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[en] Microstructure effects on densification and shape distortion in liquid phase sintering of tungsten heavy alloy were investigated. Microstructure parameters such as the solid volume fraction, dihedral angle, initial porosity, and pore size were varied to measure densification and distortion behavior during LPS using W-Ni-Cu alloys. Green compacts were formed using ethylene-bis-stearamide as a pore-forming agent with the amount of polymer controlling the initial porosity. Different initial pore sizes were generated by varying the polymer particle size. Dihedral angle was varied by changing the Ni:Cu ratio in the alloys. Finally, the solid volume fraction was adjusted via the tungsten content. Distortion was quantified using profiles determined with a coordinate measuring machine to calculate a distortion parameter. Sintering results showed that solid volume fraction and dihedral angle are the dominant factors on densification and distortion during liquid phase sintering. Distortion decreases with increasing solid volume fraction and dihedral angle, while initial porosity and pore size have no observable effect on distortion at nearly full densification. Various strategies emerge to improve distortion control in liquid phase sintering. (author)
[en] This paper presents the results of a study on the effect of alloy content (W and Ta) on microstructure and microchemistry in 9Cr-W-Ta-0.1C steels during short term thermal exposures. Detailed electron microscopy studies have been carried out to understand the kinetics of tempering in normalised steels. Steels normalised at 1253 K showed a fully martensitic structure with undissolved carbides, which were identified as Cr rich M23C6 and V and Ta rich MX. With increase in W and Ta concentration, the size and number density of undissolved carbides increased, which resulted in a decrease in prior austenite grain (PAG) size from 25 to 11 μm and lath width from 0.36 to 0.25 μm associated with an increase in hardness. Increasing the tempering temperature from 923 to 1033K and time from 30 minutes to 10 hours resulted in a decrease in hardness in all steels. However, saturation was attained after 2 hours of thermal exposure at all temperatures. Through an Arrhenius analysis of the hardness variation, activation energy for the tempering process was determined to be ∼ 0.7- 1eV, corresponding to the interstitial diffusion of C in α-Fe. Tempered steels consisted of a microstructure of lath martensite with coarse M23C6 on PAG and lath boundaries and fine intralath MX carbides due to substructural recovery and precipitation. Significant recovery of martensitic substructure was observed only beyond 1033K after 1 hour, while temperatures lower than 973K did not induce significant recovery even after 10 hours. Coarsening of M23C6 carbides was observed with time and temperature of tempering. The extent of change in substructure and microchemistry of M23C6 (W to Fe ratio) at any temperature was influenced by the W and Ta content, and was maximum for the steel with 2 wt% W and 0.14 wt% Ta due to the kinetics of dissolution of carbides. Detailed microstructural analysis will be presented in the paper
[en] Highlights: • The effects of Cr, Mn and Ni on the formation of ordered ω phases were studied. • Mn suppressed the formation of ordered ω phases. • ωo and βo phases co-existed in the Cr-containing alloy after annealing. • A Ti-Al-Ni ternary phase was identified in Ni-containing TiAl alloy after annealing. Ordered ω phases are equilibrium phases at intermediate temperatures in TiAl alloys. However, a systematical research on the effects of alloying elements on the ordered ω phase transformations is still lacking. The effects of alloying elements on the ordered ω phase formation should be considered in future design of TiAl alloys. In this study, the effects of Cr, Mn and Ni on the ordered ω phase formation are investigated. The results show that the precipitation of ordered ω phase is promoted by addition of Ni, i.e., the sizes of ordered ω phase were in micron-level. However, the addition of Mn greatly suppresses the growth of ordered ω phases. During thermal exposure at 850 °C, a Ti-Al-Ni ternary phase formed within the original βo phase areas in the Ni-containing alloy. Meanwhile, no ordered ω phase existed in an annealed Mn-containing alloy. In both as-cast and annealed conditions, the effects of Cr seemed to be similar to those reported in Mo and W-containing alloys. Differential scanning calorimetry (DSC) experiments suggested that the precipitation temperatures of ordered ω phase in these three alloys were affected by minor alloying elements, which fitted well with the observed phenomena in the electron microscopy study.
[en] In thin wires of a dilute W-Hf alloy, the precipitation rate of hafnium in form of hafnium oxide particles upon oxygen uptake is controlled by the inward diffusion of oxygen, because the oxygen uptake from the vacuum environment proceeds at low solute oxygen levels. The dissolution rate of the hafnium oxide particles in the tungsten matrix is controlled by the outward diffusion of oxygen, when the concentration of solute hafnium in interfacial equilibrium with hafnium oxide is much lower than the hafnium content of the alloys. 14 refs., 1 fig., 2 tabs. (Author)
[en] The results are presented of measurements of the reflection coefficient of an electromagnetic radiation in the spectral region of 100-300 nm for mono- and polycristalline molybdenum, and the coefficient of reflection in vacuum ultraviolet at a wavelength of 58.4 nm for single crystals of tungsten. The reflection coefficient (R) is measured for surfaces parallel to the crystallographic planes (100), (110), and (111). It has been established that the difference in the values of R for the planes (110), (111) and the plane (100) may be as large as 20%, which greatly exceeds the random error of measurements. The obtained data on the measurement of the reflection coefficient for single crystals of tungsten suggest anisotropy of the optical properties of single crystals of tungsten