粉料表面氧含量对GPS烧结氮化硅陶瓷显微结构的影响

通过研究GPS烧结氮化硅陶瓷的室温和高温抗弯强度、晶界相含量和成分以及晶界析晶相随氮化硅粉料表面氧含量的变化规律，发现当粉料表面氧含量低于1．35mg／m^2时，氮化硅陶瓷在室温下的抗弯强度基本保持不变。试样在1200℃时的抗弯强度明显低于室温强度，且随着粉料表面氧含量的增加有一最高值。由于烧结助剂引入的O和Si在烧结过程中的还原气氛下发生反应而损失，烧结体中晶界相的实际含量显著低于粉料中烧结助剂的加入量。随粉料表面氧含量的增加，氮化硅陶瓷烧结体中的二次析晶相α-Y2Si2O7和β-Y2Si2O7消失，只有β-Si3N4晶相和晶界玻璃相存在。     

氮化硅粉添加剂对反应烧结氮化硅的影响

以Mg(OH)2·4MgCO3·6H2O和Al(OH)3为烧结助剂,在起始原料硅粉中添加等轴状α-Si3N4和纤维状β-Si3N4,通过调整原料配比,采用反应烧结制备了不同气孔率和抗弯强度的氮化硅陶瓷.考察了氮化硅粉体形貌和添加量对多孔氮化硅陶瓷微观组织和力学性能的影响.借助X射线衍射(XRD),扫描电子显微(SEM)和三点弯曲法对氮化硅陶瓷的微观组织和力学性能进行了研究.XRD分析表明适量氮化硅的添加有利于提高硅粉氮化率,但同时抑制了新生成的氮化硅从α相向β相的转变.SEM表明β-Si-N4纤维的添加造成了陶瓷显微结构不均匀,因而导致陶瓷抗弯强度下降.     

Csf／Si3N4复合材料的力学性能和微波介电性能研究

采用热压烧结的方法，以α-Si3N4粉和短切碳纤维为主要原料，以Y2O3和La2O3为烧结添加剂，制备Csf／Si3N4复合材料，研究了其力学性能和微波介电性能。结果表明，该Csf／SiaN。复合材料的抗弯强度随纤维含量的增加呈现下降的趋势，是由于碳纤维氧化所产生的缺陷所致；当纤维含量较低时，断裂韧性随纤维含量增大而增大，由于纤维氧化产生的缺陷阻止了裂纹进一步的扩展或使裂纹发生了偏转。当纤维含量超过1％（质量分数）后，随着纤维含量的增大，氮化硅显微结构发生变化，氮化硅陶瓷本身断裂韧性减小，使Csf／Si3N。复合材料断裂韧性逐步降低。当碳纤维含量在0～1％（质量分数）时，随着碳纤维含量的增加，介电常数ε’，ε＂和介电损耗tanδ均随着纤维含量的增加而增大，而且所制得的复合材料表现出较强的频散效应：当纤维含量继续增加时，在相同的频率下，介电常数下降，材料的频散效应逐渐消失。     

含稀土助烧剂氮化硅陶瓷的热导率、强度及电学性能

高热导率氮化硅陶瓷作为基板材料有着广泛的应用前景.如何在尽可能保持氮化硅陶瓷机械性能的前提下,提高其热导率是其实际应用的关键,而选择适当的烧结助剂是提高热导率的一个重要途径.本文研究了稀土氧化物种类及CaO、MgO烧结助剂对氮化硅陶瓷的热导率及电学和机械性能的影响,分别采用Y2O3-MgO,Y2O3-CaO,CeO2-MgO,CeO2-CaO,La2O3-MgO和La2O3-CaO 6种烧结助剂,采用放电等离子烧结后热处理的工艺制备氮化硅陶瓷.研究结果表明:氮化硅陶瓷的热导率随着烧结助剂稀土元素阳离子半径的增大有减小的趋势;与添加MgO助烧结相比,添加CaO助烧结不利于氮化硅柱状晶的生长,热导率及强度普遍较低,但硬度较高.采用Y2O3-MgO助烧剂和适当的烧结工艺,可以得到热导率高于80 W/m·K、抗弯强度大于1000 MPa、体电阻率大于1×1013Ωm、介电常数小于10、介电损耗小于3×10-3的氮化硅陶瓷.     

Csf/Si3N4复合材料的力学性能和微波介电性能研究

采用热压烧结的方法,以α-Si3N4粉和短切碳纤维为主要原料,以Y2O3和La2O3为烧结添加剂,制备Csf/SiN4复合材料,研究了其力学性能和微波介电性能.结果表明,该Csf/Si3N4复合材料的抗弯强度随纤维含量的增加呈现下降的趋势,是由于碳纤维氧化所产生的缺陷所致;当纤维含量较低时,断裂韧性随纤维含量增大而增大,由于纤维氧化产生的缺陷阻止了裂纹进一步的扩展或使裂纹发生了偏转.当纤维含量超过1%(质量分数)后,随着纤维含量的增大,氮化硅显微结构发生变化,氮化硅陶瓷本身断裂韧性减小,使Csf/Si3N4复合材料断裂韧性逐步降低.当碳纤维含量在0～1%(质量分数)时,随着碳纤维含量的增加,介电常数ε',ε"和介电损耗tanδ均随着纤维含量的增加而增大,而且所制得的复合材料表现出较强的频散效应;当纤维含量继续增加时,在相同的频率下,介电常数下降,材料的频散效应逐渐消失.     

TiB2含量和烧结温度对B4C/Al2O3基复合陶瓷的影响

利用原位反应热压工艺制备了B4C/Al2O3基复合陶瓷,研究了TiB2含量和烧结温度对B4C/Al2O3基复合陶瓷力学性能和微观结构的影响.结果表明,当TiB2含量低于8.7％时,随原位反应生成的TiB2含量的增加,有效的促进了B4C/Al2O3/TiB2复合陶瓷的烧结,提高相对密度,改善了力学性能.当烧结温度低于1900℃时,其力学性能随烧结温度增加而提高;当超过1900℃时,其力学性能随烧结温度的提高而降低.在1900℃,60 min时,B4C/Al2O3/TiB2复合陶瓷获得最佳综合力学性能,其硬度、断裂韧性和抗弯强度分别为24.8 GPa、4.82 MPa·m1/2和445.2 MPa.     

CeO2,Dy2O3,Yb2O3及Y2O3烧结助剂对氮化硅陶瓷热导率的影响

研究了CeO2-MgO(CeM),Dy2O3-MgO(DyM),Yb2O3-MgO(YbM)及Y2O3-MgO(YM)体系烧结助剂对氮化硅陶瓷的热导率及电学性能的影响.结果表明与YM烧结助剂相比,添加DyM烧结助剂的氮化硅陶瓷热导率较低;添加CeM烧结助剂的氮化硅陶瓷的热导率最高,为85.4W/m·K;添加YbM烧结助剂的氮化硅陶瓷也有较高的热导率,但体电阻率较低.组织分析表明,添加不同稀土元素的烧结体组织形貌基本相同,晶界相不同,添加DyM,YbM和YM烧结助剂的氮化硅陶瓷在烧结过程中分别形成了含氧晶界相Dy2Si3N4O3,Yb2Si3N2O5和Y2Si3N3O4.     

气氛对NiFe2O4陶瓷烧结致密化的影响

采用不同的烧结工艺制备了NiFe2O4陶瓷材料,研究了真空、大气、N2 3种气氛对NiFe2O4陶瓷材料烧结致密度的影响,解决了烧结过程中NiFe2O4陶瓷的离解问题.结果表明:在制备NiFe2O4陶瓷过程中,烧结气氛严重影响着陶瓷的离解和致密化;真空烧结将导致NiFe2O4陶瓷的离解,N2气氛保护烧结所制备出的NiFe2O4陶瓷样品的密度较大气气氛烧结所制备出的样品的密度高出14.6%～32.6%.分析结果表明:因NiFe2O4陶瓷高温失氧所带来的表面能和晶体缺陷的差异是影响其致密化的关键;无论在何种气氛下烧结,提高烧结温度都有利于提高NiFe2O4样品的烧结密度.     

陶瓷基复合材料的微观结构和力学性能(英文)

采用添加剂热压烧结制备了短纤维增韧氮化硅基复合材料,并对材料的力学性能和微观结构进行了分析和讨论.结果表明:Y_2O_3-La_2O_3添加剂促进了α-Si_3N_4→β-Si_3N_4的相转变,这个体系经过1800 ℃的热压烧结后,其中的碳纤维产生退化.而经过1600 ℃热压烧结的含LiF-MgO-SiO_2添加剂的体系中,纤维保持完好,晶粒没有发生相转变.两个体系的复合材料的断裂韧性值均高于氮化硅基体的值,其提高幅度均接近20%,这归因于纤维拉拔、裂纹偏转和界面松解机制.     

Si3N4陶瓷材料的高温氧化理论及其抗氧化研究现状

从热力学、动力学和整体控速过程探讨了氮化硅陶瓷材料高温氧化理论和氧化性质,在空气中的热化过程, 因为PO2>Psio, 是纯化氧化过程,氧化产物为SiO2和N2.氧扩散是控制氧化速度的主导因素.氧在Al2O3中的高温扩散系数为10-15~10-14量级,而在SiO2中扩散系数为10-8量级,因此人们探索各种方法在Si3N4陶瓷表面改性以提高其抗氧化性能,其中, 制备金属氧化物的效果显著.在试验沉积Al2O3薄膜的基础上,讨论了Al2O3薄膜对氮化硅陶瓷抗氧化性能的作用,并且展望了精密氮化硅陶瓷表面形成具有良好结合力的金属氧化物-氧化硅混合结构表层的表面改性技术.     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.     

Theoretical Analysis for the Motion and Temperature of Melt Droplets in Spray Degassing Process

The motion of melt droplets in spray degassing process was analyzed theoretically. The height of the treatment tank in spray degassing process could be determined by the results of theoretical calculation of motion of melt droplets. To know whether the melt droplets would solidify during spraying process, the balance temperature of melt droplets was also theoretically analyzed. Then proof experiments for theoretical results about temperature of melt droplets were carried. In comparison, the experimental results were nearly similar to the calculation results.     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.