查看更多>>摘要：? 2022 Elsevier LtdIn this study, a novel intragranular self-lubricating nanostructure was proposed for preparing a potential ceramic cutting tool material. Nano-sized CaF2, which meets the requirements of such a material, was prepared and used in the synthesis of ceramic materials with intragranular self-lubricating nanostructure. Based on the obtained results, an Al2O3/TiC/CaF2 intragranular self-lubricating nanostructure ceramic material was prepared by vacuum hot-pressing sintering. As the nano-sized CaF2 content increased, the flexural strength first increased and then decreased, hardness decreased, and fracture toughness increased. The results indicated that the formation of intragranular nanostructures could significantly improve the friction performance of this material. The friction coefficient of this material against a 40Cr steel ball was 0.12–0.25, and decreased as the CaF2 content increased. The wear rate also decreased as the CaF2 content increased from 3.3–10 vol%, but the friction coefficient decreased only slightly. Furthermore, the wear rate increased when 13.3 vol% CaF2 was added.
查看更多>>摘要：? 2022 Elsevier LtdIn order to study the dynamic mechanical properties of molybdenum under high strain rates, experiments are conducted using Hopkinson dynamic compression testing of a small-sized sample and explosion impact dynamic X-ray study of a flat shaped-charge liner. Experimental data agrees well with theoretical calculations and numerical simulations. Results demonstrate that strain hardening, thermal softening, and strain rate strengthening effects during the impact response process of the molybdenum material vary significantly with different impact conditions in a temperature range of 297–1273 K and a strain rate range of 0.0001–48,000 s?1. Strain rate strengthening and thermal softening effects are particularly apparent in the molybdenum material under impact at medium and high strain rates. At extremely high strain rates and large deformations, the dynamic mechanical response provides evidence of thermal softening and strain hardening effects. Stresses that result in damage to the molybdenum material are primarily attributed to the latter.
查看更多>>摘要：? 2022 Elsevier LtdIn this work, pure Nb processed by Powder Metallurgy Hot Isostatic Pressing (PM HIP) was investigated as a potential alternative to C-103 Nb-based alloy for the manufacture of near-net-shape thruster combustion chambers. Three Nb powders with varied particle size range (fine, mid-range and coarse) and C-103 powder were investigated to understand the differences in particle size distribution (PSD), morphology and oxygen (O) content present in the alloy chemistry. The as-HIPed microstructures of pure Nb and C-103 are characterised by a near to fully dense microstructure and the absence of PPBs. Microstructural analyses performed on pure Nb highlighted the influence of particle size on the average as-HIPed grain size. Additionally, it was observed that O content plays a crucial role in the microhardness of pure Nb. Tensile tests performed on Nb mid-range and on Nb mid-range sieved showed that a simple sieve operation was effective in increasing the strength of the material while maintaining good levels of elongation. Alternatively, it was also demonstrated that pure Nb powder heat treatment can be regarded as an effective way to increase the O levels in pure Nb powders as witnessed by the high hardness levels. On the other hand, as-HIPed C-103 powder showed superior tensile properties if compared to the minimum specifications for wrought C-103. Finally, to summarise, the work performed on pure Nb, a simple structure-property relation model was developed to predict the YS of pure Nb based on the O levels and grain size of the as-HIPed microstructure.
查看更多>>摘要：? 2022 Elsevier LtdFast Breeder Test Reactor (FBTR) is a unique Indian fast reactor and irradiation facility for materials testing. As a part of its life extension activities, high density tungsten mono carbide (α-WC) is considered as a candidate material for the lower axial neutron shielding of the grid plate. Spark plasma sintering technique is used to synthesize high density (99.5%) tungsten carbide pellets under optimized conditions without a binder phase and subjected to high temperature thermal property characterization. The dilatational strain, thermal diffusivity as well as the resonance frequency of flexural vibration is measured as a function of temperature and the corresponding values of linear thermal expansion, thermal conductivity and the dynamic Young's modulus are estimated. The measured values are analyzed in combination with quasi-harmonic Debye-Grüneisen model to obtain a comprehensive database on vibrational thermodynamic properties from 0 to 1273 K. The observed Debye temperature for tungsten carbide under ambient conditions is 630 K which gradually decreases to 586 K at 1273 K. The estimated thermal Grüneisen parameter varied in the range of 1.41–1.45.
查看更多>>摘要：? 2022Tungsten carbide?cobalt materials are useful in a variety of extreme applications due to a desirable blend of properties, yet the technology has not significantly changed since their initial development in the 1920s. The mechanical properties of this class of materials is highly dependent on two variables, the size of the tungsten carbide grains, and the amount of binder phase present in the final body. In this study, the amount of binder phase is isolated across three commercial materials from the same manufacturer with three different grain sizes to investigate the effect on mechanical properties. The mechanical properties investigated are indentation hardness, flexure and tensile strength, as well as fracture toughness. In general, an increase in hardness and tensile strength with decreasing grain size was observed, while the fracture toughness showed the opposite trend with toughness increasing with increasing grain size. The flexure strength results did not show a correlation to grain size. Fractographic analysis identified the dominant strength-limiting flaw for each sample, which largely were in the form of porosity. Other flaws types, such as inclusions from the milling process, clusters of large grains, and machining cracks from the surface finishing process, were also identified. Weibull analysis was performed and deemed appropriate for analysis of these materials, but strength-size scaling was not conducted due to the variability in the strength-limiting feature between the different specimen geometries.
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