Microstructural development during the liquid-phase sintering of VC-Co alloys

A study was made of the microstructural changes that occur during the liquid-phase sintering of VC-Co alloys. It was found that considerable growth of the carbide grains takes place and that this growth can be described by the equation: $$\bar d^3 - \bar d_o^3 = Kt$$ The value of the rate constant,K, At 1450°C was found to be in good agreement with that predicted by a modified Wagner analysis for a solution/precipitation process controlled by diffusion through the liquid. The observed variations in the growth rate both with temperature, between 1380 and 1500°C, and with the volume fraction of the liquid phase are also consistent with this theory. The contiguity of the carbide grains at the start of sintering is relatively high and is affected by the pre-sintering history of the specimen. During the early stages of sintering, however, the contiguity falls rapidly to a low value which subsequently remains relatively constant. Observations were made which suggest that the carbide does not form a continuous skeleton during sintering.     

Microstructural development and sintering of NdFeB alloys

The reactions and phase changes occurring during sintering of NdFeB permanent magnet alloys were studied by differential thermal analysis and scanning electron microscopy. The powders were produced by hydrogen decrepitation and on heating, hydrogen evolution occurred in two stages: firstly from the matrix phase (170 °C) and then from the neodymium-rich phase (350–750 °C). The neodymium-rich phase melted at 630 °C and no significant microstructural changes occurred below this temperature. Above 630 °C the neodymium phase is mobile, wetting the matrix grains and leading to their magnetic isolation. Densification occurs by a solution/precipitation mechanism.     

Microstructural development of ZnO varistor during reactive liquid phase sintering

The microstructural evolution, grain growth and densification for the varistor systems ZnO-Bi₂O₃ (ZB), ZnO-Bi₂O₃-Sb₂O₃ (ZBS), ZnO-Bi₂O₃-Sb₂O₃-MnO-Cr₂O₃-CoO (ZBSCCM) were studied using constant heating rate sintering, scanning electron microscopy (SEM) andin situ phase formation measurement by high temperature X-ray diffraction (HT-XRD). The results showed that the densifying process is controlled by the formation and decomposition of the Zn₂Bi₃Sb₃O₁₄ pyrochlore (PY) phase for the ZBS and ZBSCCM systems. The addition of transition metals (ZBSCCM system) alters the formation and decomposition reaction temperatures of the pyrochlore phase and the morphology of the Zn₇Sb₂O₁₂ spinel phase. Thus, the spinel grains act as inclusions and decrease the ZnO grain growth rate. Spinel grain growth kinetics in the ZBSCCM system showed ann value of 2.6, and SEM and HT-XRD results indicate two grain growth mechanisms based on coalescence and Ostwald ripening.     

Microstructural evolution of Y123/Ag composites during sintering

The grain-growth kinetics of YBa2Cu3O7–xX (Y123) and the coarsening kinetics of silver inclusions in Y123/Ag composites during sintering were investigated. The sintering was carried out in the temperature range 900–950°C. The addition of silver lowered the formation of a liquid phase and the grain growth of Y123 in Y123/Ag composites was thus enhanced at the beginning of sintering. However, as the effective silver content was increased, more silver inclusions became interconnected, and the grain growth kinetics and coarsening kinetics slowed down significantly. This may be due to the mass transportation paths progressively changing from the grain boundaries to interfacial boundaries as the amount of interconnected silver networks is increased. The grain growth kinetic constant was calculated and compared with other published data. Because the inclusion was ripened with the growth of matrix grains, the coarsening of silver inclusions was deduced to be a coalescence process.     

Direct observation of liquid-phase sintering in the system iron-copper

The hot-stage of a scanning electron microscope has been used to observe liquid-phase sintering in the system iron-copper. The densification behaviour of compacts of Fe and Cu particles were determined. The influence of particle size of both components and the amount of liquid phase developed were investigated. In samples with about 20 vol % liquid phase, the densification kinetics as observed by direct observation shows that no rearrangement takes place. In samples with 40 vol % liquid phase and particle sizes of 10 to 20 µm, some rearrangement was observed.     

Direct observation of liquid-phase sintering in the system tungsten carbide-cobalt

The hot-stage of a scanning electron microscope has been used to observe liquid-phase sintering in the system tungsten carbide-cobalt. Densification behaviour and the mechanism for the first, fast stage of sintering have been determined; the influence of particle size and the amount of liquid phase has been investigated. In all samples the densification kinetics is that of a rearrangement process; direct observation confirmed this result.     

Laser sintering of Cu-Sn-C system P/M alloys

The Cu-Sn-C system P/M alloys were sintered by laser. The influence of laser sintering on the properties of the laser-sintered materials was investigated. The properties, such as the density, impact toughness, hardness and wear resistance of the laser-sintered materials are desirable. A maximum impact toughness of 11.7 J/cm² was achieved and the wear resistance of the laser-sintered materials was tremendously increased compared with conventional sintering. The green compact made from Cu-Sn-C powders, with a thickness of 10 mm, can be penetratively sintered by using a suitable processing condition. The laser sintering was characterised by a thermal cycle which was accomplished in a relatively short time, resulting in a relatively high temperature (950 °C). The curves showing temperature profiles for the top surface and bottom of the laser-sintered specimen were recorded by thermocouples. Differences in temperatures between the top surface and bottom were very small, less than 60 °C. Some intermetallics/phases such as -Cu, Cu₃₁Sn₈(), Cu₄Sn() and Cu₆Sn₅() were most readily produced in the laser-sintered materials and their distributions were more homogeneous than conventional sintering, owing to the better flow ability of the melt caused by a relatively high sintering temperature. The behaviour of diffusion between Cu and Sn was also studied by means of simulation diffusion couple.     

Microstructural development in relation to hot working of titanium alloys

Abstract

The relationships between titanium alloy microstructures and mechanical properties are reviewed. The influence of hot working on microstructure development is discussed, and developments in high temperature processing, including superplastic forming and isothermalforging, and the use of hydrogen as a temporary alloying addition are considered. Recent work aimed at modelling the high temperature deformation and microstructure development is also examined.

MST/1265     

Microstructural development in the oxidation-induced phase transformation of Fe-Al-Cr-Mn-C alloys

Electron microscopic analysis has been used to investigate the interfacial phenomena in a high-temperature alloy system. During high-temperature oxidation of Fe-8.9Al-3Cr-31 Mn-0.87C alloy at 800 and 1000° C, an oxidation-induced transformation a layer was observed between the matrix and the oxide scale. The morphology of the oxidized sample was studied by scanning electron microscopy, and the elemental redistribution of the constituents was evaluated using an electron microprobe. The concentration of elements was detected with the ZAF-corrected quantitative program. It is believed that the formation of the layer was caused by the selective oxidation of manganese during the oxidation process. The thickness of the layer increased with both oxidation time and temperature, the temperature having the greater influence on this transformed layer. In addition, the interfacial concentrations at the/ and/oxide boundaries were employed to investigate the selective oxidation of manganese.     

Microstructural evolution during liquid-phase sintering of high-speed steel-based composites containing TiN-coated Al2O3, TiC,or Al2O3 particles: influence on wear properties

A high-speed steel containing 10 vol% TiN-coated Al₂O₃, Al₂O₃, or TiC particles was liquid-phase sintered with the addition of copper-phosphorus. The mixtures were sintered in a furnace with a controlled axial temperature gradient, and their microstructural evolution during sintering was studied using quantitative image analysis. Additional samples were fully densified and tested using a pin-on-disc tribometre. Interfaces were characterized by transmission electron microscopy. The final microstructure depended on the behaviour of the liquid phases surrounding the ceramic particles during sintering, which was different for the three types of particles used, and influenced the wear resistance.     

Microstructural evolution during the supersolidus liquid phase sintering of nickel-based prealloyed powder mixtures

A novel concept for full-density sintering is described. Two prealloyed powders with slight compositional differences are tailored to separate the solidus temperatures into high-melt and low-melt compositions. A mixture of these two powder compositions allows full-density sintering at a temperature between the two solidus temperatures. For these experiments, the two powders were nickel-based alloys, where the low-melt powder contained boron. The mixed powders were sintered at temperatures above the solidus of the low-melt powder to form a transient liquid that promoted rapid densification of the mixture. Microstructure evolution during sintering was assisted using quenching experiments. Variables in this study included the heating rate, peak temperature, hold time, and powder ratio. Interdiffusion between the two powders controls microstructure evolution, with a dominant role associated with boron diffusion and reaction. The transient liquid phase responsible for densification is linked to boron diffusion and subsequent compound precipitation.     

Effect of WC nanoparticle size on the sintering temperature,density, and microhardness of WC-8 wt % Co alloys

Using X-ray diffraction, scanning electron microscopy, and density measurements, we have studied the effect of WC particle size (20 to 150 nm) on the optimal sintering temperature of the WC-8 wt % Co alloy and the effect of sintering temperature (800–1600°C) on its phase composition, density, and microhardness. The results indicate that, during sintering of the starting powder mixture, the first to form is the ternary carbide phase Co₆W₆C. At sintering temperatures of 1100°C and above, this phase reacts with carbon to form Co₃W₃C. Sintering above 1000°C leads to the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co〈WC〉, along with the ternary carbide phases. The density and microhardness of the alloy have been measured as functions of sintering temperature. The use of WC nanopowder has been shown to reduce the optimal sintering temperature of the WC-Co alloy by about 100°C.     

VC/Cr3C2添加剂与烧结温度对超细WC-12Co硬质合金的影响

为了有效控制烧结过程中WC晶粒的长大,获得高强度高硬度的超细硬质合金,采用扫描电镜、拉伸机和洛氏硬度仪研究了不同质量分数及配比的VC/Cr3C2晶粒长大抑制剂和烧结温度对超细WC-12Co硬质合金的显微组织及力学性能的影响,并结合试验结果分析了超细硬质合金中VC/Cr3C2晶粒长大抑制剂的作用机理.结果表明,添加适量VC/Cr3C2晶粒长大抑制剂的超细硬质合金中WC晶粒尺寸分布集中,不存在明显的组织缺陷,合金具有细而均匀的微观组织及优异的力学性能.当晶粒长大抑制剂(质量分数)为0.2%VC/0.5%Cr3C2,1450℃烧结制备WC-12Co超细硬质合金的抗弯强度为3710MPa,硬度(HRA)为91.5.VC/Cr3C2晶粒长大抑制剂的作用机理为:VC主要与WC反应生成(W,V)C固溶体聚集在WC/Co界面,降低WC/Co界面能,Cr3C2主要固溶在粘结相中,导致WC在粘结相中的溶解度降低,二者的综合作用减缓了粘结相中WC溶解-析出过程,从而抑制烧结过程中WC晶粒的长大.     

Optical dilatometry for the control of microstructure development during sintering

An optical method for the in situ measurement of sintering shrinkage has been developed. It relies on a simple and robust CCD camera technique and is applicable up to high temperatures (2000 °C) and heating rates (10 K/min) in industrial furnace atmospheres. A resolution of less than 1 μm for a sample diameter of 10 mm has been achieved using a special geometrical arrangement and a smart image analysis algorithm. Systematic errors are small (<0.1%) and can be corrected by an additional measurement. A high reproducibility of shrinkage measurement has been observed in various sintering experiments. The optical dilatometer is very flexible: it has been used also for an accurate temperature calibration and for wetting experiments and it can be adjusted to different furnace types. Received: 17 May 2000 / Reviewed and accepted: 26 June 2000     

Aspects of phase equilibria in Fe/Co/2.5 to 3.0% V alloys

Remendur alloys, one of which is used as the base material in remanent reed sealed contacts for the communications industry, are ternary alloys containing approximately equal iron and cobalt with 2 to 4 wt % vanadium. The available equilibrium diagram for this system does not provide precise positioning of the phase boundaries in the regions of commercial interest and, consequently, does not permit accurate determination of the amounts of the phases present or their compositions. This paper reports on the precise determination of five tie lines in theα ₁+γ two-phase field in the region of interest, by metallographic and microprobe techniques. In the 900 to 950° C range, this field was found to be narrower than expected from published data. Submicron fcc (γ) particles form during annealing at 600° C by decomposition of a non-equilibrium bcc (α₂) phase into a secondary ordered bcc phase (α′₁) and stabilizedγ. Deformation, by drawing and by stamping, enhances coercivity in these alloys by promoting a more uniform, more finely divided dispersion ofγ particles. Annealed microstructures are especially sensitive to vanadium content, annealing temperature, and annealing time.