查看更多>>摘要：In this study, we have firstly proposed a high-speed electron beam sintering technique for producing fully dense submicron-grained WC-8Co hardmetals. A distinctive feature of the sintering process is the controlled stress-strain state by varying thermal stresses during local electron beam heating at different rotation speed (0.1; 1.0; 2.5 mm/s) of a cylindrical powder compact. As a result, the sintering time of porous compacts to full density does not exceed 150 s. The experimental and calculated results indicate that the main requirement for obtaining well-sintered material is the shrinkage rate should be less or equal to the rate of thermal expansion of the sample segments subjected to repeated local heating.
查看更多>>摘要：The deformation behavior and strengthening mechanism for Mo-3Nb single crystal with 111 orientation and polycrystal have been investigated and disclosed comprehensively in a wide temperature range by quasi-static compression with 7% plastic strain. The slip traces of single crystal and polycrystal at room temperature show different features, which long continuous slip traces appear on the surface of deformed Mo-3Nb single crystals while some branch-off slip lines can only be observed in large grains for polycrystals. The conjugate slip planes of single crystals are activated at the elevated temperature, while the slip lines of polycrystals are difficult to be observed because of the grain boundary sliding effect. The compressive stress of single crystals and polycrystals shows different results at different deformation temperatures, suggesting that grain boundaries in polycrystals play an important role in the strengthening mechanism. TEM analysis shows that dislocation entanglement is the main strengthening mechanism both in single crystal and polycrystal during the room temperature deformation. With the increase of temperature, the strengthening mechanism caused by dislocation entanglement becomes ineffective, while grain boundaries continue to hinder the dislocation movement and the strengthening effect greatly weakened due to the dynamic recovery.
查看更多>>摘要：The current study is focused on the preparation of Mo-10 vol%La2O3 and Mo-10 vol% La2Zr2O7 composite powders via low- and high-energy ball milling approaches as potential candidates for near-future high-temperature structural applications. The mechanical milling parameters play a critical role on the final powder's microstructure. When using the high-energy milling mode (using 800 rpm, ball-to-powder ratio (BPR) 100: 6), the homogeneous powder agglomerates are formed with refined laminated microstructure and more uniform ceramic phase distribution in both Mo-La2O3 and Mo-La2Zr2O7 systems compared to the powders produced by means of the low-energy milling mode (using 350 rpm, BPR 100: 6), where inhomogeneous powder mixture with less embedding of ceramic phases into Mo agglomerates was obtained. This study also focuses on the evaluation of high-temperature phase and microstructural stability of the produced composite powders treated at the temperature of 1300 degrees C under the different gaseous environments, including ambient, inert and reducing atmospheres. The Mo-10 vol% La2Zr2O7 composite powder exhibited better thermal stability during the high-temperature exposure in all tested atmospheres in comparison with the Mo-La2O3 composite powder, since it revealed less intensive formation of the intermediate phases, such as lanthanum oxymolybdates. Therefore, the Mo-10 vol%La2Zr2O7 composite powder was used further for consolidation by means of spark plasma sintering at 1600 degrees C. The successful production of Mo-La2Zr2O7 composite with homogeneous distribution of ceramic phase, the grain size about of 5 mu m, and hardness of 3.4 GPa was not reported so far.
查看更多>>摘要：TiN coating was deposited on high-speed steel, cemented carbide and TiCN-based cermets using arc evaporation. The variation of substrates was to evaluate the effect of tool materials on the coating's microstructure, composition, hardness, adhesion and tribological properties. The TiN coating on all tool materials had a strong preferred TiN (111) orientation. The TiN coating on high-speed steel grew so fast in the vertical direction that the columnar crystal structure was noticeable. The columnar structure of TiN coating on TiCN-based cermets weakened for the reduced vertical growth rate. The Ti/N atomic content ratio of the TiN coating on high-speed steel was 0.84, and some N atoms formed an interstitial solid solution, which contributed to the increased hardness of the coating. However, the Ti/N atomic content ratio of TiN coating on TiCN-based cermets increased to 1.04. Due to the low residual stress, the adhesion of TiN coating on TiCN-based cermets and cemented carbide was noticeably higher than that of coating on high-speed steel. The wear mechanism of TiN coatings on all tool materials tested against with steel balls was mainly abrasive wear. An iron oxide film appeared on the coated TiCN-based sample, which directly caused the friction coefficient to decrease from 0.8 to 0.6. Therefore, the TiN coating on TiCN-based cermets had the best wear resistance.
查看更多>>摘要：Kinetics of WC coarsening during sintering of WC-10 wt% Co cemented carbide grades was examined by use of submicron WC powders with a broad grain size distribution containing much nanograin fraction, medium -finegrain powders with a narrow grain size distribution containing little nanograin fraction and medium-grain powders with a very narrow grain size distribution not containing nanograin fraction. Based on the kinetic curves re-constructed in the Arrhenius coordinates, values of the apparent activation energy for each carbide grade were obtained, which allowed the limiting stage of the WC coarsening process to be evaluated. The limiting stage of the WC coarsening process for the medium-grain grade is related to the diffusion of W and C atoms in the liquid binder during sintering. For the submicron grade the limiting stage of the WC coarsening process is related to interfacial reactions at WC/liquid interfaces. In this case, the diffusion of W and C atoms dissolved in the liquid binder is fast due to a very significant difference between sizes of the fine/nano WC grains and coarser WC grains. When increasing the sintering temperature and time above a certain level, the significant acceleration of WC coarsening takes place indicating the transition of the process to the stage of anomalous grain growth, at which the formation of abnormally large WC grains in the microstructure determines the whole WC coarsening process. The limiting stage of the WC coarsening process for the medium-fine-grain grade is mainly related to diffusion of W and C atoms dissolved in the liquid binder.
查看更多>>摘要：The paper analyzed temperature ranges of aggregate/polymorphic state changes, phase and structural transformations in Al, Ti, WC-Co, and Ti1-xAlxN, leading to the gradient of thermo-physical and physico-mechanical properties of cathodes, substrate, and coating. The critical temperatures for Al and Ti are the points of the exo- or endothermic effect occurrence at 933 K (T-mAl) and 1155.5 K (T alpha-Ti ->beta-Ti), respectively. The hardness of the WC-Co/Ti1-xAlxN composition varies with the heating temperature of the WC-Co before coating deposition. The optimal heating interval for WC-Co is 873...963 K. The structuring process of the Ti1-xAlxN coating was investigated in the temperature range 673...1073 K before the spinodal decomposition of c-TiAlN into metallic c-TiN and dielectric-semiconductor c-AlN. Physico-mechanical properties of the Ti1-xAlxN coatings were determined using a FISCHERSCOPEH100C measurement system per the standard DINENISO14577-1. Wear tests were carried out according to the "finger-disk" scheme in a coolant-cutting fluid on a laboratory friction machine. The optimal temperature conditions for the deposition of a nanostructured Ti1-xAlxN coating with a minimum phase and elemental composition gradient and microdroplets number, a complex of high physico-mechanical properties, wear, and cracking resistance have been established. The Ti1-xAlxN coatings efficiency has been increased by controlling the thermo-physical and physico-mechanical properties of Ti and Al cathodes, the substrate (hard alloy WC-Co), and the Ti1-xAlxN coating in-process of their heating and cathodic arc evaporation. Multifunctional dependency graphs of the physico-mechanical properties and the friction coefficient of the Ti1-xAlxN coatings on the Al = x content have been developed. The relationship between the resistance of the coating to plastic deformation and the coefficient of friction has been established.
查看更多>>摘要：In this paper, the wear and corrosion resistance of FeNiCoCrMox (x: atomic ratio, x = 0, 0.15, 0.20, 0.25) high entropy alloy coatings (HEACs) which prepared by laser cladding (LC) technology on 316 stainless steel (SS) was investigated. According to XRD, SEM, MicroXAM-3D, micmhardness tester and electrochemical workstation tests results, the HEACs presented typical DR-ID structure with single FCC solid solution phase. The average microhardness of MoO, Mo0.15, Mo0.20 and Mo0.25 HEACs increased by 70.1%, 77.0%, 84.9% and 90.5% relative to the substrate. Dry sliding friction and wear tests results showed that the average friction coefficient of the HEACs were significantly lower than that of 316 SS substrate. Meanwhile, it can be found that Mo0.25 HEACs had the lowest average friction coefficient which displayed the best wear resistance. Compared with Mo0 HEAC, the specific wear rates of Mo0.15, Mo0.20 and Mo0.25 HEACs decreased by 11.1%, 27.8% and 38.9% respectively. The electrochemical tests results showed that the corrosion type of HEACs was intergranular corrosion and all HEACs had a higher corrosion resistance than that of 304 SS or 316 SS. Moreover, Mo0.20 HEACs showed the best corrosion resistance due to the enhancing effect of MoO3 on Cr2O3 passivation film.
Shichalin, O. O.Buravlev, I. Yu.Papynov, E. K.Golub, A. V....
12页
查看更多>>摘要：The paper presents a method of fabrication and a comparison of the characteristics of obtained cemented carbide samples based on tungsten carbide with 10% metal binder. They were synthesized by spark plasma sintering (SPS) at different sintering temperatures: Co (1200 degrees C), Fe and Ni (1150 degrees C), Cr (1500 degrees C), and Ti (1400 degrees C). During the SPS-consolidation process, investigations of the dilatometric characteristics of samples of starting powders of identical granulometric composition obtained by mixing/grinding resulting in the fractional composition in the particle size range up to 10 mu m are presented. The mechanism of phase formation and microstructure of the obtained materials is investigated by XRD, SEM, and EDS. The possibility of synthesizing hard alloys WC - 10%Ti and WC - 10% Cr by the mechanism of reaction SPS synthesis with a reduction in the synthesis time in comparison with traditional sintering methods has been demonstrated. Analyses have shown that during sintering of alloys containing Ti and Cr the volume content of WC decreases to 47% because of dissolution and interaction, while in alloys containing Co, Ni, and Fe, the volume content of WC remains unchanged (85-86%). This led to a dramatic change in hardness, fracture toughness, and strength when replacing Co, Ni, or Fe with Ti or Cr.
查看更多>>摘要：In this paper, Hf-Mo-Nb-Ti-Zr alloy system with single BCC solid solution phase was selected as the matrix alloy. In order to design a new high temperature structural material with excellent comprehensive properties, the multiphase reinforced refractory high entropy alloys was prepared by adding metallic elements of Al or Cr and nonmetallic elements of B, C or Si into Hf-Mo-Nb-Ti-Zr alloys at the same time. The results show that the type and content of newly formed phases are closely related to the mixing enthalpy between alloying elements and the constituent elements of the matrix alloy. This provides a valuable reference for the prediction of phase formation when adding alloying elements to improve the properties of high entropy alloys.
查看更多>>摘要：For studying the evolution of the eta phase in cemented carbides with the CoNiFeCr high entropy alloy binder, we prepared low-carbon, medium-carbon, and high-carbon cemented carbides with a 0.2 wt% difference in the C content using SHIP. The results showed that the eta phase morphology was sensitive to the carbon content. When the carbon content was 4.5%, the eta phase was abundant and uniformly dispersed. When the carbon content increased to 4.7%, the eta phase amount decreased and became locally aggregated in large clusters, showing typical segregation. When the carbon content was further increased to 4.9%, the ri phase disappeared. The eta phase clusters exhibited a sharp contrast between inner and outer parts. The inner part showed the presence of the eta phase, WC with small and round grains, and a small amount of binder. The outer part had only WC with large grain and sharp grains, and a large amount of binder. In addition to the inherent eta phase brittleness, the eta phase segregation resulted in a large hardness dispersion and a further toughness decrease.
NSTL
Elsevier