Structural Changes Induced in Nonstoichiometric Titanium Carbide by High-Temperature Deformation

The microstructure evolution in nonstoichiometric titanium carbide is studied during high-temperature deformation at high strain rates and low strains (shock compression) and at slow strain rates and high strains (superplastic regime). The results demonstrate that high-temperature deformation in a broad range of strain rates offers a means of controlling the microstructure of titanium carbide. By varying deformation conditions, one can obtain materials differing in microstructure and chemical composition, in particular, with equilibrium and nonequilibrium microstructures. Accordingly, the physicochemical properties of such materials also differ.     

High-temperature deformation of fully-dense fine-grained boron carbide ceramics: Experimental facts and modeling

Boron carbide ceramics are the hardest material in Nature after diamond and the cubic phase of boron nitride. Due to this fact, their room-temperature fracture properties are the object of intense research. Paradoxically, high-temperature deformation is essentially unknown, because very high temperatures and stresses are necessarily required and high-quality specimens have not been available until recently. In this paper, the high-temperature compressive creep of fine-grained boron carbide polycrystals is reported. The breakdown of the classical power-law for high-temperature plasticity in ceramics is found. An analytical model is proposed. The model assumes that deformation is produced by dislocation glide. However, since the formation of twins is energetically favorable in this material and they act as strong barriers for dislocation glide, their motion turns to become progressively more difficult as elongation proceeds. The combination of increasing twin barriers and dislocations in mutual interaction is proposed to be the mechanism for high-temperature plasticity in this material. The model is validated with the experimental results. Final elongation of boron carbide specimens is reported to be over 100%, although this material cannot be described as a superplastic ceramic.     

The influence of high-energy impacts on the microstructure of synthesized metal ceramics

On the example of the metal-ceramic alloy of titanium carbide (TiC) with nickel-chromium (Ni-Cr) binder, the comparative analysis of the influence of different high-energy impacts on the dispersion of the internal structure and phase composition of the synthesized metal ceramics 70 vol % TiC + 30 vol % (Ni-Cr) has been performed for the first time (self-spreading high-temperature synthesis (SSHTS) under pressure, preliminary mechanical activation (MA) of metal components of the initial powder mixture titanium-carbon-nickel-chromium binder, subsequent MA of the whole powder mixture, and intense plastic deformation of the synthesis product). It has been demonstrated that, under intense plastic deformation with extrusion of the high-temperature synthesis product, there a metal-ceramic structure forms containing particles of the nanosized carbide phase of the stoichiometric composition.     

High-temperature ultra-strength of dual-phase Re0.5MoNbW(TaC)0.5 high-entropy alloy matrix composite

The dual-phase Re0.5MoNbW(TaC)0.5 composite,consisting of refractory body-centered cubic(BCC)high-entropy alloy and carbide with many fine eutectic structures,was successfully synthesized by arc melting.The phase stability,high-temperature mechanical properties and strengthening mechanism of the as-cast composite were studied.The microstructure of the composite remained stable after annealing at 1300℃for 168h.It exhibited remarkably high-temperature strength,yield strength～901 MPa,and true ultimate compressive strength～1186 MPa at 1200℃.The BCC phase and carbide exhibited a semi-coherent interface with good bonding after severe deformation at 1200℃.The dipolar dislocation walls in BCC phase,restricted dynamic interaction between defects in carbide,and the pinning effect of semi-coherent interface offered effective strengthening effects.     

Effect of high temperature thermomechanical treatment on the mechanical properties of vanadium – modified AISI 4330 steel

The relationships between the recovery and recrystallization of hot-deformed austenite grains and the precipitation of carbide on tempering in a 0.3C-Ni-Cr-Mo-V steel have been studied to examine closely the favourable effects of high-temperature thermomechanical treatment (HTMT) on the mechanical properties in steel. The results show that the strength and toughness increase with increasing degree of deformation by HTMT, particularly in which the ductile-brittle transition temperature as a measure of toughness decreases steeply with increasing degree of deformation. From the analysis of line broadening on (110)_M and hardness change with holding time after hot deformation, the dynamic recovery and recrystallization during HTMT are believed not to occur. Owing to this, the random distribution of fine carbide precipitates in martensite structure is hardly affected. Consequently, the improvement of mechanical properties by HTMT is due to the distribution of fine carbide precipitates within martensite lath and the refinement of the martensite structure.     

真空熔烧钴基合金--碳化钨复合涂层材料的耐磨性能研究

采用真空熔烧法制得钴基合金——碳化钨复合涂层材料,借助扫描电子显微镜、X射线衍射仪、显微硬度计等先进的测试手段对涂层的组织结构和表面形貌进行观察分析。应用盘销式摩擦磨损试验机对不同碳化钨含量的复合涂层材料和淬火态45钢进行了磨损试验。试验结果表明,在相同试验条件下,复合涂层的耐磨性显著高于淬火钢,且其耐磨性随碳化钨含量的增加而提高,淬火钢的耐磨性随着载荷的增加迅速降低,而复合涂层的耐磨性则变化不大。     

Dependence of the strength and plasticity of WC-6Co medium-grained hard alloys on deformation characteristics of the cobalt and carbide phases

The effect of deformation characteristics of the carbide phase and cobalt binder on the strength and plasticity of the WC-Co hard alloy has been studied using a calculation algorithm.     

Fabrication of Functionally Gradient Cemented Carbide with Ultrafine Grains

At present, the functionally gradient cemented carbide (FGCC) substrate with enrich cobalt on surface is mainly formed from medium grained WC grains. In order to further improve the properties of gradient cemented carbides, the ultrafine powder was chosen in this study and the functionally gradient cemented carbide with ultrafine grains was prepared by a two-step process, where the cemented carbide is first lower pressure pre-sintered and then subjected to a gradient sintering. The results show that it is possible to form gradient layer with enriched cobalt on surface by this method and also the grain growth can be inhibited by low pressure pre-sintering. Ultrafine grain gradient cemented carbide was fabricated after the gradient sintering, the thickness of gradient layer was about 43μm and the average grain size of WC is about 0.42μm. The formational mechanism of the functionally gradient cemented carbide with ultrafine grains are discussed through analyzing the influence of ultrafine microstructure, which was obtain by lower pressure pre-sintering, on atomic diffusion and grain growth during gradient sintering process.     

钴基合金-碳化钨复合涂层耐磨抗蚀性能研究

采用真空熔烧法制得钴基合金-碳化钨复合涂层.应用盘销式摩擦磨损试验机对不同碳化钨含量的复合涂层和淬火态45#钢进行了磨损试验.借助自设计振动装置,在10%HCl溶液及10%NaOH溶液中进行了室温全浸泡振动腐蚀试验.试验结果表明,复合涂层的耐磨性显著高于淬火钢,且其耐磨性随碳化钨含量的增加而提高,复合涂层在腐蚀液中耐蚀性也远高于45#钢.     

Effect of Particulate Silicon Carbide on Cyclic Plastic Strain Response and Fracture Behavior of 6061 Aluminum Alloy Metal Matrix Composites

In this paper, the cyclic stress response and cyclic stress–strain response characteristics, cyclic strain resistance and low-cycle fatigue life, and mechanisms governing the deformation and fracture behavior of aluminum alloy 6061 discontinuously reinforced with silicon carbide (SiC) particulates are presented and discussed. Two different volume fractions of the carbide particulate reinforcement phase in the aluminum alloy metal matrix are considered. The composite specimens were cyclically deformed using fully reversed tension–compression loading under total strain-amplitude-control. The stress response characteristic was observed to vary with strain amplitude. The plastic strain-fatigue life response was found to degrade with an increase in carbide particulate content in the metal matrix. The fracture behavior of the composite is discussed in light of the interactive influences of composite microstructural effects, cyclic strain amplitude and concomitant response stress, deformation characteristics of the composite constituents and cyclic ductility.     

碳化硅高温压力传感器的研究进展与展望

碳化硅材料由于其优良的电学、机械和化学特性,在开发适用于高温恶劣环境下的压力传感器等领域,有着广阔的应用前景,逐渐为人们所重视。简单介绍了碳化硅的材料特性,阐述了国外碳化硅高温压力传感器的最新发展成果,比较了电容式、压阻式和光学结构压力传感器的结构、特点,总结了我国碳化硅压力传感器发展问题与挑战。     

中国超细晶硬质合金及原料制备技术进展

超细晶硬质合金是WC晶粒度≤0.5μm的硬质合金,这类合金具有高强度和高硬度的优异性能。目前由超细晶硬质合金制备的高效刀具已经广泛用于航空航天、核能、汽车、发电设备、新能源和电子通讯等现代制造业。主要对中国超细晶硬质合金原料(例如超细碳化钨粉、钴粉、复合粉)和超细晶硬质合金制备技术、性能及表征方法作了系统的阐述。最后对超细晶硬质合金制备技术进行了展望。     

Production of isotropic materials by thermomechanical treatment

The general principles of thermomechanical treatment for ensuring isotropy of mechanical properties are discussed. The deformation conditions of isotropy are formulated, and an expression is derived for estimating the intensity of plastic deformation during the treatment which may consist of several cycles. As a specific example of a treatment of this kind a high-temperature thermomechanical treatment of steel ShKh15 and 37KhN3A torsion bars (involving a single deformation cycle in torsion) is considered.     

Fabrication and characterization of some novel reaction-bonded silicon carbide materials

Attempts have been made to produce modified reaction-bonded silicon carbide (RBSC) ceramics by incorporating a dispersion of other phases into the initial powder mix. ZrC, TiC, TaC and B₄C were chosen as additives together with TiB₂ as a phase likely to produce microcrack toughening in the final compact. During fabrication an important factor appears to be the possible reactions of the added phase with liquid silicon during the infiltration stage of the process. Thus, while all the carbides react with liquid silicon to form refractory silicides and new silicon carbide, this only significantly affected the reaction-bonding process if the dissolution/reaction kinetics were so fast as to disrupt the formation of the new silicon carbide framework which grows epitaxially to bond the existing silicon carbide particles together. As with conventional RBSC, the initial SiC grits play no part in any reaction except to act as nucleation sites for the new SiC. The microstructures of the various new materials have been characterized by reflected light microscopy, scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. This has led to an appraisal of the high-temperature reactions observed to have occurred and the unreliability of the high-temperature thermochemical data used to predict their occurrence. The mechanical properties of the new materials have been investigated by indentation testing (hardness and fracture toughness), including temperature-variant tests. Results are presented and the possibility for improving the properties of RBSC are discussed.     

Characteristics of diamond regrowth in a synthetic diamond compact

A transmission electron microscopic study of a commercial sintered diamond compact is reported that identifies and characterizes the diamond that has regrown between the grains of the original diamond powder during the high-pressure, high-temperature manufacturing process of the compact. The majority of the original grains are strongly deformed whereas the regrown diamond shows little or no plastic deformation. The dislocations in diamond regrown between the original grains occur in low-angle boundaries and other configurations typical of grown-in dislocations in crystals. The manufacturing process involves infiltrating the diamond aggregate by molten cobalt, and the regrown diamond is characterized by the presence of cobalt inclusions in sizes ranging from a few tenths of a micrometre down to a few nanometres, possessing the same orientation and lattice parameter as the diamond host. Graphite inclusions also occur in regrown diamond, few in comparison with cobalt inclusions and in random orientation. The graphite crystals exhibit axial ratios, (c/a), lowered by several per cent due to the containment pressure exerted by the diamond host.     

Dislocation network models for recovery creep deformation

The development of dislocation network models for recovery creep and the important results which arise from the application of the models are discussed. These models are basically aimed at describing the two simultaneous processes, namely strain hardening and recovery, which occur during high-temperature creep deformation. These processes are modelled using detailed dislocation mechanisms which occur in the deforming crystalline materials. The present models, although still being approximations, are reasonably well able to describe high-temperature recovery creep deformation of crystalline materials.     

Modelling dislocation assisted tempering during rolling contact fatigue in bearing steels

Rolling contact fatigue in bearing steels is manifested by dark-etching regions, which are attributed to deformation induced tempering. In order to quantitatively explain this phenomenon, a model is suggested for martensite tempering assisted by dislocation glide during rolling contact fatigue. In the model, dislocations transport carbon from the matrix to carbide particles, provided that the carbon is located at a certain distance range from the dislocation contributing to the tempering process. By calculating the amount of carbon in the matrix, the kinetics of carbide thickening and hardness reduction are computed. It is found that the dark-etching region kinetics can be controlled by both bearing operation conditions (temperature and deformation rate) and microstructure (type, size, and volume fraction of carbides). The model is validated against tested bearings, and its limitations are discussed.     

碳化硅泡沫陶瓷的制备

碳化硅泡沫陶瓷具有气孔率高、热稳定性好等优良性能,被广泛用作金属溶液过滤器、高温气体和离子交换过滤器、催化剂载体等.重点介绍了碳化硅泡沫陶瓷的种类,阐述了碳化硅泡沫陶瓷的制备方法和影响碳化硅泡沫陶瓷产品性能的因素,展望了碳化硅泡沫陶瓷的发展前景.     

多组元硬质合金的热力学研究及其在微观组织控制中的应用

硬质合金是一种多组元多相材料, 其制造过程涉及复杂的热力学和动力学过程。为了给硬质合金的开发提供理论依据,基于建立的热力学数据库CSUTDCC1,将通过CALPHAD计算模拟的结果与其他文献中的实验结果进行了比较,以验证热力学数据库的准确性和热力学计算方法的高效性。同时,为展示热力学计算在硬质合金设计中的应用,包括合金成分、烧结温度和N2气氛控制等的确定,通过热力学计算和实验验证,研究了Cr、V、Ta、Nb、Zr等元素在钴黏结相和M6C碳化物中的饱和溶解度,论述了热力学计算在硬质合金发展中的具体作用。     

Prospects of producing hard alloys based on submicron and nanoscale W and WC powders prepared by a chemical metallurgy process and with the use of self-propagating high-temperature synthesis

We present a brief review of research and design work aimed at producing tungsten-containing hard alloys for various applications. We examine the feasibility and prospects of using a chemical metallurgy method and self-propagating high-temperature synthesis (SHS) for the preparation of submicron and nanoscale hard-alloy powders as a key component of highly efficient state-of-the-art materials and products. Particular examples are presented of the use of submicron and nanoscale powders for the preparation of TVS tungsten-based heavy metallic alloys and VK tungsten carbide hard alloys for various applications. Their application fields are discussed and their properties are compared to those of their analogs produced by conventional powder metallurgy methods. Using the SHS of tungsten carbide as an example, we demonstrate a particular path from research to commercialization (from the discovery of SHS processes to commercialscale production) of key modern engineering materials: tungsten-based heavy alloys and tungsten carbidebased hard alloys.