Effect of Grain Size on the Thermal Expansion of Isotropic Synthetic Graphites

The relationship is analyzed between the thermal expansion coefficient of synthetic graphites and the grain size of their filler. It is shown by examining a large number of commercial graphites of different classes that their thermal expansion coefficient increases from 2 × 10⁻⁶ to (7–8) × 10⁻⁶ K⁻¹ as the grain size decreases from 3000 to 1 μm. The strength of synthetic graphites also increases with increasing grain size and correlates with thermal expansion. The likely reason for the increase in thermal expansion coefficient is the better contact between neighboring microvolumes of the material. A model is proposed for the thermal expansion of synthetic graphites which considers microvolumes 0.1 to 0.5 μm in size, with a thermal expansion coefficient on the order of 8.3 × 10⁻⁶ K⁻¹, only a fraction of the microvolumes being involved in thermal-expansion transfer. The fraction of such microvolumes decreases with increasing grain size. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 12, 2005, pp. 1456–1462. Original Russian Text Copyright ? 2005 by Samoilov, Shilo.     

Relation between Modulus of Elasticity and Compressive Strength of Ultrahigh-Strength Mortar with Mixed Silicon Carbide as Fine Aggregate

Ultrahigh-strength mortar mixed surface-oxidized silicon carbide as a fine aggregate was prepared by means of press-casting followed by curing in an autoclave. The relation between modulus of elasticity up to 111 GPa and compressive strength up to 360 MPa of mortar mixed silicon carbide was discussed and it was revealed that the contributions of the aggregate hardness and of the interfacial strength between the aggregate and the cement paste on the elasticity of mortar were imporant.     

Effect of Reagent Concentrations on the Grain Size of Chemical-Vapor-Deposited ZnSe

The effect of Zn and H₂Se concentrations on the microstructure of chemical-vapor-deposited ZnSe was studied. The average grain size was found to decrease in the flow direction and to vary nonmonotonically with depth, passing through a maximum. The factors influencing the mechanism of ZnSe deposition are discussed.     

Effect of Grain Size on the Weldability of Cast Alloy 718

Electron beam welding was conducted on cast alloy 718 with varying grain sizes obtained using Microcast (MX™) and conventional cast (CC) techniques. The average value per section of Total Crack Length (Av. TCL) was measured on each cross section and used to represent the material's weldability. It was found for the first time that the grain size had a reduced effect on the weldability of cast alloys, relative to that reported in the literature for wrought alloys; i.e., increased grain size in the range of 90-3000 microns resulted in improved weldability. This was determined to be related to the probability of welds intersecting grain boundaries and causing grain boundary microfissuring. The conclusion from the experimental analysis is corroborated by the use of a Weibull-type analysis to evaluate the probability of a weld microfissure occurring relative to the grain size. As grain size increases, the probability of the weld intersecting the grains is reduced, and thus, the likelihood of microfissuring is reduced. With regard to a single crystal, there are no grain boundaries intercepting the weld (probability = 0), and thus, microfissuring related to constitutional liquation of primary carbides, or segregation of species to grain boundaries would not occur.     

Effect of Grain Size on the Ionic Conductivity of Silver Halides

An equation is derived for the ionic conductivity of silver halide emulsion grains with dimensions comparable to the Debye length of intrinsic defects. The conductivity of the grains is shown to be inversely proportional to the grain size. The surface potential distribution depends on both the shape and size of the grains. The surface charge density, surface potential, and volume potential are estimated for octahedral, cubic, and tabular AgBr grains.     

晶粒尺寸对Cu-30%Zn合金形变孪晶厚度的影响

对于面心立方结构的纳米金属,晶粒尺寸对孪生厚度(孪生核)的影响虽已有研究,但仍有待深入。本论文以Cu-30%Zn合金为模型材料,通过高压扭转变形技术、等径角挤压连同轧制技术变形得到晶粒尺寸在5~500nm的样品。透射电子显微镜观察发现:变形孪晶的片层厚度随晶粒尺寸的减小而减小,当晶粒尺寸小于20nm以后,孪晶厚度为(111)晶面间距(层错);另外,层错存在于各个不同尺寸范围的晶粒内,表明层错不受晶粒尺寸影响。研究结果表明在低层错能超细晶材料中,孪生变形是通过从晶界连续发射不全位错(层错)形成的。     

Effect of Grain Size Distribution on the Performance of Perpendicular Recording Media

Micromagnetic simulations of perpendicular recording in hard disk storage media have been performed with model media of variable microstructural disorder. Simulations indicate that increasing disorder, either due to size and shape distribution or due to disordered packing, decreases signal and increases noise. The mechanism observed in the model is that, in a disordered microstructure, there is a distribution of magnetostatic and exchange coupling between grains that acts to create clusters of grains that act collectively. These clusters increase the auto-correlation function of the spatial distribution of magnetization that is a measure of the magnetic feature size. Consequently, the transition width between recorded bits increases and the position variation of the transition locations (jitter) increases, so that signal-to-noise ratio (SNR) falls. The results suggest that microstructurally ordered media will exhibit higher performance, and that such effects may ultimately demand the use of self-assembled or patterned media with regular packing and very narrow size distribution     

Correlation between Diffusion Parameters and the Temperature-Dependent Modulus of Elasticity of Metals

It is shown using the Debye model and an equation of state for solids that the diffusion parameters of anomalous and conventional metals correlate with the temperature variation of their shear moduli.     

The Effect of Size on the Splitting Strength of Cubic Concrete Members

In the study of concrete fractures, split‐tension specimens, such as cylinders, cubes and diagonal cubes, are frequently preferred to beams. However, experimental investigations on concrete reveal that for the same specimen geometry, the nominal strength of specimen decreases with increasing specimen size. This phenomenon is named as the size effect in the fracture mechanics of concrete. Although nominal strength is also highly affected by the width of the distributed load in the split‐tension cylinder and cube specimens, this effect can be negligible within the practical range of the load‐distributed width in the diagonal cubes. However, the number of theoretical and experimental studies with diagonal split‐tension specimens is limited. Besides, a size effect formula for estimating the split‐tensile strength of the diagonal cube specimens has not been proposed. In this study, nine series of cube and diagonal cube specimens, with three different sizes but similar geometries, were tested under different load‐distributed widths. The ultimate loads obtained from the test results are analysed by the modified size effect law. Subsequently, prediction formulas are proposed, and they are compared with historical test data from the split‐cylinder specimens.     

Effect of Cooling Conditions on Grain Size of AZ91 Alloy

The grain size of AZ91 alloy was investigated in terms of the effects of cooling rate, superheat and steel gauze. It was found that rapid cooling rate and low superheat favoured the achievement of fine grain structures. The gauze had a less profound effect on the grain size in AZ91 magnesium alloy than that in A356 aluminum alloy. The mechanisms by which these factors affect grain refinement of AZ91 have also been discussed.     

Interactive Effect of Grain Orientation and Grain Size on Magnetic Properties of Fe-78 wt% Ni Ribbons

Fe-78 wt% Ni ribbons were prepared by the melt spinning technique and the interactive contribution of the grain size and grain orientation on the magnetic properties was examined. Heat treatment at 673 K for 1 h followed by furnace cooling was performed to show the annealing impact. At three wheel speeds of 10, 20, and 30 m/s, the saturation magnetization nearly does not change. High wheel speed and heat treatment are inclined to promote the growth of <001> grains. Although the <001> orientation is not the easy axis of magnetization, the improvement of the texture in this direction makes the coercivity decrease, which counteracts the inverse effect of the grain size at high wheel speed. It indicates that for preparing soft magnetic ribbons, the interactive contribution of grain orientation variation and the grain size should be considered.     

氮化层弹性模量及残留内应力的测定

为了计算钢铁材料氮化层的弹性模量,提出了组合樑物理模型,采用等强樑的试样,测定氮化层弹性模量及残留内应力.试验结果表明:氮化层的弹性模量比母材高出约29.2%.X射线应力常数比母材提高44.4%.     

原奥氏体晶粒尺寸对珠光体钢组织及韧性的影响

探索了奥氏体晶粒尺寸对珠光体等温转变组织特征以及对韧性性能的影响规律。研究表明,在相同等温转变温度下,珠光体片层间距无明显变化,随奥氏体晶粒尺寸的增加,先共析铁素体量减少而珠光体团尺寸增加。珠光体断裂韧性受控于裂纹前沿塑性影响区尺寸(1~2)δc,其中δc为临界裂纹张开位移,当原奥氏体晶粒大于(1~2)δc时,裂纹扩展阻力主要来自穿越珠光体片层α、θ相的颈缩、破断。当原奥氏体晶粒尺寸接近或小于(1~2)δc时,裂纹主要沿晶界、珠光体团界、α+θ片层界面扩展,通过扩展路径发生多次弯折消耗能量,随原奥氏体晶粒尺寸增加,准静态断裂韧度J变化幅度较小。而冲击韧性缺口前沿塑性影响区远大于原奥氏体晶粒,大角度晶界将促使裂纹的转折而提高扩展阻力,提高裂纹前沿塑性区大角度晶界密度有利于提高冲击功,冲击韧性Ak随晶粒尺寸的增加显著下降。     

稀土氯化物对磷化膜层晶粒及膜重的影响

稀土作磷化促进剂,稳定性好,有利于环保,有利于提高磷化效率和质量.在磷化液中加入不同稀土(Re)氯化物,探讨了其对6061铝合金磷化膜膜重的影响.利用SEM和XRD观测了磷化膜晶粒的形貌和粒径.结果表明:适量(10～50 mg/L)稀土离子的加入使得磷化膜膜重增加,晶粒细化,在此范围内Re含量越大、Re离子半径越大,则膜重越大.而当稀土含量超过60 mg/L时,反而不利于磷化膜的生成,磷化膜膜重减轻.     

原奥氏体晶粒尺寸对珠光体钢组织及韧性的影响

探索了奥氏体晶粒尺寸对珠光体等温转变组织特征以及对韧性性能的影响规律.研究表明,在相同等温转变温度下,珠光体片层间距无明显变化,随奥氏体晶粒尺寸的增加,先共析铁素体量减少而珠光体团尺寸增加.珠光体断裂韧性受控于裂纹前沿塑性影响区尺寸(1～2)δc,其中δc为临界裂纹张开位移,当原奥氏体晶粒大于(1～2)δc时,裂纹扩展阻力主要来自穿越珠光体片层α、θ相的颈缩、破断.当原奥氏体晶粒尺寸接近或小于(1～2)δc时,裂纹主要沿晶界、珠光体团界、α+θ片层界面扩展,通过扩展路径发生多次弯折消耗能量,随原奥氏体晶粒尺寸增加,准静态断裂韧度J变化幅度较小.而冲击韧性缺口前沿塑性影响区远大于原奥氏体晶粒,大角度晶界将促使裂纹的转折而提高扩展阻力,提高裂纹前沿塑性区大角度晶界密度有利于提高冲击功,冲击韧性A随晶粒尺寸的增加显著下降.     

晶粒尺寸对304奥氏体不锈钢组织演变和性能的影响

研究了初始织构相近而晶粒尺寸不同的304奥氏体不锈钢在后续10%压缩变形和热处理过程中微观组织、力学和耐蚀性的变化。结果表明,具有相似织构而晶粒尺寸不同的样品变形热处理后其织构不同,粗晶在变形中织构的变化更大;织构相近时抗拉强度对晶粒尺寸的依赖较大;织构不同时,织构对硬度和抗拉强度的影响大于晶粒尺寸和微应变的影响;变形热处理后普通大角度晶界和晶内微应变的增大降低了试样的耐腐蚀性能;初始晶粒尺寸较小的试样在变形热处理后出现四种密排面平行于外表面的织构,其耐点蚀的性能更优。     

BaTiO3超微粉及其在陶瓷中的晶粒尺寸效应

用Ｓｏｌ－Ｇｅｌ法制备了ＢａＴｉＯ３超微粉并经烧结获得了不同粒径的陶瓷。用Ｘ射线衍射研究了室温晶体结构。在粉末压块和陶瓷上测得了介电常数的温度依赖性。介电常数－温度曲线表明，随着晶粒尺寸减小，居里温度降低，而四方－正交以及正交－三角相变温度升高。在晶粒尺寸减小过程中，三个介电峰逐步降低和宽化并按如下顺序最终消失：三角－正交，正交－四方和四方－立方。用介电测量确定了铁电性消失的临界尺寸，其值与文献中     

Effect of TiN Particles and Grain Size on the Charpy Impact Transition Temperature in Steels

The toughness of ferritic steels is influenced by the grain size distribution,second phase,precipitates and coarse inclusions.In this work an examination of the effect of coarse TiN particles(0.5 μm) and ferrite grain size on the Charpy impact transition temperature in high strength low alloyed steels has been carried out.Steels with high Ti content(up to 0.045 wt%),have been heat-treated and furnace cooled to obtain a ferrite-pearlite microstructure with different ferrite grain sizes.Coarse TiN particle size and ferrite grain size distributions have been measured and Charpy impact testing has been carried out.Scanning electron microscopy(SEM) analysis has been used to measure the grain boundary carbide thickness and to determine if the coarse TiN particles are acting as cleavage initiation sites by fractographic analysis.The Charpy ductile-brittle transition temperatures(DBTT) have been predicted using standard literature equations,and compared to the measured values.The relationship between the ferrite grain size and coarse TiN particle size and number density in terms of whether the coarse TiN particles act as effective cleavage initiation sites is discussed in this paper.     

Q235钢杨氏弹性模量的研究

采用实验与模型计算相结合的方法,对Q235钢的杨氏弹性模量进行了研究,通过Gleeble 1500热模拟机对700～1400℃温度区间的杨氏弹性模量进行了测量,每100℃取1个测量值,通过拟舍得到连续曲线;1400℃至零强度温度(ZST)使用Tszeng模型计算,体现液相区的真实力学行为.将得到的杨氏模量结果运用于连铸二冷段的热力耦合模拟,得到的结果与实测结果一致.