Microstructural formations and phase transformation pathways in hot isostatically pressed tantalum carbides

A series of XTa:(1 − X)C (0.5 < X < 1) compositions have been fabricated by hot isostatic pressing (HIP) of Ta and TaC powder blends. Depending upon the targeted stoichiometry, single- or multiple-phase microstructures formed. The single-phase microstructures of both TaC and Ta₂C had equiaxed grain morphologies. The multiphase microstructures had either equiaxed TaC grains with a crisscross pattern of Ta₄C₃ laths or acicular grain morphologies with rafts of TaC, Ta₄C₃ and Ta₂C laths running parallel to the major axis of the grains. The effect of phase transformations on the microstructure of these specimens is discussed and compared to those microstructures seen in a reaction diffusion couple formed between Ta and TaC powders processed under the same HIP conditions. This couple revealed the depletion of carbon from the TaC phase and its reaction with the tantalum metal to form the various Ta-rich carbide phases. The precipitation sequence was found to be paramount in controlling the grain morphology. A close-packed plane and direction orientation relationship was seen between all the phases. The crisscross pattern of Ta₄C₃ precipitation in TaC was a consequence of TaC’s multiple variant {1 1 1} orientations and had little or no effect on the grain morphology. In contrast, the single variant close-packed plane {0 0 0 1} in Ta₂C resulted in the parallel alignment of the precipitated phases within its grain and an anisotropic growth direction that facilitated the acicular grain morphology.     

Properties and microstructure of VC/Cr3C2-doped WC/Co cemented carbides

This paper deals with the effects of codoped VC/Cr3C2 and sintering temperature on the magnetic and mechanical properties of ultra-fine grained WC-12%Co alloys. Results show that the synergistic action of doped VC/Cr3C2 in optimal proportion enhances both the hardness and transverse rupture strength (TRS) of the alloys, with more homogeneous microstructure. When the alloy is sintered at 1430℃ and with 0.5% Cr3C2/0.2% VC, the TRS reaches 3786 MPa, the hardness is 91.7 HRA and the grain size srnaller than 0.6 μm. The numerical analyses on grain growth during the sintering process show that both VC precipitating on the WC grain boundary and Cr3C2 dissolving in the Co phase decrease the solid/liquid interfacial energy γ, the process of dissolution and reprecipitation is greatly retarded and the coarsening of WC grains is inhibited.     

Preparation of Co powders for cemented carbides in China

Cobalt has been used as a supreme binder in cemented carbides. As the largest production nation of cemented carbides in the world, large amounts of cobalt powders, about 1200 tons in 2005, are consumed in China market. It is known that many factors, both in terms of composition and processing techniques, affect cemented carbides properties that are correlative with the ingredient, particle size, particle shape and microstructure of the cobalt powders. The studies on preparation of Co powders for cemented carbides in China are reviewed in this paper. The fabrication method, particle size and shape of cobalt powders are described and discussed according to the processing techniques. The typically chemical and physical properties of cobalt powders in Chinese production are introduced briefly. In order to further improve the cemented carbides quality, more research works on the microstructure and composition of doped alloying elements cobalt powders should be made in China in future.     

High-temperature plastic flow behaviour in the binder of WC-Co cemented carbides

Co-rich solid solution alloys regarded as the composition of binder phasesat elevated temperatures in WC-Co cemented carbides were fabricated and the high-temperature deformation behaviour of the alloys was investigated. The logarithmic relationship between flow stress and strain rate is expressed by a single straight line with the slope of 0.15 at a constant temperature in all strain rate range examined, unlike in cemented carbides showing the sigmoidal behaviour. The solid solution hardening due to the addition of Cr₃C₂ and VC is negligible in the Co-9WC-lCr₃C₂-0.5VC alloy and the mutual relation in flow stress is different between the cemented carbides and their binder phases in region I. The plastic flow in region I in WC-Co cemented carbides cannot be explained by the flow stress or flow behaviour in the binder phase.     

超细晶粒硬质合金磨削的材料去除机理研究

在使用金刚石砂轮的平面磨床上进行了超细晶粒WC-Co硬质合金的磨削实验研究,通过扫描电子显微镜观察磨削表面形貌,利用X射线能谱仪进行磨削表面元素微区分析,对不同磨削条件下超细硬质合金的材料去除机理进行了研究。研究结果表明:超细硬质合金磨削过程中,随砂轮粒度的增大或切深的增加,材料去除方式渐由滑擦、耕犁向脆性断裂、材料粉末化转变。磨削表面黏结相分布受材料去除方式的影响,以耕犁方式去除的磨削表面Co相分布不均匀程度最大。     

Two- and three-dimensional equilibrium morphology of a misfitting particle and the Gibbs–Thomson effect

The equilibrium shapes of misfitting precipitates in elastically anisotropic systems are obtained in both two and three dimensions, and the corresponding Gibbs–Thomson equation is derived as a function of the characteristic ratio between elastic and interfacial energies, L′. The effect of elastic inhomogeneity is investigated systematically. For soft or moderately hard particles, the stable equilibrium shape bifurcates from a fourfold symmetric shape to a twofold symmetric one in 2D and from a cubic symmetric shape to a plate-like one in 3D. For a very hard particle, the shape bifurcation is not observed in 2D for the range of L′ investigated, but both plate-like and rod-like shapes are found in 3D. The computed Gibbs–Thomson equation is well approximated by a piecewise linear function of L′. Predictions are made for coarsening of many-particle systems based on an established mean-field theory. The results predict that the elastic stress has no effect on coarsening kinetics where most particles are highly symmetric (fourfold in 2D and cubic in 3D), and the exponent remains 1/3 but the rate constant increases if stress is sufficient to induce symmetry-breaking bifurcation on most particles.     

Change in relative density of WC-Co cemented carbides in spark plasma sintering process

The relative density of WC-Co cemented carbides during spark plasma sintering (SPS) was analyzed.Based on the change in displacement of the ram in the SPS system,the relative densities in the sintering process can be achieved at different temperatures.The results indicated that densification of the samples started at near 900℃,the density rapidly reached its maximum at the increasing temperature stage,in which the temperature was lower than the sintering temperature of 1200℃,and most of the densification took place in the stage.Besides,the theoretcal values were consistent with the experimental results.     

The study of ternary carbides formation during SPS consolidation process in the WC–Co–steel system

Tungsten carbide has a wide range of applications, mainly cemented carbides made of WC and Co, as wear resistant materials. However, the high cost of WC–Co powders encourages the use of a substrate to manufacture a functionally graded material (FGM) tool made of WC–Co and a tool steel. These materials join the high wear resistance of the cemented carbide and the toughness of the steel. This work deals with the study interaction of the WC–Co and H13 steel to design a functionally graded material by means of spark plasma sintering (SPS). The SPS, a novel sintering technique reaching the consolidation of the powders at relatively low temperatures and short dwell times, is a promising technique in processing materials. In this study, WC, H13 steel, WC–Co, WC–H13 steel and WC–Co–H13 steel bulk samples were investigated using scanning electron microscopy and X-ray diffraction techniques to evaluate the phase transformations involved during SPS consolidation process. The W₂C and W₃Fe₃C precipitation were identified after the SPS consolidation of the WC and WC–H13 steel samples, respectively. The precipitation of W₄Co₂C was also identified in the WC–Co and WC–Co–H13 steel samples. The WC–H13 steel and WC–Co–H13 steel were also evaluated after heat treatments at 1100 °C for 9 h, which enhanced the chemical interaction and the precipitation of W₃Fe₃C and W₄Co₂C, respectively.     

硬质合金气氛烧结过程中碳含量调整

阐明了碳含量对硬质合金质量控制的重要性以及国内外对用甲烷氢气混合气调整合金碳含量的研究情况。为了在两相区范围内对合金的碳势进行调整,从热力学上估算了不同温度下混合气体浓度的临界值,与试验结果基本符合。优化烧结气氛的试验结果显示:通过选择合适的烧结气氛,对WC-10Co合金性能的提高有明显作用。     

WC-Co硬质合金结构参数对烧结表面残余应力的影响

通过建模分析和实际测量,对WC-Co硬质合金结构参数与烧结表面残余应力之间的关系进行了研究。结果表明,硬质合金的结构参数对烧结表面残余应力有很大的影响。相同WC粒径条件下,随Co相体积分数的增大,硬质合金烧结表面残余应力的数值增大。当Co相体积分数固定时,增大WC颗粒的粒径,可明显降低硬质合金烧结表面的残余应力。实际测量结果与分析结论相吻合。     

Analysis of local composition gradients in the hard-phase grains of cermets using a combination of X-ray diffraction and electron microscopy

Combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used for investigation of local composition gradients in the hard-phases of cermets. XRD revealed distribution of lattice parameters in hard-phase grains, from which the composition gradients in the hard-phases were estimated using an appropriate microstructure model. This microstructure model was build with the aid of SEM micrographs, which were taken with back-scattered electrons (BSE) and completed by the registration of the electron back-scatter diffraction (EBSD) patterns and characteristic X-ray spectra. SEM/BSE yielded the first information about the spatial distribution of elements in individual hard-phase grains, SEM/EBSD about the morphology, the size and the size distribution of these grains. The final interpretation of the distribution of lattice parameters, which was obtained from the X-ray line profile analysis, was done with the aid of the local elemental analysis that was performed using SEM with the energy dispersive analysis of the characteristic X-ray spectra (EDX) and the known dependence of the lattice parameters on concentration.     

Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides

In this work, the effects of 1.0 wt.% additions of Mo and Co on the microstructure and properties of WC-TiC-Ni cemented carbides were investigated using scanning electron microscope, mechanical properties tests, corrosion resistance and abrasion resistance tests. The results show that 1.0 wt.% Mo addition can refine the WC grains and increase the hardness. Moreover, with the addition of minor Mo, the corrosion resistance and abrasion resistance of alloys improved significantly. The addition of 1.0 wt.% Co can inhibit the growth of WC grains, improve the density and hardness slightly, and enhance the abrasion resistance of cemented carbides. However, the minor Co has negative effect for the corrosion resistance.     

电冶稀土WC钢结硬质合金中碳化物特征的研究

采用电渣熔铸冶金工艺,用回收的碳化钨钢结硬质合金作原料,再加入微量的稀土,制备了新型的钢结硬质合金。用透射电镜、扫描电镜、金相显微镜及X射线衍射等测试手段研究了新材料中的碳化物的特征。结果表明：在该材料的显微组织中存在原始颗粒区和扩散区。原始颗粒区组织中碳化物的特征是存在大量变化甚微的角状大尺寸原始WCp及具有明显反应层的长椭圆状WC颗粒,界面反应产物为Fe3W2C。在粗大碳化钨颗粒附近能够原位生成先共晶析出相（WC和W2C）,在较远处的钢基体中分布着细网状碳化物,同时有（Cr,Fe）7C3等条状复式碳化物生成;而扩散区主要细小W2C小颗粒、WC颗粒及再结晶W-Fe-C小颗粒组成。稀土的加入可提高WC颗粒分散性。     

Interfacial strained structure and orientation relationships of the nanosized oxide particles deduced from elasticity-driven morphology in oxide dispersion strengthened materials

Fe-Cr-Ti-Y₂O₃ oxide dispersion strengthened materials have a high density of nano-oxides (Y, Ti, O) that are stable at high temperature. In this work based on transmission electron microscopy it is shown that the interfacial strained structure and orientation relationship can be determined by using the elasticity-driven morphology of the nanosized particles after coarsening at 1573 K. In a Fe-14 Cr-1 W-0.3 Ti-0.3 Y₂O₃, most of the coarsened phases found are Y₂Ti₂O₇ particles adopting a cubical shape. They are embedded with a cube-on-cube orientation relationship with the matrix, while a few Y₂O₃ particles are embedded in both cube-on-cube and cube-on-edge orientation relationships with the matrix. This morphological transformation could reveal the coherent misfitting character of the nanosized parent phases. The kinetics of the elasticity-driven morphology is different for both phases. Based on the supersphere approach, the elastic and interface energy calculation is proposed. The corresponding value of the (1 0 0) interface energy is 260 mJ m⁻² for Y₂Ti₂O₇ particles and 350 mJ m⁻² for Y₂O₃ particles. The particle evolution during thermal annealing and (Y, Ti, O) coarsening resistance system is then discussed.     

Effect of WC particle size on the microstructure, mechanical properties and fracture behavior of WC–(W, Ti, Ta) C–6 wt% Co cemented carbides

This study deals with the microstructure and mechanical properties of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides fabricated by conventional sintering. The conventional WC particles of 4 μm size and ultrafine particles of 0.2 μm were introduced in the system with varying ratio. The ratios of conventional WC particles to ultrafine WC particles were 2:1, 1:1, and 1:2. The microstructures of sintered WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides were sensitively dependent on the ratio of conventional WC particles to ultrafine WC particles. The rim phase increased with the increase in the amount of ultrafine particles. Hardness of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbide increased with increase in the amount of rim phase and decrease in the average grain size of WC particles. The bending strength showed the similar trend of the hardness. The fracture morphologies are reported. The fracture behavior changed from mixed mode to transgranular fracture mode, when the ratio of conventional WC particles to ultrafine WC particles was changed from 2:1 to 1:2.     

Application of a wax-based binder in PIM of WC–TiC–Co cemented carbides

A wax-based binder was developed for powder injection molding of WC–TiC–Co cemented carbides. The critical powder loading and the rheologic behavior of the feedstock were determined. It was found that the critical powder loading could achieve up to 62.5 vol% and the feedstock exhibited a pseudo-plastic flow behavior. The injection molding, debinding and sintering processes were studied. The dimensional deviation of the sintered samples could be controlled in the range of ±0.2% with the optimized processing parameters. The mechanical properties were better than or equivalent to those of the same alloy made by the conventional press-sintering process.