Microstructural characterization of self-propagating high-temperature synthesis/ dynamically compacted and hot-pressed titanium carbides

Titanium-Carbide produced by combustion synthesis followed by rapid densification in a high-speed forging machine was characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The density of the combustion synthesized/dynamically compacted TiC reached values greater than 96 pct of theoretical density, based on TiC_0.9, while commercially produced hot-pressed TiC typically exceeded 99 pct of theoretical density. The higher density of the hot-pressed TiC was found to be attributable to a large volume fraction of heavy element containing inclusions. The microstructure of both TiCs consists of equiaxed TiC grains with some porosity located both at grain boundaries and within the grain interiors. In both cases, self-propagating high-temperature synthesis (SHS)/dynamically compacted (DC) and hot-pressed, the TiC is ordered cubic (NaCl-structure,B ₁; Space Group Fm3m) with a lattice parameter of ≈0.4310 nm, indicative of a slightly carbon deficient structure; stoichiometric TiC has a lattice parameter of 0.4320 nm. For the most part, the grains were free of dislocations, although some dislocation dipoles were found associated with the voids within the grain interiors. In one SHS/DC specimen, a new, complex Ti-Al-O(C) phase was observed. The structure could not be matched with any previously published phases but is believed to be hexagonal, with a c-axis/a-axis ratio of ≈6.6, similar to the AlCTi₂ phase which has a point group 6 mmm. In all other SHS/DC TiC samples, the grains and grain boundaries were devoid of any second-phase particles. The hot-pressed TiC exhibited a greater degree of porosity than the SHS/densified specimens and a large concentration of second-phase particles at grain boundaries and within grains. The structure and composition of these second-phase particles were determined by con-vergent beam electron diffraction (CBED) and X-ray microanalysis. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

连续冷却过程中铜含量对析出的影响

 研究了一系列含铜钢在连续冷却过程中的析出行为。采用1℃/s的冷却速度以及中间淬火取样方法获得铁素体组织。利用金相显微镜和透射电镜研究不同铜含量对析出的影响。结果表明：当铜的质量分数高于1.5%时,随着铁素体的产生,在铁素体内产生第二相析出颗粒。当铜的质量分数低于1%时,并未观察到第二相析出。第二相析出物为富铜颗粒,以相间沉淀方式析出。     

Segregation structures of melt-spun CuSiB alloys and their high temperature deformation behaviour

Rapidly-solidified microcrystalline alloys typically possess a fine-grained matrix with solute distributed in a cellular arrangement, either in the form of enriched cell-wall regions or as second-phase particles at the original cell walls. It is unclear whether optimal strength would be achieved when a second phase is distributed as discrete particles or as a continuous film. The microstructures and mechanical properties of a series of CuSiB alloys were examined both at room temperature and at high temperature. Depending on the alloy considered, fairly stable amorphous films, or discrete particles, arranged in a cell structure, are formed by rapid solidification. The continuous second-phase film is shown to lead to higher average internal stresses and material deformation process, during hot testing, appears to be the same as that controlling deformation in the particle-strengthened materials.     

重熔对Al-Ti-B-RE细化剂的影响

采用熔铸法制备Al-Ti-B-RE细化剂,并就重熔温度及次数对该细化剂合金组织第二相粒子形貌、分布规律及细化效果的影响进行了研究。结果表明:细化剂重熔后原弥散分布的第二相粒子呈现偏聚现象,随着重熔温度升高,第二相粒子由细小颗粒状逐渐团聚为块状、短棒状、针片状;细化剂受重熔次数的影响不大,多次重熔后仍具有一定的细化能力。     

Effect of Thorium Additions on Metallurgical and Mechanical Properties of Ir-0.3 pct W Alloys

Metallurgical and mechanical properties of Ir-0.3 pct W alloys have been studied as a function of thorium concentration in the range 0 to 1000 ppm by weight. The solubility limit of thorium in Ir-0.3 pct W is below 30 ppm. Above this limit, the excess thorium reacts with iridium to form second-phase particles. Thorium additions raise the recrystallization temperature and effectively retard grain growth at high temperatures. Tensile tests at 650 °C show that the alloy without thorium additions (undoped alloy) fractured by grain-boundary (GB) separation, while the alloys doped with less than 500 ppm thorium failed mainly by transgranular fracture at 650 °C. Intergranular fracture in the doped alloys is supressed by GB segregation of thorium, which improves the mechanical properties of the boundary. The impact properties of the alloys were correlated with test temperature, grain size, and heat treatment. The impact ductility increases with test temperature and decreases with grain size. For a given grain size, particularly in the fine-grain size range, the thorium-doped alloys are much more ductile and resistant to GB fracture. All of these results can be correlated on the basis of stress concentration on GBs by using a dislocation pileup model.     

Ti微合金钢热变形后连续冷却相变及第二相析出行为

采用热力模拟试验技术、金相显微镜(OM)、扫描电镜(SEM)及透射电镜(TEM),并结合宏观硬度测试,研究了钛微合金钢形变奥氏体的连续冷却相变和组织演变规律,建立了试验钢连续冷却转变曲线,探讨了不同冷速对第二相析出的影响。结果表明,不同冷速下,试验钢获得了不同的微观组织,随冷速增加,第二相析出量增多,尺寸更细小,但冷速过高,析出被抑制。     

船板钢不同N含量第二相粒子变化规律

利用萃取复型和焊接热模拟技术,对不同N含量的EH36船板钢第二相粒子的形态、尺寸及分布情况进行了分析。结果表明,高N含量钢中的第二相粒子数量多于低N含量钢,并且粒子分布更加弥散。热模拟时间为50 s时,粒子在热循环过程中以小粒子的消失为主要特征,150 s时,粒子不仅发生溶解和长大,在钢中还出现了独立形核的第二相粒子。     

三元第二相平衡固溶的计算方法及其工程应用

 钢中第二相的合理控制对于提高钢铁材料的综合性能具有重要意义,而第二相控制的关键在于准确掌握第二相在铁基体中的固溶与析出行为。多元第二相平衡固溶的定量计算是第二相固溶与析出行为研究中的一个难点问题。本文以铌、钒、钛的微合金碳氮化物为例讨论了钢中三元第二相平衡固溶的热力学模型与计算方法及其在含钛渗碳齿轮钢产品开发中的应用。由于该方法求解计算量较大,我们在其基础上开发了一套可以自动计算和对比分析其中各个控制参量的通用分析计算软件,该软件可以有效应用于微合金钢的成分设计、生产工艺控制和钢种开发等,具有重要的工程应用价值。     

Nb-Ti微合金钢热变形后组织演变及第二相粒子析出行为

利用GLEEBLE-2000对试验钢进行热模拟试验,并结合显微硬度测试,研究了一种Nb-Ti微合金钢热变形奥氏体的变形后冷却相变行为.利用JEM-2000FX电镜对碳膜萃取复型法获取的第二相析出物进行分析,从而对热变形后冷却相变中的第二相析出行为进行了研究,探讨了不同冷却速度对第二相析出的影响,结果表明,有大量弥散分布的细小粒子析出,析出相主要以Nb-Ti碳氮化物复合相形式存在,一般呈方形、椭圆形、圆形以及不规则形状,并且随着冷却速度的增加,第二相析出增多且变得细小.     

Effect of clustering of precipitates on grain growth

The present work shows that clustering of particles promotes deviation in the classical mathematical expressions describing the grain growth control by second-phase particles. On the basis of experimental results and theoretical laws, a semiempirical expression to predict the limiting grain size is presented. The latter expression takes account of agglomeration phenomena and can be extended to large volume fractions of particles, conditions under which classical theories clearly fail. The equation remains valid as far as the nucleation of precipitates takes place at random. From a practical point of view, it is shown that volume fractions larger than 0.12 cannot significatively control the grain size because of the increased probability for clustering.     

Effect of clustering of precipitates on grain growth

The present work shows that clustering of particles promotes deviation in the classical mathematical expressions describing the grain growth control by second-phase particles. On the basis of experimental results and theoretical laws, a semiempirical expression to predict the limiting grain size is presented. The latter expression takes account of agglomeration phenomena and can be extended to large volume fractions of particles, conditions under which classical theories clearly fail. The equation remains valid as far as the nucleation of precipitates takes place at random. From a practical point of view, it is shown that volume fractions larger than 0.12 cannot significatively control the grain size because of the increased probability for clustering.     

微合金钢第Ⅲ脆性区的形成机理

为研究微合金钢第Ⅲ脆性区形成机理及其影响因素,控制连铸坯的表面裂纹,采用Gleeble热力模拟机测定了S355微合金钢在不同温度下的抗拉强度及断面收缩率。使用扫描电镜对拉伸断口进行观察分析,同时采用透射电镜对析出物进行观察分析。在此基础上对拉伸试样进行金相实验,对第二相析出进行热力学计算,分析了组织状态及第二相析出规律对脆性区的影响。结果表明,在第Ⅲ脆性区(660~850℃)内,拉伸断口呈冰糖状,韧窝较浅,形貌表现为沿晶脆性断裂。铁素体网膜沿奥氏体晶界优先析出、第二相沿晶界析出是第Ⅲ脆性区形成的主要原因。     

Influence of severe plastic deformations on secondary phase precipitation in a 6082 Al-Mg-Si alloy

The role of severe plastic deformation on the second-phase stability in a 6082 Al-Mg-Si alloy was studied using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The alloy was fully annealed prior to undergoing up to six equal channel angular pressing (ECAP) passes using route C. The Orowan strengthening mechanism was calculated on the basis of TEM inspections for the two hardening second-phase precipitates: Mg₂Si and Si. The former had a major tendency to be cut and fragmented by dislocations, while in the latter, a dissolution process was induced by severe plastic deformation. Accordingly, the second-phase Si particles became progressively less effective with increasing deformation (i.e., additional ECAP passes). The increase in hardness with the ECAP passes was mostly due to the grain refining mechanism and to dislocation tangles within the newly formed grains. The expected, though if limited, contribution of second-phase hardening was prevalently accounted for by the Mg₂Si particles.     

Microstructure of second-phase particles in Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd alloy

The microstructure of second-phase particles in the Ti-55 alloy (Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd) was studied by scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM) observations. The second-phase particles in the conventional ingot-cast Ti-55 alloy of 1 to 15 μm in diameter and uniform distribution in matrix were observed, where the majority of these particles are elliptical. The mean free path between the particles is about 46 μm, and the volume fraction (pct) is 2.35. The second-phase particles typically contain Nd, Sn, and O in substantial amounts, and the content of Nd is the largest in the three elements. The elements Ti, Al, Zr, Mo and Si are depleted in the particles. The second-phase particle consists of either a dark or bright matrix and some small dark blocks dispersed within the matrix. Dark blocks match SnO (orthorhombic, a=0.500 nm, b=0.572 nm, and c=1.120 nm), and the matrix consists of a nanocrystalline phase with a stoichiometric Nd₃Sn structure having a space group of Pm3m and lattice parameter of a=0.344 nm. The grain size of the nanocrystalline Nd₃Sn phase is about 3 to 15 nm. The melting range of the second-phase particle is estimated to be 1042 °C to 1600 °C. The microstructure of the second-phase particles in the quenched Ti-55 alloy was also studied. Fine and uniform dispersoids (6 to 15 nm in diameter) were observed in the as-quenched state. Some lenslike particles occur at the grain boundaries, other elliptical particles appear within the grains, and some particles within the grains form rows which are parallel to the advancing liquid-solid interface. After annealing at 980 °C (1 to 10 hours), of the as-quenched Ti-55 alloy, coarse particles are 17 to 42 nm in average diameter, and the growth of the particles is very slow. The dispersoids in the as-annealed Ti-55 alloy are identified as nanocrystalline Nd₅Sn (orthorhombic, Pnmn, a=0.814 nm, b=1.732 nm, and c=0.814 nm) intermetallic compound, and the interface between the Nd₅Sn₄ phase and the matrix is a typical high-angle grain boundary.     

Precipitation of austenite particles at grain boundaries during aging of Fe-Mn-Ni steel

Precipitation of austenite particles at grain and lath boundaries after aging treatment of a Fe-8Mn-7Ni alloy was investigated by selected area electron diffraction (SAD), X-ray energy dispersive spectrometry (EDS) in a scanning transmission electron microscope (STEM), and high-resolution (HRTEM) analysis. High spatial-resolution (2 to 5 nm) EDS analysis revealed no significant segregation of alloying elements at grain boundaries but the precipitation of very fine particles of Mn- and Ni-rich phase. Detailed EDS, SAD, and HRTEM analyses all confirmed that these particles are austenite phase, which have a Kurdjumov-Sachs (K-S) orientation relationship with one of the adjacent grain. The concentration of Mn and Ni in austenite, measured by EDS, varied from ∼15 pct to a maximum of ∼30 pct. Low-voltage scanning electron microscopy (SEM) fractographs also revealed the presence of very fine, second-phase precipitates on the fracture surface of the embrittled alloys.     

The influences of multiscale-sized second-phase particles on ductility of aged aluminum alloys

Commercially aged aluminum alloys commonly contain second-phase particles of three class sizes, and all contribute appreciably to the mechanical properties observed at the macroscopic scale. In this article, a multiscale model was constructed to describe the individual and coupling influences of the three types of second-phase particles on tensile ductility. The nonlinear relationships between the parameters of particles, including volume fraction, size, aspect ratio, shape, and ductility, were then quantitatively established and experimentally validated by the measured results from disc-shaped precipitate containing Al-Cu-Mg alloys and needle-shaped precipitate containing Al-Mg-Si alloys, as well as by using other researchers’ previously published results. In addition, we discuss extending this model to predict the fracture toughness of aluminum alloys.     

The plasticity of particle-containing polycrystals

The lattice misorientations adjacent to second-phase particles of silicon in a polycrystalline aluminium matrix deformed in compression have been measured by a TEM microtexture technique. The results have been analyzed in terms of a simple model which is based on a modification of Taylor's polycrystalline plasticity theory. A model in which independent deformation zones are formed in the vicinity of each particle for each active slip system, gives reasonable agreement with the experiments if overlap of the deformation zones is taken into account. The number of deformation zones formed at a particle and the size of the misorientation are found to be functions of particle size as well as strain.     

B7-1 (CD80), B7-2 (CD86), interleukin-12 and transforming growth factor-beta mRNA expression in CSF and peripheral blood mononuclear cells from multiple sclerosis patients

Injection of formalin into the hind paw of mice produced a biphasic nociceptive response consisting of immediate (first-phase) and tonic (second-phase) components. Although the duration of the first-phase response was significantly longer in diabetic mice than in nondiabetic mice, the second phase was significantly shorter in diabetic mice. The first-phase response was dose-dependently and significantly reduced by pretreatment with calphostin C (0.3 to 3 pmol, i.t.), a specific protein kinase C inhibitor, in diabetic mice. The second-phase response was markedly increased when diabetic mice were pretreated with calphostin C. However, calphostin C (3 nmol, i. t.) had no significant effect on either the first-phase or second-phase response in nondiabetic mice. On the other hand, pretreatment with phorbol-12,13-dibutyrate (50 pmol, i.t.), a protein kinase C activator, significantly enhanced the first-phase response in nondiabetic mice. These results suggest that the change in the formalin-induced nociceptive response in diabetic mice may be due, at least in part, to the modification of nociceptive transmission in the spinal cord by the activation of protein kinase C.     

DISSIMILAR-SURFACE CONTACTS AND THEIR INFLUENCE ON SINTERING CHARACTERISTICS OF POWDER COMPOSITE COMPACTS

Abstract

In compacts containing metal and oxide particles, metal/oxide surface contacts modify the diffusional processes. Different distributions of such dissimilar-surface contacts and their influence on the sintering characteristics of some metal-oxide compacts are considered.

By selecting appropriate volume fractions and diameters of matrix and second-phase particles, structures containing different types of dissimilar surface contact distribution have been produced. Isochronal sintering of these structures clearly demonstrates that (a) the presence of chemically inert second-phase particles always hinders densification of the composite, (b) the sintering densification (for a given volume fraction of second-phase particles) is a strong function of (d_m/d_s), where d_m and d_s are the diameters of the matrix and second-phase particles, respectively, and (c) the nature of dissimilar-surface contact distribution has a significant effect on the densification of the composite.     

Effects of alloy modification and thermomechanical processing on recrystallization of Al-Mg-Mn alloys

The 5083 Al alloy (Al-4.75Mg-0.8Mn) holds potential for superplastic forming (SPF), but slow rates of forming limit its use for many applications. Higher strain rates are believed possible through the development of finer grained microstructures or stabilized subgrain structures. Grain sizes after recrystallization and recrystallization characteristics are known to be dependent on the amount and distribution of second-phase particles in the matrix. In this study, the concentration and sizes of such particles were varied by additions of particle-forming elements of Mn and Zr and by modifications of the rolling and aging schedules (thermomechanical processing (TMP)). The investigation involved studying recrystallization kinetics at different temperatures and correlating the grain sizes with particle sizes and volume fractions. The addition of Mn and Zr, for the composition ranges and TMP methods studied, resulted in a substantial reduction of the recrystallization kinetics, but complete suppression of static recrystallization (or subgrain stabilization) was not observed. However, statically recrystallized grain sizes as small as 6 μm were achieved.