Preparation of Nb-silicide based alloy powder by hydrogenation-dehydrogenation reaction
Abstracts:Nb-16Si-25Ti-8Hf-2Cr-2Al alloy powders were prepared by a hydrogenation-dehydrogenation reaction. The Nb alloy ingot, which was prepared by vacuum arc melting, was composed of a (Nb, Ti) solid solution, (Nb, Ti, Hf)5Si3, and (Nb, Ti, Hf)3Si phases. For hydrogenation, the ingot was annealed in the hydrogen atmosphere and it underwent self-crushing. After hydrogenation, the hydrogen concentration increased from 0.003% to 1.30% and the (Nb, Ti) solid solution phase was completely transformed into the (Nb, Ti)H2 hydride phase. The hydrogenated ingot could be easily pulverized by milling due to its brittle nature. For dehydrogenation, the hydrogenated powders were annealed in a vacuum atmosphere, whereby the hydrogen concentration decreased to 0.002% and the (Nb, Ti)H2 hydride phase was transformed back to the (Nb, Ti) solid solution phase. By using this method, Nb alloy powders having sizes less than 20 μm could be easily fabricated.
Constitutive modelling and processing map analysis of tungsten heavy alloy (92.5 W-5.25Ni-2.25Fe) at elevated temperatures
Abstracts:Tungsten Heavy Alloys (WHA) are finding increasing applications in the kinetic energy penetrators. For developing the accurate simulation models of deformation processes on WHA, it is essential to properly understand the flow stress behavior of these alloys considering the combined effects of strain, strain rate and temperature. This paper focuses on developing various constitutive models for 92.5 W-5.25Ni-2.25Fe alloy by using the flow stress data at six different temperatures (298 K, 573 K, 773 K, 973 K, 1173 K, 1373 K) and three different strain rates (1600, 2500, 4000 s−1). Four constitutive models, namely, modified Johnson-Cook (m-JC), modified Zerilli-Armstrong (m-ZA), modified Arrhenius (m-Arr) and modified Khan-Huan-Liang (m-KHL) models have been developed to predict the flow behavior. The predictions of these constitutive models have been compared with the experimental values using statistical measures like correlation coefficient, average absolute error and its standard deviation. Based on these statistical measures, m-Arr and m-ZA models have been found to be better models for predicting the flow stress values. In addition, using the flow stress curves, the strain rate sensitivity values have been computed for determining the efficiency of power dissipation and instability parameter for the deformation process. Superimposing the efficiency map over the instability map, the processing maps have been developed for better understanding of response of a material to the imposed experimental parameters.
Increase of surface hardness and substrate toughness due to migration of grain growth inhibitors in the ultrafine-grained layer during sintering of functionally graded cemented carbides
Abstracts:This work is devoted to increasing the hardness of the surface and toughness of the substrate of the functionally graded hard alloy obtained from sintering the medium-grained (WC-15Co) and ultrafine-grained (WC-8Co-0.4VC-0.4Cr3C2) layers. The migration of the liquid phase from the medium-grained layer to the ultrafine-grained layer during sintering leads to the displacement of grain growth inhibitors (VC + Cr3C2) dissolved in cobalt to the surface of the ultrafine-grained layer. Local slowing of grain growth increases the hardness of the surface of the ultrafine-grained layer up to 1680 HV. An increase in the local concentration of cobalt near the interface in the medium-grained layer increases the toughness to 18.9 MPa√m. Studies on cobalt and grain growth inhibitors concentrations, average grain diameters, contiguity, hardness and toughness over the depth of samples sintered at 1350 °C, 1370 °C, 1390 °C, and 1410 °C were carried out. The obtained results were compared with theoretical estimates based on the Gurland equations.
Compressive behavior of liquid phase sintered 90 W-7Ni-3Fe heavy alloy at high temperature and low strain rate condition
Abstracts:The compressive behavior of 90 W-7Ni-3Fe heavy alloy prepared by liquid phase sintering is investigated systematically at strain rates 0.001, 0.01 and 0.1 s−1 and temperatures 298, 1073, 1273 and 1473 K. Compressive yield strength of the material increases with the increase of strain rate and decreases with the increase of temperature. The compressive deformation is affected by the coupling of work hardening and thermal softening; plastic deformation is dominated by work hardening at 298, 1073 and 1473 K below strain 0.5; plastic deformation is dominated by work hardening at 1473 K below strain 0.2. The work hardening rate decreases with the increase of strain and then tends to a relatively stable level. Microstructure evolution is sensitive to temperature and strain rate. The mechanical properties of specimen deteriorated significantly and the abnormal growth of W particles occurs at low strain rate (0.001 s−1) and high temperature (1473 K). The shear band with angle of about 45° to the compression direction appeared at the corner of samples at 1073 K. A large number of defects on phase interface appeared due to inconsistent deformation.
The effect of TiO2 additive on the electrical resistivity and mechanical properties of pressureless sintered SiC ceramics with Al2O3-Y2O3
Abstracts:In this study, SiC-TiC composite was fabricated by the reaction between TiO2 and SiC and in addition to the effect of TiO2 additive, influence of sintering temperatures on the electrical resistivity, the relationship between electrical resistivity and microstructure, density, indentation fracture resistance and hardness were investigated. The main goal of this study was to improve electrical resistivity while preserving mechanical properties of SiC body. The results showed that with 10 wt% Al2O3-Y2O3, the electrical resistivity reached to 9 × 108 Ω·m. Increasing the amount of TiO2 particles from 2.5 to 10 wt% and changing the sintering temperature from 1850 °C to 1950°C made the electrical resistivity to be variable in the range of 2.2×105 Ω·m to 9 × 108 Ω·m. In this report, the highest density, hardness and indentation fracture resistance for the samples containing 5 wt% additive which were sintered at 1900 °C were 96.2%, 24.4 GPa and 5.8 MPa.m1/2, respectively. Microscopic images showed that if the grain boundary phase is located in the triple points or multiple points of grain boundary, the electrical resistivity will decrease and if it is located in full circumference of the SiC particles, due to failure of conducting pathways through SiC grains, the resistance of ceramics will increase instead.
Microstructure and properties of W-doped self-lubricated Ti(C,N)-based cermets
Abstracts:Since wear usually occurs on the surface, it is not necessary to fabricate self-lubricated bulk materials. In the paper, Ti(C,N)-based cermets with self-lubricated gradient surfaces are prepared after carburizing treatment based on W-doped carbon-deficient cermets. The microstructures, mechanical properties and wear behavior of the W-doped self-lubricated Ti(C,N)-based cermets were investigated. The results showed that when W is added into the cermets, the carbon-deficient cermets were prepared. This carbon-deficient system hinders the inward immigraion of carbon atoms and thus changes the distribution of other elements in the cermets which results in the formation of a gradient layer enriched in graphite phase but deficient in Ni binder phase. Besides, with the increase of W content, the amount of graphite decreases. Moreover, the TRS and the hardness of cermets is enhanced due to the addition of W and the hardness of the graded cermets was obviously different along the depth direction. Compared with the self-lubricated bulk cermets without the addition of W, the self-lubricated cermets with 2 wt% W addition show the best tribological properties due to the lubricated effect of the gradient layer with high volume fraction of graphite as well as a superior tough substrate.
Low-temperature sintered porous Si3N4 ceramics using two fluorides as sintering aid
Abstracts:Porous Si3N4 ceramics with a unique interlocking microstructure have been fabricated by pressureless liquid-phase sintering with YbF3 and CaF2 sintering additives at a relatively low temperature (1550 °C). Phase transition and mass diffusion in the low-temperature sintering process are facilitated by YbF3 and CaF2 producing plenty of liquid phase owing to their extremely low melting points. As the content of YbF3 increases from 8.0 wt% to 9.5 wt% and the CaF2 decreases from 2.0 wt% to 0.5 wt%, the conversion rate of the phase transition from α-Si3N4 to β-Si3N4 increases from 71.1% to 81.2% and the largest average aspect ratio of the β-Si3N4 grains reaches 10.05. At the same time, the porosity declines from 40.4% to 34.9% and the flexural strength increases from 168.5 MPa to 232.1 MPa.
Refractory high entropy alloys: CrMoNbTiVWZr and AlxCryNbMoTiVzZry(x = 0,0.6;y = 0.3,z = 0,0.6)
Abstracts:Four potential High Entropy Alloys, CrMoNbTiVWZr, Cr 0.3 NbMoTiZr 0.3 , Cr 0.3 NbMoTiV 0.6 Zr 0.3 and Al 0.6 Cr 0.3 NbMoTiV 0.6 Zr 0.3 have been designed using the indications provided by atomic radius and electronegativity mismatch parameters. Alloys were synthesized by arc melting and characterized in the as-cast state and after heat-treatment by X-ray Diffraction and Scanning Electron Microscope experiments. In the as-cast state all alloys have dendritic microstructure, although with varied shapes of dendrites, accompanied by a cubic Laves phase and an hcp phase in the region between the dendrites. After annealing at 1350 °C for 3 h it is shown that only non-equimolar compositions are constituted of bcc high entropy phase complying with the requirements for defining them as HEAs, with a small amount of secondary phases which help in providing high hardness.
The preparation of tungsten carbides and tungsten powders by reaction of tungsten trioxide with methanol
Abstracts:In the present study, a new and simple method to prepare tungsten and tungsten carbides powders was proposed by using WO3 powders as the raw material and CH3OH vapor as the reduction and carbonization agent. Both thermodynamic and experimental studies were carried out, and based on the results, the reaction routes were obtained. It was found that at low and moderate temperatures, the final product of WC was formed via intermediates of W18O49, WO2, partial metallic W and W2C; while at high temperature, WO3 was first reduced to WO2 with the intermediate phase of W18O49, and then reduced to W. The morphology of final products almost retained the same shape as that of raw materials WO3 at low temperature, however, the large particles with blocky shape disappeared and the products were all small W particles with polyhedral shape at high temperature. Furthermore, the reaction mechanisms were proposed: the pseudomorphic transformation mechanism dominated the reaction at low temperature, but at high temperature, the reduction obeyed the chemical vapor transport mechanism, which is beneficial for grain refining of final product.
Application of surface modification techniques during hard turning: Present work and future prospects
Abstracts:Hard turning process involves machining of materials of very high hardness, and thus, the process is associated with high machining forces resulting in the generation of a large amount of heat. The prevailing machining conditions during hard turning cause high tool wear. Thus, various surface modification techniques have been adopted to reduce the friction during the hard turning process resulting in the reduction of tool wear and thereby increasing the cutting tool durability. In this context, the present review deals with the surface modification techniques adopted during hard turning applications to improve the machining performance of cutting tools. It was observed that the nanocomposite and nano-crystalline coatings excelled in machining performance among all the coatings during hard turning. Also, texturing on the surface of cutting tools resulted in the reduction of friction and thus, helped in improving the tool life of the cutting tools. Further, the present review lists various surface modification techniques and methods by identifying the research gap that can be considered for future research in hard turning.