Optical and radiant characteristics of tungsten at high temperatures

It is proposed that thermal radiation spectra of solid materials at high temperatures be treated by the method of moments.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 1, pp. 131–134, July, 1989.     

Laser surface alloying of case hardening steel with tungsten carbide and carbon

Abstract

The laser surface alloying process was used to introduce two different alloying materials, tungsten carbide (WC/Co) and carbon, into the molten surface of a case hardening steel (16MnCrS5), to improve its hardness and wear resistance. The chemical composition and the resulting microstructure in the alloyed layers were of particular interest in this investigation, because the strengthening mechanism was strongly dependent upon the type and amount of the alloy material. For laser alloying with carbon the increase in hardness and wear resistance was based on the martensitic transformation in the composition range concerned. For alloying with tungsten carbide it was necessary to consider two different strengthening mechanisms, namely, martensitic transformation and precipitation of carbides. In both cases the grain refinement in the laser affected zone had an additional effect. Resistance to dry abrasive sliding wear was measured using a conventional pin-on-disc wear testing machine. For both alloy materials the wear rate was substantially lower than that of a substrate that had been laser remelted without alloying additions.

MST/1556     

Dielectric anomaly and low frequency dispersion in ferroelectric materials at high temperatures

A dielectric anomaly related to the observed departure from the Curie–Weiss law in several Aferroelectric materials has been characterized in this work in ferroelectric BaTiO₃. The electrical properties of the material were investigated using the a.c. analysis technique. From the analysis of the permittivity versus frequency curves collected at various temperatures, we discuss the anomalous behaviour as a result of a low frequency dispersion phenomenon due to the characteristics of the conduction processes at the material inhomogeneities.     

Phase relationship on Mo-S system at high temperatures

Changing the partial pressure of sulfur Ps₂ at temperatures of 750° and 950°C, phase equilibria on the Mo-S system by solid-gas reaction were investigated. Hexagonal 2H-MoS₂ and monoclinic Mo₂S₃ phases were identified from the x-ray powder diffraction pattern. The 2H-MoS₂ had a slight homogeneity range, i.e. MoS_1.978 to MoS_2.0 at 950°C, MoS_1.983 to MoS_2.0 at 750°C. No remarkable variation of lattice parameters for the MoS₂ was observed. The composition of the Mo₂S₃ phase was not stoichiometric MoS_1.5 but MoS_1.457 at 950°C. At 750°C the composition of the Mo₂S₃ phase could not be determined since it was quite difficult to establish the equilibrium state between the gas and the condensed phases. This finding agreed well with the result of Morimoto and Kullerud.     

Thermal diffusivity and conductivity of pyrolytic titanium carbide,columbium carbide,and titanium nitride at high temperatures

Results are shown of thermal diffusivity measurements and of thermal conductivity calculations pertaining to monolithic specimens of TiC_0.94, CbC_0.77, and TiN_0.94 produced by chemical precipitation from a gas phase, and covering the temperature range from 1500 to 2200–2400°C.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 2, pp. 294–296, February, 1973.     

Breakup of eutectic carbide network of white cast irons at high temperatures

The fracture toughness of white cast irons is related to the morphology of eutectic carbides,being better when isolated than when network-like. In this paper observations on the breakup of eutectic cementite network during annealing treatment of white cast irons are reported using a high temperature microscope and scanning electron microscopy (SEM). Dissolution-induced breakup and capillarity-induced breakup are identified. The former occurs through growth of holes and pre-existing fissures as well as through fragmentation at narrow necks and narrow roots of branches. The latter is observed through growth of perturbations. Dissolution-induced breakup is closely associated with the morphology of the as-cast eutectic cementite. A combination of solidification processing and heat treatment thus produces a more positive breakup effect.     

On the work hardening behaviour of metallic materials with spherical second-phase particles at high temperatures

The work hardening behaviour of metallic materials with spherical second-phase particles at elevated temperatures has been discussed theoretically, based on the continuum mechanics model which incorporated the effect of the dynamic recovery by diffusion of atoms. It was found that the theoretical model developed in this study could satisfactorily explain the experimental results of tensile tests in an austenitic heat-resisting steel with M₂₃C₆ carbides. The model was also applied to the understanding of the internal stress during high-temperature creep in this steel.     

Dielectric relaxations and phase-transition-like behavior in SmAlO3 ceramics at high temperatures

SmAlO₃ ceramics samples were prepared via the solid-state reaction route. The low-frequency (20–10⁷ Hz) dielectric properties were investigated in the temperature range from 100 to 800 °C. It was found that SmAlO₃ shows a flat dielectric plateau with the value about 20 independent frequency below 220 °C. In the temperature range higher than 220 °C, two oxygen-vacancy related Debye-like relaxations (P _A, P _B) and a non-relaxation hump (P _H) were observed. Our results indicate that: (1) the low-temperature relaxation (P _A) is associated with the bulk effect due to hopping motions of oxygen vacancies; (2) the high-temperature relaxation (P _B) results from the sample/electrode contact effect; and (3) P _H is suggested to be a phase-transition-like behavior as a result of the oxygen-vacancy ordering process.     

Analytic evaluation of hydrogen-assisted void growth at high temperatures

We propose a model of grain-boundary growth of voids caused by diffusion in the presence of hydrogen. The influence of hydrogen is simulated by taking into account its pressure in the cavities. To evaluate the pressure of hydrogen, we perform the thermodynamic analysis of two-phase metal-cavity systems. Unlike traditional approaches, the newly proposed equation takes into account changes in the surface energy depending on the concentration of hydrogen and gives much more moderate estimates for the intermal pressure of hydrogen. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 4, pp. 75–86, July–August, 1997     

Plastic flow and fracture of tantalum carbide and hafnium carbide at low temperatures

Knoop and Vickers hardness measurements have been made on tantalum carbide and hafnium carbide single crystals. The hardness varies with orientation of the indenter in the crystal, but indentations in the two carbides are of very different character, TaC behaves in a relatively ductile manner and deforms plastically before cracking, while HfC exhibits extremely limited plastic flow and cracks on indentation. Moreover, the preferred slip plane is {111} in TaC but is {110} in HfC. These results are related to the reported physical properties of these carbides. In particular, the observed mechanical behaviour of TaC appears to be consistent with the more metallic nature of this carbide.     

Vickers indentation behavior of several commercial glasses at high temperatures

The Vickers indentation behavior of five commercial glasses has been investigated as a function of temperature. The glasses included: (i) soda-lime-silica Float glass, (ii) lead-alkali silicate, (iii) 7740 Pyrex™ borosilicate, (iv) potassium phosphate, and (v) lanthanum borate. A recording microindentation system was constructed to allow Vickers indentation testing to be conducted at temperatures significantly above room temperature. The Vickers hardness was observed to decrease continuously with increasing temperature for all glasses, with the exception of the 7740 Pyrex™ glass. Decreases in hardness were attributed to decreases in elastic moduli and bond strength with increasing temperature. The lengths of median-radial cracks around indentations in several glasses were observed to first increase, and then decrease, with increasing temperature. The first of this behavior was attributed to initial increases of the crack driving force, characterized by the quantity (E/H), as well as to a decrease in fracture surface energy. Viscous flow at higher temperatures was believed responsible for a reduction in the crack driving force and crack tip stress, resulting in an eventual decrease in crack length. Viscoelastic behavior of the Float glass was characterized by a rate-dependent hardness and indentation crack pattern. Load–displacement traces indicated an increase in the work of indentation and residual indentation depth with increasing temperature for all glasses.     

Effectiveness of hardening by methods of plastic surface deformation of components operating at high temperatures

Strength of Materials -     

Grain growth and strength degradation of SiC monofilaments at high temperatures

The microstructural stability of SCS-6 SiC monofilaments was determined by measuring the average grain size, tensile strength and critical length of the fibres as a function of various annealings in vacuum (0.1 Pa) at different temperatures (1400–1600 °C). The average grain size, calculated from X-ray diffraction line broadening, increased from 23 nm for the as-received fibres to 46 nm for fibres annealed for 2 h at 1600 °C. The corresponding tensile strength measured at room temperature dropped from 3.6 GPa for the as-received fibres to 2 GPa for the treated fibres. Simultaneously, the average critical lengths, measured using the glass-slide technique decreased from 0.37 mm for the as-received fibres to an average of 0.23 mm for the heat-treated fibres. The degradation of the mechanical properties was attributed to a combination of coarsening of the -SiC grains as well as interactions with the annealing environment, namely the vacuum hot-press chamber.     

Solid-state synthesis of tungsten carbide from tungsten oxide and carbon, and its catalysis by nickel

Tungsten carbide has been produced by heating a mixture of tungsten oxide and carbon powder at 1300 °C for 2 h. Further batches were made with additional KCl, KCl + Ni, or KCl + Fe. The products were compared by XRD and SEM. A mixture of WC and W₂C was produced from the plain WO₃/carbon reaction, but adding 1 wt.% nickel assisted the formation of a pure WC phase. Both Ni and Fe assisted the growth of larger WC crystals.