation,and the effective control for the deformation of precision de-vices has been not yet mature.In this work,the phase field method was used to investigate the shape memory effects(SMEs)of NiTi SMAs undergoing grain size(GS)engineering,to obtain tunable one-way and stress-assisted two-way SMEs(OWSME and SATWSME).The OWSME and SATWSME of the systems with various gradient-nanograin structures and bimodal grain structure,as well as that with geometric gradients were simulated.The simulated results indicate that due to the GS dependences of martensite transformation and reorientation,the occurrence and expansion of martensite reorientation,martensite transformation and its reverse can be efficaciously controlled via the GS engineering.When combining the GS engineering and geometric gradient design,since the effects of GS and stress gradient can be su-perimposed or competing,and the responses of martensite reorientation,martensite transformation and its reverse to this are different,the OWSME and SATWSME of the geometrically graded systems with various nanograin structures can exhibit different improvements in controllability.In short,the reorienta-tion hardening modulus during OWSME is increased and the transformation temperature window during SATWSME is widened by GS engineering,indicating the improved controllability of SMEs.The optimal GS engineering schemes revealed in this work provide the basic reference and guidance for designing tun-able SMEs and producing NiTi-based driving devices catering to desired functional performance in various engineering fields.

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.

表面活性剂对金属铜微切削过程中微沟槽显微形貌影响机制的研究

本研究对微沟槽横截面微观组织进行了观察,阐明了表面活性剂对纯铜微切削加工表面的机理影响。研究结果表明:添加表面活性剂后,切削力和推力明显降低,并发现具有物理化学效应的微沟槽表面粗糙度为12nm,没有物理化学效应的微沟槽表面粗糙度为17nm。受物理化学效应影响的样品的微沟槽区内的平均晶粒尺寸为67.9μm,而无物理化学效应影响的样品平均晶粒尺寸为48.3μm,此外,还发现无物理化学效应影响微沟槽附近晶粒尺寸大于远离微沟槽表面的晶粒尺寸。通过电子背散射衍射显微镜观察,受物理化学效应影响的横截面表面晶粒呈各向异性,而无物理化学效应影响的横截面表面晶粒取向主要为{101}方向。通过对几何必要位错计算和分析,推断出在微切削过程中,具有物理化学效应的样品中残余应力和温度较高,可以为再结晶提供足够的驱动能量。

CYCLIC HARDENING BEHAVIOR OF POLYCRYSTALS WITH PENETRABLE GRAIN BOUNDARIES:TWO-DIMENSIONAL DISCRETE DISLOCATION DYNAMICS SIMULATION

A two-dimensional discrete dislocation dynamics （DDD） technology by Giessen and Needleman （1995）, which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocationpenetrable grain boundaries （GBs） on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.

Surface grain refinement mechanism of SMA490BW steel cross joints by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a postweld technique for improving the fatigue strength of welded joints. This technique makes use of ultrasonic vibration to impact and plastically deform a weld toe and can achieve surface grain refinement of the weld toe, which is considered as the main reason for the improvement of fatigue strength. In this paper, the microstructure of the surface of a treated weld toe was observed by metallographic microscopy and transmission electron microscopy (TEM). The results show that UIT could produce severe plastic deformation on the surface layer of the weld toe and the maximum depth of plastic deformation extended to approximately 260 μm beneath the treated surface. Repeated processing could exacerbate the plastic deformation on the surface layer, resulting in finer grains. We can conclude that the surface grain refinement mechanism of SMA490BW welded joints is related to the high density of dislocation tangles and dislocation walls. © 2017, University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg.

Modeling of material deformation behavior in micro-forming under consideration of individual grain heterogeneity

This study aims to develop a model to characterize the inhomogeneous material deformation behavior in micro-forming.First,the influence of individual grain heterogeneity on the deformation behavior of CuZn20 foils was investigated via tensile and micro-hardness tests.The results showed that different from thick sheets,the hardening behavior of grains in the deformation area of thin foils is not uniform.The flow stress of thin foils actually only reflects the average hardening behavior of several easy-deformation-grains,which is the reason that thinner foils own smaller flow stress.Then,a composite modeling method under consideration of individual grain heterogeneity was developed,where the effects of grain orientation and shape are quantitatively represented by the method of flow stress classification and Voronoi tessellation,respectively.This model provides an accurate and effective method to analyze the influence of individual grain heterogeneity on the deformation behavior of the micro-sized material.

Thickness-related synchronous increase in strength and ductility of ultrafine-grained pure aluminum sheets

To explore the specimen size effect of mechanical behavior of ultrafine-grained(UFG)materials with different structures,UFG Al sheets processed by equal channel angular pressing(ECAP)were selected as target materials and the dependency of tensile behavior on sheet thickness(t)was systematically investigated.The strength and ductility of ECAPed UFG Al sheets were improved synchronously as t increased from 0.2 to 0.7 mm,and then no apparent change occurred when t reached to 0.7 and 1.0 mm.The corresponding microstructure evolved from dislocation networks in equiaxed grains into the walls and subgrains and finally into the dominated cells in elongated grains or subgrains.Meanwhile,dense shear lines(SLs)and shear bands(SBs)were clearly observed and microvoids and cracks were initiated along SBs with the increase of t.These observations indicated that the plastic deformation of UFG Al sheets was jointly controlled by shear banding,dislocation sliding,and grain-boundary sliding.Furthermore,the propagation of SBs became difficult as t increased.Finally,the obtained results were discussed and compared with those of annealed UFG Al and UFG Cu.

河床表层冻融作用下推移质输移特征研究

高纬度河流系统常会经历季节性冻结与融化。为揭示冻融作用对床面推移质运动的影响,本研究以未经历冻融过程的常规室温(20℃)推移质输沙实验结果为基准,保证实验条件不变,开展了均匀沙与非均匀沙床表层冻融后的室温(20℃)推移质输沙实验,综合比较了冻融作用影响下推移质输沙率与级配组成特征。结果表明,河床表层冻融后的推移质输沙率均高于无冻融作用的推移质输沙率,据此提出了有冻融影响与无冻融影响下的推移质输沙率比值ζ。研究显示,冻融作用下的推移质输沙率呈有规律变化:剪切力较小时ζ较大,剪切力较大时ζ减小并逐渐趋于1.0;非均匀沙中,随着泥沙粒径增大,各粒径组的ζ逐渐减小;低流量驱动时,冻融作用对非均匀输沙级配组成的影响显著,并随着流量增大而逐渐减弱。本研究剖析了冻融作用对推移质输沙率与级配组成的影响,有助于认识冰冻区河流的河床稳定性与河床演变情势。