Effects of temperature on fracture behavior of Al-based in-situ composites reinforced with Mg_2Si and Si particles fabricated by centrifugal casting

An aluminum-based in-situ composites reinforced with Mg2Si and Si particles were produced by centrifugal casting A1-20Si-5Mg alloy. The microstructure of the composites was examined, and the effects of temperature on fracture behavior of the composite were investigated. The results show that the average fraction of primary Si and Mg2Si particles in the composites is as high as 38%, and ultimate tensile strengths （UTS） of the composites first increase then decrease with the increase of test temperature. Microstructures of broken specimens show that both the particle fracture and the interface debonding affect the fracture behavior of the composites, and the interface debonding becomes the dominant fracture mechanism with increasing test temperature. Comparative results indicate that rich particles in the composites and excellent interface strength play great roles in enhancing tensile property by preventing the movement of dislocations.

Stress initialization methods for dynamic numerical simulation of rock mass with high in-situ stress

In the context of deep rock engineering,the in-situ stress state is of major importance as it plays an important role in rock dynamic response behavior.Thus,stress initialization becomes crucial and is the first step for the dynamic response simulation of rock mass in a high in-situ stress field.In this paper,stress initialization methods,including their principles and operating procedures for reproducing steady in-situ stress state in LS-DYNA,are first introduced.Then the most popular four methods,i.e.,explicit dynamic relaxation(DR)method,implicit-explicit sequence method,Dynain file method and quasi-static method,are exemplified through a case analysis by using the RHT and plastic hardening rock material models to simulate rock blasting under in-situ stress condition.Based on the simulations,it is concluded that the stress initialization results obtained by implicit-explicit sequence method and dynain file method are closely related to the rock material model,and the explicit DR method has an obvious advantage in solution time when compared to other methods.Besides that,it is recommended to adopt two separate analyses for the whole numerical simulation of rock mass under the combined action of in-situ stress and dynamic disturbance.

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.

Measurement and study of the distributing law of in-situ stresses in rock mass at great depth

To solve the technical cruxes of the conventional system in deep rock mass, an automatic testing system for hydraulic fracturing that includes a single tube for hydraulic loop, a pressure-relief valve, central-tubeless packers, and a multichannel real-time data acquisition system was used for in-situ stresses measurement at great depths （over 1000 m） in a coalfield in Juye of Northern China. The values and orientations of horizontal principal stresses were determined by the new system. The virgin stress field and its distributing law were decided by the linear regression from the logged 37 points in seven boreholes. Besides, the typical boreholes arranged in both the adjacent zone and far away zone of the faults were analyzed, respectively. The results show that a stress concentration phenomenon and a deflection in the orientation of the maximal horizontal stress exist in the adjacent zone of the faults, which further provides theoretical basis for design and optimization of mining.

Microstructure of coupled Al/gray cast iron obtained by molding and its effect on mass loss

This study aims to consolidate the surface of gray cast iron with aluminum deposition by developing a method that combines the preparation and surface treatment in a single operation. The effect of the wall thickness of the castings on the microstructure of the formed layers was studied, and two thicknesses, 8 mm and 25 mm, were studied. The formation of a continuous and homogeneous rich aluminum layer on the surface of the cast iron was observed. The formed layer is composed of two successive zones identified as two proeutectoid phases FeAl+FeAl/FeAl2 and single-phase FeAl, which significantly increases the surface hardness. Furthermore, this change in surface composition makes it possible to reduce the mass of the immersed samples in a 1 M hydrochloric acid solution during different exposure times(1 to 4 days). Consequently, a clear improvement in the corrosion resistance of the treated layers is highlighted.

Heat transfer behavior of AZ80-1%Y alloy during low-frequency electromagnetic casting

Heat transfer behaviors of AZ80?1%Y alloy during low frequency electromagnetic casting (LFEC) and direct chilling casting were studied by in-situ temperature measurement. The results demonstrated that the low frequency electromagnetic field (EM) caused forced convection in the melt during LFEC. The forced convection led to uniform solidification velocity and temperature field. EM frequency, excitation current intensity and casting temperature could control the heat transfer behavior. The forced convection could improve the microstructure and degrade the difference in microstructure between the edge and center of billet. Appropriate parameters of low frequency EM for casting Mg alloy are 20 Hz of frequency and 60 A of electric current intensity.

冷拉拔变形量对Cu-20 mass%Ag合金线材组织性能的影响

采用三室真空冷型竖引连铸+多道次冷拉拔变形工艺制备了不同线径的Cu-20 mass%Ag合金线材,测试了合金线材在不同变形量下的导电率和抗拉强度,分析了其显微组织的演变规律。结果表明:铸态Cu-20 mass%Ag合金杆坯(ϕ7.84 mm)的导电率为80.3%IACS,抗拉强度为263 MPa;当拉拔变形量达到98.4%(ϕ1 mm),线材的导电率为69.6%IACS,抗拉强度达到889 MPa。铸态Cu-20 mass%Ag合金杆坯的共晶组织在横截面上呈现为连续的网状结构,在纵截面上呈现为鱼骨状。随着变形量的增加,Cu-20 mass%Ag合金的共晶组织沿拉拔方向形成长纤维并沿轴向排列,纤维间距逐渐减小。在拉拔变形过程中,Cu-20 mass%Ag合金的主要强化机制是位错强化和细晶强化。

SinterCast蠕墨铸铁的大规模生产

介绍了SinterCast公司开发的用于蠕墨铸铁的SinterCast铸造工艺,它利用现代计算机技术可确保蠕墨铸铁大规模生产的稳定性。分析了SinterCast蠕墨铸铁的化学成分、力学性能和物理性能。欧美等国家已将这一技术成功地应用于蠕墨铸铁发动机缸体、缸盖的大规模生产,可满足发动机技术特别是柴油机技术发展的需要。

In-situ observations of damage-fracture evolution in surrounding rock upon unloading in 2400-m-deep tunnels

The damage-fracture evolution of deep rock mass has obvious particularity,which is revealed in 2400-mdeep tunnels by field tests.The evolution of the excavation damaged zone depth is consistent with that of the fractured zone depth.The ratio of the excavation damaged zone depth to the excavation fractured zone depth is greater than 2.0 in a rock mass with both high strength and good integrity,but less than1.5 in a rock mass with lower strength or poor integrity.Zonal disintegration in a rock mass with high strength and fair integrity is more likely to occur when it contains more than two groups of primary fractures in damaged zones.Fractures develop outward in zonal disintegration but are totally different from the single-zone fracture,in which the fractures develop inward,and it is the starting position of the fractured zone when the excavation surface of the middle pilot is 7–9 m close to the pre-set borehole and it stops after the excavation surface of the baseplate is 11–14 m away.The most intense evolution occurs around 2–4 m from the pre-set borehole in the sidewall expansion stage.The research results provide a reference for the monitoring scheme and support design of CJPL-Ⅲin its future construction.

In-situ SEM observation on fracture behavior of austempered silicon alloyed steel

Crack initiation, propagation and microfracture processes of austempered high silicon cast steel have been investigated by using an in-situ tensile stage installed inside a scanning electron microscope chamber. It is revealed that micro cracks always nucleate at the yielding near imperfections and the boundary of matrix-inclusions due to the stress concentration. There are four types of crack propagations in the matrix: crack propagates along the boundary of two clusters of bainitic ferrite; crack propagates along the boundary of ferrite-austenite in bainitic ferrite laths; crack propagates into bainitic ferrite laths; crack nucleates and propagates in the high carbon brittle plate shape martensite which is transformed from some blocky retained austenite due to plastic deformation. Based on the observation and analysis of microfracture processes, a schematic diagram of the crack nucleation and propagation process of high silicon cast steel is proposed.

Study on in-situ WC particles/tungsten wire reinforced iron matrix composites under electromagnetic field

By applying electromagnetic field to a system consisting of tungsten wires and grey cast iron melt,the grey cast iron matrix composite reinforced by either in-situ WC particles or the combination ofin-situ WC particles and the residual tungsten wire was obtained.By means of differential thermal analysis(DTA),the pouring temperature ofiron melt was determined at 1,573 K.The microstructures of the composites were analyzed by using of X-ray diffraction(XRD),scanning electron microscopy(SEM) equipped with an energy dispersive spectrum(EDS) and pin-on-disc abrasive wear test.The obtained results indicated that,with the enhancing frequency of electromagnetic field,the amount ofin-situ WC particles gradually increases,leading to continuous decrease of the residual tungsten wires.When the electromagnetic field frequency was up to 4 kHz,tungsten wires reacted completely with carbon atoms in grey cast iron melt,forming WC particals.The electromagnetic field appeared to accelerate the elemental diffusion in the melt,to help relatively quick formation of a series of small FeW-C ternary zones and to improve the kinetic condition ofin-situ WC fabrication.As compared with the composite prepared without the electromagnetic field,the composite fabricated at 4 kHz presented good wear resistance.

Development of a multivariate empirical model for predicting weak rock mass modulus

Estimating weak rock mass modulus has historically proven difficult although this mechanical property is an important input to many types of geotechnical analyses. An empirical database of weak rock mass modulus with associated detailed geotechnical parameters was assembled from plate loading tests per- formed at underground mines in Nevada, the Bakhtiary Dam project, and Portugues Dam project. The database was used to assess the accuracy of published single-variate models and to develop a multivari- ate model for predicting in-situ weak rock mass modulus when limited geoteehnical data are available. Only two of the published models were adequate for predicting modulus of weak rock masses over lim- ited ranges of alteration intensities, and none of the models provided good estimates of modulus over a range of geotechnical properties. In light of this shortcoming, a multivariate model was developed from the weak rock mass modulus dataset, and the new model is exponential in form and has the following independent variables： （1） average block size or joint spacing, （2） field estimated rock strength, （3） dis- continuity roughness, and （4） discontinuity infilling hardness. The multivariate model provided better estimates of modulus for both hard-blocky rock masses and intensely-altered rock masses.

Mass ratio design based on compaction properties of backfill materials

The backfill-mining mass ratio is the ratio of the mass of the backfill materials in the goaf to the mass of the produced raw coal during solid backfill mining and it is regarded as a direct control index of the backfill effect in solid backfill mining. To design the backfill-mining mass ratio in a solid backfill mining panel, the backfill-mining mass ratio was defined on the basis of the basic principle of solid backfill mining. In addition, the density-stress relationship of backfill materials under compaction was obtained for five types of materials to derive a design formula for backfill-mining mass ratio. Moreover, the 6304-1 backfill panel under the large-scale dam of Ji′ning No. 3 coal mine was taken as an engineering case to design the backfill-mining mass ratio. In this way, it is found that the designed backfill-mining mass ratio is 1.22, while the mean value of the measured backfill-mining mass ratio is 1.245. Besides, the maximum roof subsidence is only 340 mm which effectively guarantees the backfill effect in the panel and control of strata movement and surface subsidence.

Centrifugal slip casting for preparing open-cell Al_2O_3 foam

Centrifugal slip casting in prefabricated template was used to prepare open-cell Al2O3 foams. Aqueous α- Al2O3 slurries with up to 50% solid contents (volume fraction) were prepared. The sedimentation of slurries during centrifugation was discussed with respect to the hydronamic conditions at large particle concentrations. The effect of solid contents on mass segregation was observed. Segregation phenomena were hindered for slurries with high solid loadings exceeding 50% (volume fraction). The dried and sintered behaviors of samples were analyzed. The cell struts in green bodies had good particles packing and showed high green density (63.4% theory density). After sintered at 1500 ℃, the cell struts of final products had high sintered density (98.8% theory density) and homogeneous microstructure. The porosity of final products is 75.6%

High temperature mechanical properties of low carbon silicon-bearing steel by twin-roll strip casting

The tensile equipment of the advanced confocal scanning laser microscope （CSLM） was used to research the high temperature mechanical properties of low carbon silicon-bearing steel by twin-roll strip casting. The results show that, at the strain rate of 0. 000 5 s ^-1, the strip clearly shows signs of brittleness at around 600 ℃ and its plasticity falls considerably between 750 ℃ and 600 ℃. This is because during the transformation from austenite to ferrite, the low strength ferrite at the austenite grain boundaries greatly reduces the steel＇ s high temperature plasticity. The subsequent strip coiling process should be controlled at less than the brittle temperature of approximately 600 ℃, so cracks at the coiling stage can be prevented.

Effect of CaO/SiO_(2) Slag Mass Ratio on Dissolution Rate of Alumina-based Refractory Ceramics

The dissolution of alumina-based refractory ceramics in CaO-Al2O3-SiO_(2)slag melts was performed based on the in-situ observation system of an ultra-high-temperature laser confocal microscope,and the effect of the CaO/SiO_(2)slag mass ratio(C/S ratio)on the dissolution rate of alumina-based refractory ceramics was investigated.The results indicate that the dissolution rate increases with an increase of the C/S ratio and is mainly controlled by diffusion.During the early stage of dissolution,for all C/S ratios,the dissolution process conforms to the classical invariant interface approximation model.During the later stage of dissolution,when the C/S ratio is≥6,the dissolution process is significantly different from the model above because of the formation of a thick interfacial layer,which can be explained by dissolution kinetics.

Empirical Development of the Rock Mass Deformation Modulus

Rock mass deformation modulus is a fundamental factor for a safe and economical design of rock structures like large underground openings, tunneling, and open pit mine as well as foundations in both the initial state of stresses act on rock mass and its strength characteristics. The rock mass deformation modulus recently has been measured by in-situ loading tests and has been estimated by use of empirical equation based on classification systems and data of laboratory tests. In-situ tests to measure modulus directly are so expensive, times consuming and the reliability of the results of these tests is sometimes doubtful; subsequently, many researches have been carried out to estimate this parameter based on classification systems. In this study, a new empirical equation was proposed by use of statistical analyses based on a database of more than 142 in-situ tests, like plate load tests, dilatometer tests, flat jack tests, and classification systems; in addition, properties of the intact rock.

超宽鱼腹式现浇箱梁混凝土裂缝控制

近年来,国内站城综合体TOD开发成为一种新型建筑模式,地铁、市政道路、商业综合体等多种大体量建筑形态交叉同步施工成为工程实践难题。基于此,结合已经实施的成都博览城综合交通枢纽工程,解析福州路桥鱼腹式多幅四箱室闭合型超宽箱梁,采取优化混凝土配合比、U形冷却管布置、后浇带布置、支架沉降控制、箱梁浇筑顺序等5种措施,解决大体积C60梁体混凝土防开裂难题,并取得明显实践成果,获得国家优质工程奖和土木工程詹天佑奖。

江阴靖江长江隧道现浇大体积混凝土的抗裂技术与应用

在原材料优选和配合比优化的基础上,通过协同调控水化速率与膨胀历程,设计制备了低温升高抗裂混凝土(M0不掺抗裂剂、M1和M2掺抗裂剂),对比研究了其力学性能、绝热温升和自生体积变形。在此基础上,依托江阴靖江长江隧道现浇大体积C40混凝土工程,采用基于水化-温度-湿度-约束多场耦合机制的收缩模型对该工程不同结构部位的开裂风险进行了定量评估。试验结果表明:掺入适量抗裂剂能有效改善混凝土的绝热温升和自生体积变形,且对抗压强度影响不大。开裂风险评估计算结果表明:底板、侧墙可选择M1配合比,并控制入模温度不大于28℃,同时在侧墙中心布置冷却水管;顶板可选择M2配合比,并控制倒角处和其他部位的入模温度分别不大于28℃和32℃。工程现场持续跟踪监测6个月,未发现混凝土出现开裂、渗漏,达到预期目标。

挤压力及热处理对体育器材用7A09铝合金组织和性能的影响

采用挤压铸造工艺制备了7A09铝合金,研究了挤压力及时效工艺对7A09铝合金组织和性能的影响。结果表明:以挤压力为25、50和75 MPa的挤压铸造工艺制备的7A09铝合金铸件内无气孔、缩孔等缺陷,其显微组织比采用普通铸造工艺制备的铝合金铸件致密;挤压铸造的合金晶粒和沉淀相比普通铸造的合金的细小,随着挤压力的增大,合金的硬度和抗拉强度升高,磨损质量损失减小;经120℃时效130 min后,挤压铸造的合金的磨损质量损失比普通铸造的合金显著减小,这是由于挤压铸造合金凝固时间缩短、沉淀相密度提高、晶粒较细小和共晶组织分布较均匀从而耐磨性显著改善所致。