In-situ production and microstructures of iron aluminide/TiC composites

In this study we have tried to produce the titanium carbide reinforced iron aluminide composites by in-situ reaction between titanium and carbon in liquid iron–aluminum alloy doped with titanium and carbon. A homogeneous distribution of titanium carbide particles in the iron aluminide matrix up to about 16 vol% of titanium carbide was intended without agglomeration. The composition of TiC formed during in-situ reaction was investigated by ICP analysis and the Combustion-Infrared Absorption method after chemical dissolution of the iron aluminide matrix. It is found that the composition of titanium carbide formed during melt processing is an average of Ti–48.4 mol% C. In addition, titanium carbide has very low solubility of Fe and Al. The microstructure of composites consists of three different regions; primary large TiC particles of 5–40 μm, matrix with small dendritic TiC particles of about 1 μm and particle-free regions around primary large TiC particles. The formation of this complex microstructure can be explained by assuming the Fe₃Al–TiC pseudo-binary system containing the eutectic reaction. Particle-free regions are halos of iron aluminide phase and the formation of halos is explained by coupled zone concept. Subsequent heat treatment at 1373 K for 48 h induces spheroidization and/or coarsening of small TiC particles, while microstructure after heat treatment at 973 K for 48 h exhibits the additional formation of small TiC precipitates. Though excess 1 mol% Ti addition over the Ti content for TiC formation is soluble to Fe–28 mol% Al, excess 1 mol% C addition forms the secondary Fe₃AlC phase during melt processing.     

乘用车用新型冷作模具钢Cr8真空气淬后的组织与性能

采用硬度计、光学显微镜和扫描电镜等对比分析了国产HNC53和日本KD11max两种Cr8型冷作模具钢真空气淬处理后的组织和性能,并分析了两种钢热处理前后的碳化物演变规律对性能的影响。结果表明,由于HNC53钢降低了P、S含量,控制在标准范围内,与KD11max钢相比原始组织中大块状共晶碳化物尺寸变小,且均匀分布在基体中;两种钢在1030 ℃真空气淬+520 ℃高温回火之后组织为马氏体和大颗粒状共晶碳化物及细小二次析出碳化物,与热处理前相比大块状共晶碳化物颗粒尺寸减小,数量减少;520 ℃回火后HNC53钢中析出较多的二次碳化物颗粒,均匀分布在马氏体基体上。HNC53钢与KD11max钢的硬度和韧性相差不大。     

Effect of specific pressure on microstructure and mechanical properties of squeeze casting ZA27 alloy

The effect of specific pressure on the microstructure and mechanical properties of ZA27 squeezed castings with high height-to-thickness ratio was studied. The results of DTA and SEM show that at high specific pressure the eutectic reaction of squeeze casting ZA27 alloy is restrained, and the final solidified structure is (η+ε) phases instead of eutectic phase (β+η+ε). At the same time, the primary reaction is promoted in squeeze casting ZA27 alloy solidified at high pressure, and the fine microstructure is obtained with the increase of pressure. Al and Cu elements are homogeneously distributed in matrix of squeeze casting ZA27 alloy. The homogenously distributed high-density fine ε phase can effectively hinder dislocation motion, and then the strength and plasticity of squeeze casting ZA27 alloy are increased.     

Development of boron white cast iron

Abstract

With boron substituting for carbon in cast iron composition and eutectic borides substituting for eutectic carbides in microstructure as the hard wear resistant phase, a new kind of wear resistant white cast iron has been developed. The microstructure and mechanical properties of this new white cast iron both in the as cast state and after appropriate heat treatments were studied. The results show that the as cast microstructure of the boron white cast iron comprises a dendritic matrix and interdendritic eutectics, and the eutectic compound is that of M₂B or M′_0˙9Cr_1˙1B_0˙9 type, where M represents Fe, Cr or Mn and M′ represents Fe or Mn. The morphology of the eutectic borides is much like that of carbide in high chromium white cast iron, but the hardness of boride is higher than that of carbide. The matrix in as cast microstructure comprises martensite and pearlite. After austenitising and quenching, the matrix mostly changes to lath type martensite and the eutectic borides remain unchanged. In addition, two different sizes of particles, with different forming processes during heat treatment, appear in the matrix. The boron white cast iron possesses higher hardness and toughness than conventional white cast iron and nickel hard white cast iron, and has a better balance between hardness and toughness than high chromium white cast iron.     

Effect of potassium on as-cast microstructure of a hypereutectic high chromium cast iron

The present work mainly evaluates the effect of potassium(K) on as-cast microstructure of a hypereutectic high chromium cast iron by means of a field emission scanning electron microscope(FESEM) and an X-ray diffractometer using CuKα radiation with a 2θ range of 30-130°.Results showed that,with the addition of K-containing modifier,the large lath-like and/or rod-like primary M7C3 carbides can be modified to the hexagonal prisms,and the eutectic carbides can also be refined.In addition,the carbides are distr...     

WC颗粒增强高铬钢基表层复合材料的制备和组织

采用真空实型铸渗法(V-EPC)工艺,成功制备了以高铬钢为基材,WC颗粒为增强颗粒的表层复合材料。结果表明,用含有WC颗粒和高碳铬铁颗粒的预置块制备的不同WC颗粒体积分数的高铬钢基表层复合材料,WC颗粒均匀分布于复合层中,复合层在颗粒熔化、元素扩散互溶、金属液渗入的共同作用下形成由WC、W2C共晶组织,未溶解的高碳铬铁颗粒和各种析出的碳化物组成的组织。     

Effect of ferrovanadium inoculation on microstructure and properties of high speed steel

The effect of ferrovanadium inoculation on the microstructure and properties of high speed steel （HSS） used for rolls was studied. The results showed that the as-cast eutectic carbide network tends to be broken after ferrovanadium inoculation, and the carbides are changed to rod-like or nodular shape. After heat treatment, the carbides in the inoculated HSS are spheroidized and distributed more uniformly in the matrix. The impact toughness of high speed steel with ferrovanadium inoculation is obviously improved. The action mechanism of ferrovanadium inoculation on the microstructure of the alloy is also discussed.     

Effect of ferrovanadium inoculation on microstructure and properties of hign speed steel

The eriect of ferrovanadium inoculation on the microstructure and properties of high speed steel (HSS)used for rolls was studied.The results showed that the as-cast eutectic carbide network tends to be broken after ferrovanadium inoculation,and the carbides are changed to rod-like or nodular shape.After heat treatment,the carbides in the inoculated HSS are spheroidized and distributed more uniformly in the matrix.The impact toughness of high speed steel with ferrovanadium inoculation is obviously improved.The action mechanism of ferrovanadium inoculation on the microstructure of the alloy is also discussed.     

Correlation of microstructure with the wear resistance and fracture toughness of white cast iron alloys

The objective of this investigation was to set down (on the basis of the results obtained by the examination of white cast iron alloys with different contents of alloying elements) a correlation between chemical composition and microstructure, on one hand, and the properties relevant for this group of materials, i.e., wear resistance and fracture toughness, on the other. Experimental results indicate that the volume fraction of the eutectic carbide phase (M₃C or M₇C₃) have an important influence on the wear resistance of white iron alloys under low-stress abrasion conditions. Besides, the martensitic or martensite-austenitic matrix microstructure more adequately reinforced the eutectic carbides, minimizing cracking and removal during wear, than did the austenitic matrix. The secondary carbides which precipitate in the matrix regions of high chromium iron also influence the abrasion behaviour. The results of fracture toughness tests show that the dynamic fracture toughness in white irons is determined mainly by the properties of the matrix. The high chromium iron containing 1.19 wt% V in the as-cast condition, showed the greater fracture toughness when compared to other experimental alloys. The higher toughness was attributed to strengthening during fracture, since very fine secondary carbide particles were present mainly in an austenitic matrix.     

Structure and ductility of eutectic type iron-carbon alloys

Conclusion The ductility of iron-carbon alloys of the eutectic type is governed by the structure of eutectic carbides and it may be increased by two methods. The first envisages formation during prior heat treatment of dislocations in eutectic carbides and creation of subgrain boundaries along which during deformation there is carbide fragmentation. This method, as a result of the specific effect of the metal base on formation of dislocations in carbides and prevention of carbide failure under the action of compressive stresses from the surrounding solid solution, may only be used for alloys in which the carbide phase reinforces a metal matrix. The second method involves a marked increase in carbide ductility as a result of transformation occurring in them under the action of deformation [10]. This method may be used to increase the ductility of cast irons around the eutectic composition with eutectics whose matrix phase is carbide. In this way forming may be accomplished by rolling in the range of rates used in metallurgical production practice.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 5–9, September, 1984.     

多步热处理对GH4742合金组织性能的影响

采用多步热处理,研究了不同固溶温度和高温时效温度对GH4742高温合金组织性能的影响。结果表明,固溶温度显著改变晶粒尺寸和一次γ′相形态。固溶温度为1090 ℃时,基体中存在大量未溶的一次γ′相,可以有效阻止晶粒长大,同时这种粗大γ′相以及后续时效过程中形成的细小γ′相共存的组织使得合金具有良好的综合力学性能。高温时效处理对组织形貌影响较小,但可以改变γ′相的尺寸,显著影响晶界碳化物的形态,通过Mo置换一次碳化物MC中的Nb、Ti,使晶界连续分布的碳化物发生溶解形成颗粒状的碳化物,显著降低了GH4742高温合金的高温持久性能。     

原位自生TiC和(Ti,W)C增强Fe基复合材料的研究

利用块体原材料原位合成10%TiC-Fe和(Ti,W)C-Fe两种复合材料,采用扫描电镜分析了复合材料的微观结构,利用X射线分析了相组成.结果表明,在TiC-Fe复合材料中,TiC作为唯一的第二相呈现粒状和条状两种形态.分析认为,粒状相为亚共晶相,而条状第二相为共晶相.通过用W替代部分Ti,成功地制备了10%(Ti,W)C-Fe复合材料,其中(Ti,W)C作为唯一的第二相比较均匀地分布在Fe基体中,其形态大部分呈粒状,条状相较少.在粒状(Ti,W)C相中,中心富Fe,而边缘W、Ti和C元素的分布是均匀的.与TiC相比,(Ti,W)C的密度更接近Fe,更适合作为大型铸件的增强相.     

Characterization of quasi-nano-sized TiCx-Ni-Fe thin composite sheet prepared by using self-propagating high-temperature synthesis reaction and electroforming

Thin TiC_x-Ni-Fe composites sheet was prepared by self-propagating high-temperature synthesis (SHS) and electroforming. The quasi-nano-sized titanium carbide particles were prepared by self-propagating high temperature synthesis (SHS) followed by mechanical milling and ultrasonic floating agitation for classifying particles. The composite sheet was fabricated by co-deposition of the classified titanium carbide particles in a modified Watts nickel bath containing iron chloride during nickel-iron electro-forming. Neutron diffraction showed that the non-stoichiometric number of titanium carbides formed by the SHS reaction were in the range of 0.68 to 0.97, which depended on the initial carbon sources. X-ray diffraction and electron probe micro-analysis revealed that co-deposition of the carbides in Ni-Fe bath during the electroforming process produced a thin TiC_x-Ni-Fe composite sheet, in which quasi-nano-sized titanium carbides were embedded about 7 at.%. The average surface resistance of the thin composite sheet was 1.053 ohm/sq. The corrosion potential and rate of the composites in a 50% NaOH solution were ?920.6 mV_SHE and 8.4×10^?6 Acm^?2, respectively.     

Study of cemented carbonitrides with nickel as binder: Experimental investigations and computer calculations

Cobalt is the most common binder in cemented carbides industry. However, there are some interests in use of alternatives. The similarity in properties has made nickel the first choice. In the present work, the effect of initial composition on modern hardmetals containing transition metal carbides/carbonitrides that are called “cemented carbonitrides” with nickel as binder was investigated. Change in quantity of additive carbides and tungsten to carbon (C/W) weight ratio through applying metallic tungsten powder in primary powder mixture had some effects on final hardness, transverse rupture strength, and microstructure of studied alloys. Addition of vanadium carbide not more than 0.2 wt.%, increased the final hardness. Application of (Ta,Nb)C solid solution carbide cancelled the grain refinement effect of VC. Formation of eta (η) phase was observed in SEM micrographs as a result of increase in W/C weight ratio. Surface modified layers with thicknesses ranging from 55 to 65 μm called Cubic Free Layer (CFL) was observed in vacuum sintered specimens. DICTRA® module from Thermo-Calc® software package was applied for simulation of CFL formation process in studied alloys. Formation of eta phase and consumption of metallic binder was predicted using calculations of Thermo-Calc® ver. P software. A state of the art technique was developed to prove linear relationship between Labyrinth factor (λ) and binder phase volume fraction (f).     

Local microstructure and its influence on precipitation behavior in hot deformed Nimonic 80a

The influence of local microstructure on the kinetics of heterogeneous carbide precipitation was investigated in hot deformed, commercial nickel-based superalloys Nimonic 80a. The as-deformed alloys were aged at 1073 K for different times, and precipitation characteristics were examined by transmission electron microscopy and small-angle neutron scattering. M₂₃C₆ precipitates were found to nucleate at grain boundaries, but in particular also on dislocation lines and boundaries of subgrains. In the early stages of ageing, the kinetics of carbide precipitation is enhanced and the average carbide size is smaller as a result of deformation. An influence of the degree of recrystallization on overall carbide kinetics is also found, which is attributed to differences in the amount of bulk carbides as compared to carbides at grain boundaries. In contrast to the behavior of carbides, the kinetics of the homogeneously distributed γ′ precipitates is not influenced by prior deformation.     

Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.     

Aging of cast Ni-base heat resisting alloy

Abstract

The behaviour of a Ni-base heat resisting alloy during aging was studied by mechanical testing and by optical and electron microscopy in samples aged for up to 6 weeks at 750°C. The as cast microstructure consists of an austenitic matrix and a network of two types of primary carbide (chromium and niobium). As aging proceeds, the main changes detected were precipitation of secondary chromium carbides and decomposition of primary niobium carbide in Ni rich silicide. The primary chromium carbides were largely unaltered. The mechanical tests indicate that these phenomena degrade the properties of the material.     

钛和冷速对过共晶Fe-4.5B-1.0C合金组织细化的影响

借助扫描电镜和透射电镜,研究了钛和冷速对过共晶Fe-4.5B-1.0C合金显微组织的影响.该合金加入钛后,可与钢液中的碳在高温下生成大量的TiC.TiC对初生硼化物和共晶硼化物均有细化作用:一部分作为初生硼化物异质形核核心使其细化,另一部分在基体中嵌入初生硼化物阻碍其生长;嵌入初生硼化物的TiC改变了共晶硼化物依附初生硼化物的特点,使共晶硼化物分布细小均匀.初生硼化物对冷速敏感,髓冷速加快,组织细化显著.     

Zr和Y对铸态M951镍基高温合金显微组织的影响(英文)

应用光学显微镜、扫描电镜、电子探针和X射线衍射研究Zr和Y对镍基高温合金铸态组织的影响。结果表明,随着镍基合金中Zr和Y含量的增加,合金中的γ+γ′共晶数量明显增多。同时,随着Y和Zr含量的增加,合金的初生MC碳化物形貌也发生明显变化,由针状或片状转化为以孤立的块状为主。XRD衍射测试发现,加入一定量的Zr和Y后,MC碳化物的晶格常数变大。电子探针结果表明,针状或片状碳化物主要含Nb和C,而块状的碳化物则平均含有39.2%Zr和39.6%Nb。这两种元素对铸态显微组织的影响可以归结于它们所具有的原子尺寸效应。