Infra-red studies of vitreous silica at elevated temperatures

The purpose of this paper is to present a new approach to the problem of structure of the ordered regions in X-ray amorphous silica glass. The problem is open because of the differences in scientific opinion as to which polymorph of silica is the true crystalline model of vitreous silica. Infra-red absorption spectra of cristobalite and silica glass taken at temperatures ranging from room temperature to 500° C were used to identify a characteristic low-high temperature polymorphic transition relaxed in X-ray amorphous silica after heating at 210° C for at least 10 h. The infra-red spectrum of silica glass from various sources was identified as the spectrum of high-cristobalite, the spectrum after the polymorphic transition in silica glass is identical with that of low-cristobalite. The results give a strong support to the high-cristobalite/cubic/model of normal silica glass, with ordered regions in metastable form at room temperature.     

Tensile deformation behaviour of two aluminium alloys at elevated temperatures

Abstract

The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.     

Electrical conductivity and sintering in iron oxides at high temperatures

Two raw materials were chiefly used, namely laboratory ferric oxide and a high grade hematite. All experiments were carried out in an oxygen atmosphere, in the range 600 to 1300° C.The conductivity/temperature determinations described are of two kinds,viz those on loose powders and those across fractures in previously compacted and sintered material. In the former, indications were obtained of differences in electrical behaviour above 1100° C which could be correlated with the appearance (red or black) of the powder. In the latter, unstable electrical behaviour was noted above the Tammann temperature in the hematite, but not in the ferric oxide.Additionally, isothermal sintering experiments were carried out, at 975 to 1200° C on red ferric oxide, and at 1350 to 1400° C on black ferric oxide. The temperature dependence was approximately the same for both materials and the results are compatible with the electrical data, on the assumption that conductivity in general reflected particle surface conditions, whereas shrinkage was dependent on internal lattice movements.     

Enhanced reduction of nitrate by zero-valent iron at elevated temperatures

Kinetics of nitrate reduction by zero-valent iron at elevated temperatures was studied through batch and column experiments. It was hypothesized that under increased solution temperatures, the zero-valent iron may accelerate the reduction of nitrate by overcoming the activation energy barrier to nitrate reduction. The results of the batch experiment showed the synergistic effects of elevated temperature (75 degrees C) and a buffered condition (pH 7.4 with 0.1 M HEPES) to enhance the rate of nitrate reduction by zero-valent iron from 0.072+/-0.006 h(-1) ((0.35+/-0.03) x 10(-4) L m(-2) h(-1)) at room temperature to 1.39+/-0.23 h(-1) ((1.03+/-0.07) x 10(-3) L m(-2) h(-1)). Complete nitrate removal was obtained in a Fe(0) column after 30 min under both buffered and unbuffered conditions at 75 degrees C. These results indicate that a temperature increase could overcome the energy barrier. We suggest that an iron reduction process at moderately elevated temperature (50-75 degrees C) may be a suitable method for removing nitrate from industrial discharges.     

Enhanced reduction of perchlorate by elemental iron at elevated temperatures

Kinetics of perchlorate reduction by elemental iron was examined at elevated temperatures using microwave heating and conventional block heating. It was hypothesized that increasing the solution temperature may accelerate the reduction of perchlorate by overcoming the high activation energy barrier. Results from microwave heating study showed that 98% of aqueous perchlorate was removed in 1 h at 200 degrees C. Similar results observed in control experiments with a block heater indicated that the enhancement in the extent and rate of perchlorate removal by elemental iron was mostly due to heat energy at high temperature. The rapid and complete reduction of perchlorate by elemental iron at elevated temperatures suggests that iron reduction process at elevated temperature may be an option to consider for complete removal of perchlorate from industrial discharges.     

Carbon solubilities in iron at elevated temperatures analysed by statistical thermodynamics

Abstract

Carbon solubilities in iron at elevated temperatures (>1000 K) are analysed on the basis of statistical thermodynamics. A parameter Q which refers to the extent of stabilization of a carbon atom in the iron lattice is estimated for α-, γ-, and molten FeC_x phases. The results suggest that the carbon atom is most stable in molten FeC_x and least stable in (α-FeC_x. In addition, the value of Q_C^α for (α-FeC_x at high temperatures appears to be different from that at lower temperatures. This observation can be interpreted as the effect of the magnetic transition of the iron atom around the Curie temperature. The values for Q estimated for the other interstitial elements X (X = H, N, P, S) in iron lattices are compared.

MST/586     

Deformation of PC/ABS alloys at elevated temperatures and high strain rates

The objective of this paper is to experimentally study the deformation behavior of the alloys of polycarbonate (PC) and acrylonitrile–butadiene–styrene (ABS) at elevated temperatures and high strain rates. Four kinds of PC/ABS alloys with the ratio of PC to ABS being 80:20, 60:40, 50:50 and 40:60 and three different strain rates 8.0 × 10² s⁻¹, 2.7 × 10³ s⁻¹ and 1.0 × 10⁴ s⁻¹ are considered. The Split Hopkinson Pressure Bar (SHPB) experiments are carried out at 293 K and 343 K, respectively. The curves of engineering stress and engineering strain and true stress and true strain are obtained for the PC/ABS alloys at different temperatures and different strain rates, respectively. The effects of temperature, strain rate and the fraction of ABS on the deformation behavior of PC/ABS alloys are discussed in details, and then a temperature and strain rate-dependent phenomenological constitutive model for PC/ABS alloys is developed.     

Fracture-resistant ultralloys for space-power applications: Normal-spectral-emissivity and electron-emission studies of tungsten, rhenium, hafnium-carbide alloys at elevated temperatures

A series in this journal on high-temperature properties of “fracture-resistant ultralloys for space-power systems” preceded the present paper: the antecedent publications covered tungsten(W), rhenium(Re) alloys with and without thoria(ThO₂) (W, 23Re; W, 27Re; W, 30Re and W, 30Re, 1ThO₂). This paper reports radiative and thermionic effects of hafnium carbide(HfC) and Re variation in W alloys: normal spectral emissivity(ε_λ) is used in pyrometry to determine the true temperature of a surface. Effective work function (φ_e) is an important consideration in the selection of the electrode materials for high-temperature thermionic energy converters in space-power applications. The _0.535μ, ε_0.65μ and φ_e trends of W, Re, 0.35HfC with 5–20% Re were measured in the range of 1700–2500K. The results indicate that ε_λ decreases with increasing temperatures and Re contents. The presence of HfC produced higher ε_λ values than those of sintered materials with comparable W,Re alloy contents. The results also indicate that φ_e increases with rhenium contents. This can be explained as growth of the potential barrier at the metal, vacuum boundary associated with a volume effect—the decrease in the lattice constant of W.     

Reactions of Hf-Ag and Zr-Ag alloys with Al2O3 at elevated temperatures

We are studying reactions of Ti, V, Zr, and Hf with ceramics as part of a program to understand fundamental reaction and bonding mechanisms in active metal brazing of ceramics. In this paper we present results of experiments with model systems comprising Ag alloys that contain different amounts of Hf or Zr that were reacted with sapphire or 99.6% alumina for different times and temperatures in a controlled atmosphere furnace. In these alloys the Ag functions as an inert solvent, which allowed us systematically to determine the effects of changes in concentration of the active element. We observed qualitative wetting and spreading tendencies of the alloys during heating and examined cross sections after cooling using electron analytical techniques. For all reaction times studied, the Hf/Ag alloys formed a discontinuous reaction layer, which was consistent with earlier high-resolution electron microscopy that showed sub-micrometer HfO₂ particles embedded in the surfaces of the Al₂O₃ grains. By contrast, initial reaction of the Zr/Ag alloys with Al₂O₃ produced a continuous interface layer. With longer reaction times, the ZrO₂ reaction product became much thicker and exhibited three distinct zones at the interface. The results suggest that the rate limiting step in the Zr/Ag reaction is the chemical reaction at the interface, whereas with Hf/Ag reaction diffusion of products away from the interface is rate limiting.     

The determination of surface tension at elevated temperatures by drop image analysis

The advantages of image analysis as applied to surface tension measurements by the sessile drop technique are discussed. It is demonstrated that valuable effective improvements of dimensional resolution are obtained. Errors in surface tension arising from various unavoidable sources such as chemical reactions or optical distortions are shown to be the main difficulties in applying the sessile drop technique at high temperature. This suggests a limiting spatial resolution of the digitizer beyond which no further improvements to the accuracy of the surface tension results are possible.

The technique has been applied to determine the surface tension of liquid Cu, Bi and Ag as well as a series of sodium borosilicate glasses.     

In-situ surface transformation of magnesium to protect against oxidation at elevated temperatures

The native oxide thin scale on magnesium(Mg) surface appears continuous and crack-free, but cannot protect the Mg matrix from further oxidation, especially at elevated temperatures. This thermal oxidation process is witnessed in its entirety using a home-made in-situ heating device inside an environmental electron transmission microscope. We proposed, and verified with real-time experimental evidence, that transforming the native oxide scale into a thin continuous surface layer with high vacancy formation energy(low vacancy concentration), for example MgCO_3, can effectively protect Mg from high-temperature oxidation and raise the threshold oxidation temperature by at least two hundred degrees.     

Structure and morphology studies of chromium film at elevated temperature in hypersonic environment

This paper presents the after shock heated structural and morphological studies of chromium film coated on hypersonic test model as a passive drag reduction element. The structural changes and the composition of phases of chromium due to shock heating (2850?K) are characterized using X-ray diffraction studies. Surface morphology changes of chromium coating have been studied using scanning electron microscopy (SEM) before and after shock heating. Significant amount of chromium ablation and sublimation from the model surface is noticed from SEM micrographs. Traces of randomly oriented chromium oxides formed along the coated surface confirm surface reaction of chromium with oxygen present behind the shock. Large traces of amorphous chromium oxide phases are also observed.     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.