Fracture behavior and microstructure analysis of Al2O3–MgO–CaO castables for steel-ladle purging plugs

Three different castables based on the Al₂O₃–MgO–CaO system were prepared as steel-ladle purging plug refractories: corundum- based low-cement castable (C-LCC), corundum-spinel-based low-cement castable (C-S-LCC), and corundum-spinel no-cement castable (C-S-NCC) (hydratable alumina (ρ-Al₂O₃) bonded). The fracture behavior at room temperature was tested by the method of “wedge-splitting” on samples pre-fired at different temperatures; the specific fracture energy G′_f and notched tensile strength σ_NT were obtained from these tests. In addition, the Young’s modulus E was measured by the method of resonance frequency of damping analysis (RFDA). The thermal stress resistance parameter R′′′′ calculated using the values of G′_f, σ_NT, and E was used to evaluate the thermal shock resistance of the materials. According to the microstructure analysis results, the sintering effect and the bonding type of the matrix material were different among these three castables, which explains their different fracture behaviors.     

Effects of stress ratio on the temperature-dependent high-cycle fatigue properties of alloy steels

This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue (HCF) properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they function as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.     

Microstructure evolution and thixoforming behavior of 7075 aluminum alloy in the semi-solid state prepared by RAP method

The effects of isothermal treatments on the microstructural evolution and coarsening rate of semi-solid 7075 aluminum alloy produced via the recrystallization and partial remelting (RAP) process were investigated. Samples of 7075 aluminum alloy were subjected to cold extrusion, and semi-solid treatment was carried out for 5–30 min at temperatures ranging from 580 to 605°C. A backward-extrusion experiment was conducted to investigate liquid segregation during the thixoforming process. The results revealed that obvious grain coarsening and spheroidization occurred during prolonged isothermal treatments. In addition, higher soaking temperatures promoted the spheroidization and coarsening process because of the increased liquid fraction and the melting of second phases. Segregation of the liquid phase caused by the difference in fluidity between the liquid and the solid phases was observed in different regions of the thixoformed specimens.     

Thermodynamic simulation of the effect of slag chemistry on the corrosion behavior of alumina–chromia refractory

The corrosion behavior of alumina–chromia refractory against two kinds of industrial slags (coal slag and iron smelting slag) at 1550°C was investigated via thermodynamic simulations. In the proposed simulation model, the initial slag first attacks the matrix and surface aggregates and subsequently attacks the inner aggregates. The simulation results indicate that the slag chemistry strongly affects the phase formation and corrosion behavior of the refractory brick. Greater amounts of alumina were dissolved and spinel solid phases formed when the brick interacted with iron smelting slag. These phenomena, as well as the calculated lower viscosity, may lead to much deeper penetration than that exhibited by coal slag and to more severe corrosion compared to that induced by coal slag. The thermodynamic calculations well match the experimental observations, demonstrating the efficiency of the proposed simulation model for evaluating the corrosion behavior of alumina–chromia refractory.     

Effect of SrCO<Subscript>3</Subscript> addition on the dynamic compressive strength of ZTA

Ceramic parts usually experience dynamic load in armor applications. Therefore, studying the dynamic behaviors of ceramics is important. Limited data are available on the dynamic behaviors of ceramics; thus, it is helpful to predict the dynamic strength of ceramics on the basis of their mechanical properties. In this paper, the addition of SrCO₃ into zirconia-toughened alumina (ZTA) was demonstrated to improve the fracture toughness of ZTA due to the formation of the SrAl₁₂O₁₉ (SA6) phase. The porosity of ZTA was found to be increased by the addition of SrCO₃. These newly formed pores served as the nucleation sites of cracks under dynamic load; these cracks eventually coalesced to form damaged zones in the samples. Although the K IC values of the samples were improved, the dynamic strength was not enhanced because of the increase in porosity; in fact, the dynamic strength of ZTA ceramics decreased with the addition of SrCO₃.     

超立方体网络递归结构的性质及应用

讨论超立方体结构的递归分解结构及性质,运用这种分解可为并行算法分析和设计提供较好的框架和理论基础,特别适用于递归算法的设计和分析。     

考虑压杆稳定性的桁架拓扑优化设计

目的 初探用进废退思想在桁架拓扑优化设计中的应用,开发相应的优化程序并用算例进行验证。方法 在桁架拓扑优化设计中考虑结构中受压杆的稳定性,同时考虑材料应力约束和压杆的稳定应力约束,通过杆件的实际受载情况及其应力约束值来调整各杆件的截面积,剔除优化结果中截面积小于阈值的杆件,以得到最优的拓扑结构和杆的截面大小,使各杆在保证结构稳定的前提下各尽所能,物尽其用。结果 优化结果表明桁架达到减重和安全稳定的     

均匀化热处理及其冷却速度对镍基GH141合金热塑性的影响

研究了均匀化热处理时保温后的冷却速度对ＧＨ１４１合金热塑性的影响·研究结果表明,冷却方式显著影响均匀化热处理效果·保温后缓冷过程中析出大尺寸、方形γ′粒子,超过了位错与γ′粒子交互作用方式的临界尺寸,合金变形时位错以Ｏｒｏｗａｎ机制绕过γ′粒子,从而使合金变形抗力降低,热塑性提高·     

峰峰仙庄寒武系层序地层分析

在露头层序地层学研究的基础上,对峰峰仙庄寒武系剖面进行了层序地层分析。峰峰寒武系共划分出１１５ 个副层序、６ 个三级层序,每个层序均仅由海侵体系域和高水位体系域构成。     

Au/TiO2的制备方法对其SPR的影响

纳米金粒子沉积在TiO₂表面制得具有表面等离子体共振效应(SPR)的Au/TiO₂,是目前提高TiO₂光催化剂对太阳能的利用率和量子效率的重要途径之一,本文以氯金酸(HAuCl₄·3H₂O)和P25型纳米TiO₂为原料,采用紫外可见分光光度计探讨了温度对化学还原制备金溶胶的SPR的影响; 并以紫外漫反射进行表征.比较了化学还原热沉积法、化学还原光沉积法和光还原沉积法制得Au/TiO₂的SPR差异,结果表明,光还原沉积法是一种高效、经济且环境友好型制备较强SPR的Au/TiO₂的方法.