The thermal behavior of mechanically activated galena by thermogravimetry analysis

The thermal decompositions of mechanically activated and nonactivated galenas were studied by thermogravimetry analysis (TGA) at the heating rate of 10 K min⁻¹ in argon. Results indicate that the initial temperature of thermal decomposition (abbreviated as T _di ) in the TGA curves for different galenas decreases gradually with increased grinding time. The specific granulometric surface area (S _G ), the structural disorder, and the content of elemental sulfur of mechanically activated galenas were analyzed by an X-ray diffraction (XRD) laser particle-size analyzer, XRD analysis, and the gravimetric method, respectively, which shows that the specific granulometric surface area of mechanically activated galenas remains almost constant after a certain grinding time, but the lattice distortions (ɛ) rise, the crystallite sizes (D) decrease, and the elemental sulfur contents of mechanically activated galenas increase with increased grinding time. The results imply that the decrease of the initial temperature of thermal decomposition in the TGA curves for mechanically activated galenas is mainly caused by the increase of lattice distortions, and the formation of new dangling bonds resulted from the production of elemental sulfur of mechanically activated galenas with increased grinding time. Finally, the differences in the thermal-decomposition reactivity between nonactivated and mechanically activated galenas were also discussed.     

The thermal behavior of mechanically activated galena by thermogravimetry analysis

The thermal decompositions of mechanically activated and nonactivated galenas were studied by thermogravimetry analysis (TGA) at the heating rate of 10 K min⁻¹ in argon. Results indicate that the initial temperature of thermal decomposition (abbreviated as T _di) in the TGA curves for different galenas decreases gradually with increased grinding time. The specific granulometric surface area (S _G), the structural disorder, and the content of elemental sulfur of mechanically activated galenas were analyzed by an X-ray diffraction (XRD) laser particle-size analyzer, XRD analysis, and the gravimetric method, respectively, which shows that the specific granulometric surface area of mechanically activated galenas remains almost constant after a certain grinding time, but the lattice distortions (ε) rise, the crystallite sizes (D) decrease, and the elemental sulfur contents of mechanically activated galenas increase with increased grinding time. The results imply that the decrease of the initial temperature of thermal decomposition in the TGA curves for mechanically activated galenas is mainly caused by the increase of lattice distortions, and the formation of new dangling bonds resulted from the production of elemental sulfur of mechanically activated galenas with increased grinding time. Finally, the differences in the thermal-decomposition reactivity between nonactivated and mechanically activated galenas were also discussed.     

Effect of aging conditions on the leaching of mechanically activated pyrite and sphalerite

The leaching of mechanically activated pyrite and sphalerite exposed to nitrogen (99.999 vol pct) or air at ambient temperature or 573 K was investigated. The results indicate that at the same leaching time, the iron-leaching ratio of mechanically activated pyrite or sphalerite aged in nitrogen at both ambient temperature and 573 K decreases slightly with increasing aging time and remains constant after a certain aging period. The iron-leaching ratio of mechanically activated pyrite exposed to ambient air varies with the exposure period. But, at the same leaching time, the zinc-leaching ratio of mechanically activated sphalerite aged at ambient temperature does not change with the aging atmosphere. The structures of mechanically activated pyrite and sphalerite after being aged were determined. The specific granulometric surface area of mechanically activated pyrite and sphalerite decreases with increasing aging time, but keeps constant after a certain aging period. The X-ray diffraction patterns of mechanically activated pyrites aged in nitrogen do not change with aging time; neither do the X-ray diffraction patterns of mechanically activated sphalerites aged either in air or in nitrogen. For mechanically activated pyrite exposed to ambient air for 3 and 6 months, new phases were found. The lattice distortion and the elemental sulfur content of pyrite and sphalerite mechanically activated in nitrogen were also investigated. The results indicate that the elemental sulfur content of mechanically activated pyrite rises noticeably, and its lattice distortion (ε) rises slightly, with increasing grinding time. The elemental sulfur content of mechanically activated sphalerite remains constant at 0.5 mg elemental sulfur per gram of sphalerite, and its lattice-distortion ratio increases apparently with increasing grinding time. These observations provide further evidence for our opinion that the formation of dangling bonds on the surface of mechanically activated pyrites and the lattice distortion on the surface of mechanically activated sphalerites may mainly result in the enhancement of hydrometallurgical process for corresponding sulfide minerals.     

Temperature and strain rate dependence of the portevin-le chatelier effect in a rapidly solidified Al alloy

The Portevin-Le Chatelier (PL) effect was studied for a rapidly solidified aluminum alloy in a temperature range of 193 to 423 K and a strain rate range of 10~5 to 10~’ s’1. Emphasis was put on the dependence of the critical strain (ɛ _c) for the PL effect on temperature (7) and strain rate (ɛ). It is found that the overall ɛ_c -T-ɛ relation is grouped into two categories: ɛ_c increases with increasing ɛ but with decreasingT in the low T/high ɛ regime and ɛ_c increases with increasingT but with decreasing ɛ in the high T/1ow ɛ regime. The underlying dynamic strain aging (DSA) mechanism is discussed to account for the two different yet unified categories by calculating activation energies and by introducing the transition conditions. Formerly with the Department of Materials Science, Delft University of Technology.     

Oxygen solubility in liquid indium and oxygen diffusivity in liquid indium and tin

The solubility of oxygen in liquid indium, C_o, at 973 and 1073 K in equilibrium with its oxide was determined by an isopiestic equilibration technique in order to resolve discrepancies reported in the literature. The present results, C_o = 0.0092 at. pet at 973 K and 0.0377 at. pet at 1073 K, agree with those obtained by Otsuka, Sano, and Kozuka using a modified coulometric titration method. Oxygen diffusivity in liquid indium from 873 to 1073 K and in liquid tin from 973 to 1273 K was measured utilizing a combined potentiostatic and emf method using the following double electrochemical cells: Fe,FeO/ZrO₂(+CaO)/O in Me(I)/ZrO₂(+CaO)/O in Me(II). The present results are D_O(In) = 6.6 ( _-1.6 ^+2.0 ) x 10⁻³ exp[(-3-600 ± 5600)/RT]873 K ≤T ≤ 1073 K and D_O(Sn), = 8.7( _-5.7 ^+13.5 ) x 10⁻⁴ exp[(-18800 ± 6700)/RT]973 K ≤T ≤ 1273 K. The present results are of the same order of magnitude with the self-diffusivity of the liquid metals, and are about two orders of magnitude greater than the oxygen diffusivity reported by Stevenson and co-workers. The ratio of oxygen diffusivity to self-diffusivity of the solvent was found to be correlated to the enthalpy of formation per mole of oxygen of the respective oxide at 298 K.     

Oxidation kinetics of niobium in the temperature range of 873 to 1083 K

Oxidation kinetics of niobium (columbium) in oxygen are investigated in the temperature range of 873 to 1083 K. The observed abnormal dependence of the linear oxidation rate on temperature is suggested to be due to the relative amount of NbO₂ in the oxide scale. The amount of oxygen dissolved in the metal is calculated and it constitutes a small fraction of the total oxygen uptake. From hardness measurements, the oxygen diffusion coefficient in Nb can be expressed as:D = 2.72 × 10⁻³ exp ( − 95.4/R T) with the activation energy in kjoule/mole.     

Desulfurization of iron alloys in vacuo-free energy of formation of SiS (v)

The rate of desulfurization of Fe-C-Si-S meltsin vacua (10 ώm pressure) has been investigated using 15 kg alloys melted by induction heating. For the sulfur contents from 0.008 to 0.1 pct investigated, the rate is a first-order type with respect to sulfur. Over this sulfur concentration range and in the presence of carbon up to saturation and silicon from 0 to 5 pct, the sulfur activity is sufficiently high that the fractional surface coverage by adsorbed sulfur is within 0.75 < θ_S < 1. The rate equation derived to satisfy the experimental findings for the limiting case of θ_S → 1 indicates that sulfur is evolved primarily via two activated reactions involving (SiS₂)° and (S₂)°. These complexes then produce SiS and S vapor species. At 1600°C the rates of formation of these volatile species are about one fourth of those for free vaporization of SiS and sulfur, respectively. The apparent heats of activation are ∼47 kcal for SiS and ∼37 kcal for sulfur vapor. Using an apparatus involving a Knudsen cell and the Bendix mass spectrometer, the enthalpy of reaction SiS₂(s) + Si(s) = 2SiS(v) has been measured, giving 102.2 kcal. Combining this with other thermodynamic data, the free energy of formation of SiS vapor is evaluated as Si(s) + 1/2S₂(v) = SiS(v) δF° = 15,500-19.5T from 1000° to 1686°K Si(l)+ 1/2S₂(v)= SiS(v) δF°= 3500 -12.4T above 1686°K     

Oxygen and sulfur solubilities in Ni-Fe-S-O melts

Oxygen and sulfur solubilities were determined in Ni-Fe-S-O melts under the following conditions: 10^-11.50 ≤ P_O2 ≤ 10^-8.50 atm; 10^-3.00 ≤ P_{s 2} ≤ 10^-2.00 atm; 0.19 ≤ Ni/(Ni + Fe) ≤0.85; and 1473 K ≤T ≤ 1573 K. The oxygen solubility was found to increase with increasing partial pressure of oxygen up to a maximum value at oxide saturation and to decrease with increasing equilibrium partial pressure of sulfur. The ferrous metal content enhanced oxygen solubility. The trends in dissolution behavior of sulfur were opposite to those of oxygen with respect to changing P_{O 2} and P_{S 2} and to the Ni/(Ni + Fe) ratio; however, at high matte grades Ni/(Ni + Fe) > approximately 0.5, sulfur solubility appeared to decrease as a function of the Ni/(Ni + Fe) ratio, as did oxygen solubility. The standard Gibbs energy of oxygen dissolution in Ni-Fe-S-O melts Ni/(Ni + Fe) = 0.47 in the temperature range 1473 to 1573 K can be described by ΔG° = −202.5 + 0.0660 T(K) (±1.5 kJ/mol)     

Microstructural stability on aging of an α + β titanium alloy: ti- 6ai- 1.6zr-3.3mo- 0.30si

The development of the microstructure on aging of an (α + β) type titanium alloy containing 6A1-1.6Zr-3.3Mo-0.3Si (VT9) (in weight percent) has been studied. The β-transus temperature of this alloy is approximately 1243 K. Solution treatment in the β-phase field of the alloy followed by quenching in water at room temperature resulted in the formation of a single-phase martensite struc-ture. The martensitic structure was confirmed to be orthorhombic (α″) using X-ray diffraction. The water-quenched (WQ) specimens were subjected to aging treatments at temperatures of 823, 873, and 973 K for various lengths of time. Aging at 823 K for times between 24 and 100 hours did not bring about any noticeable change in the microstructure. Aging at 823 K for 200 and 300 hours resulted in the heterogeneous precipitation ofs ₂ silicide particles and thin films of β sandwiched between the interplatelet boundaries of martensite. Electron diffraction analysis confirms that the crystal structure of silicide particles is hexagonal with lattice parameters α= 0.70(1) nm andc = 0.36(8) nm. Aging at 873 K for 12 and 24 hours resulted only in the precipitation ofs ₂ silicide particles, while aging at the same temperatures for longer times (48, 100, and 200 hours) and also at 973 K for 6 to 100 hours resulted in the precipitation of silicides and also thin films of β and acicular martensite. The relative sizes of silicide precipitates and width of thin films of β phase increase with increasing aging time. The sites for silicide precipitation are mainly at α′-α′ boundaries, α-β interfaces, and sometimes within regions of transformed β. The kinetics ofs ₂ silicide precipi-tation in this alloy is faster than in commercial near-α titanium alloys. This is attributed to the presence of Mo, a strong β stabilizer. Formerly Reader, Department of Metallurgical Engineering, Centre of Advanced Study, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India     

Dynamic fracture toughness of a Ti-45Al-1.6Mn alloy at high temperature

Dynamic fracture toughness of a reactive sintered γ-base TiAl alloy is studied in the temperature range from 298 to 1073 K. The stop block method is employed in order to observe the crack paths and microcrack distribution ahead of a main crack tip under dynamic loading conditions at high temperature. Fracture surface, crack path, and microcrack observations are carried out using a scanning electron microscope (SEM). Microcrack initiation criteria and crack-tip stress shielding effect caused by crack deflection are discussed. The experimental results demonstrate that the dynamic fracture toughness, J _Id , increases with increasing temperature, and after attaining the maximum value at 873 K, the toughness decreases. Crack path morphology varies with temperature. The stress shielding effect at the crack tip caused by main crack deflection was found to affect the difference in crack extension energy for each temperature. The number of microcracks ahead of a main crack varies with temperature. The stress shielding effect at the crack tip caused by microcracking was found to contribute to toughening around 873 K.     

Low-temperature superplasticity of ultra-fine-grained Ti-6Al-4V processed by equal-channel angular pressing

The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting (coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10⁻⁵ to 10⁻² s⁻¹. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and 10⁻⁴ s⁻¹. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation theory.     

Influence of solid state properties on ferric chloride leaching of mechanically activated galena

The reaction of mechanically activated galena with ferric chloride solution was investigated in the concentration range [Fe³⁺] =0.01–0.6 M at temperatures between 303 and 338 K. The mechanical activation lasting 5–30 min was carried out in a planetary mill. It has been found that this reaction is sensitive to the microstrains produced in the structure of galena during grinding as well as to the concentration of Fe³⁺ in the solution. The influence of Fe³⁺ concentration is in operation practically only in the region [Fe³⁺] < 0.2 M.     

Hydrogen diffusion in Al-Li alloys

The diffusion coefficients of hydrogen in binary Al-Li alloys containing 1,2, and 3 wt pct Li have been determined from desorption curves of samples saturated with hydrogen at 473 to 873 K. Within this temperature range, the diffusivity of hydrogen in the binary Al-Li alloys investigated has an Arrhenius-type temperature dependence and follows the equation of the general formD = DT) whereD ₀exp(-Q/R is the diffusion coefficient (m²/s),D ₀ is the preexponential or frequency factor (m²/s), R is the gas constant (J/K mol),Q is the activation energy (J/mol), andT is absolute temperature (K). The rate of diffusion of hydrogen in aluminum decreases with increase in lithium additions. This is provisionally attributed to the stronger local binding energy between hydrogen and lithium atoms in the aluminum metal lattice.     

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10⁻⁵ s⁻¹ to 3×10⁻³ s⁻¹. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10⁻⁵ s⁻¹ and 3×10⁻³ s⁻¹ and beyond 923 K for 3×10⁻⁴ s⁻¹ strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.     

Tensile and Thermal Properties in Compacted Graphite Irons at Elevated Temperatures

Tensile and thermal properties of compacted graphite irons (CGIs), prepared with various molybdenum additions and solidification rates, have been investigated for temperatures between room temperature and 873 K (600 °C). A slower solidification rate resulted in larger and fewer graphite particles as well as in an increase of intercellular cementite, or carbides. Molybdenum is a carbide stabilizing element; i.e., increasing additions of molybdenum increased the amount of carbides. Young’s modulus decreased with increasing temperature, and a lower solidification rate increased this parameter slightly. Both increasing content of carbide and increasing nodularity increased the Young’s modulus. Strength parameters such as yield strength and ultimate tensile strength (R _m ) were affected in similar ways by temperature and solidification rate. The strength values were generally quite temperature independent for temperatures below 573 K (300 °C) but decreased rapidly for higher temperatures. Increasing nodularity increased the strength, while increasing content of carbide had little influence on the values. The thermal conductivity decreased with increasing content of carbide and increasing nodularity. The thermal conductivity generally showed a maximum value at 573 K (300 °C). A contradictory linear relationship was found between yield strength and thermal conductivity.     

Leaching of marine manganese nodules by acidophilic bacteria growing on elemental sulfur

This article describes the bioleaching of manganese nodules by thermophilic and mesophilic sulfuroxidizing bacteria, in which oxidized sulfur compounds are biologically produced from elemental sulfur added to liquid medium and are simultaneously used to leach nodules. The thermophile Acidianus brierleyi solubilized the manganese nodules faster at 65 °C than did the mesophiles Thiobacillus ferrooxidans and Thiobacillus thiooxidans at 30 °C. Leaching experiments with A. brierleyi growing on elemental sulfur were used to optimize various process parameters, such as medium pH, initial sulfur-liquid loading ratio, and initial cell concentration. The observed dependencies of the leaching rates at a pH optimum on the initial amounts of elemental sulfur and A. brierleyi cells were qualitatively consistent with model simulations for microbial sulfur oxidation. Under the conditions determined as optimum, the leaching of nodule particles (−330+500 mesh) by A. brierleyi yielded 100 pct extraction of both copper and zinc within 4 days and high extractions of nickel (85 pct), cobalt (70 pct), and manganese (55 pct) for 10 days. However, the iron leaching was practically negligible.     

Variation in crystallinity of hydroxyapatite and the related calcium phosphates by mechanical grinding and subsequent heat treatment

The crystallinity of hydroxyapatite (HAp) and the related calcium phosphates for regenerating hard tissues was controlled by the mechanical grinding (MG) method and subsequent heat treatment. The HAp, carbonate-apatite (CO₃Ap), fluorapatite (FAp), and α- and β-tricalcium phosphates (α-TCP and β-TCP, respectively) and tetracalcium diphosphate monoxide (TTCP) were used as initial materials. Variations in crystallinity and crystal structure were examined by the X-ray diffraction (XRD) method during MG and the following heat treatment. The crystallinity, based on crystallite size and crystal elastic strain, decreased with grinding time, and the decreasing rate depended on the type of calcium phosphate; crystallographic diffraction peaks disappeared more rapidly in CO₃Ap than in FAp. The change in the morphology of powder during MG was influenced by the primary particle size of the first-stage product; α-TCP, β-TCP, and TTCP powders composed of large particles were predominantly shattered into small pieces and then gathered during MG, while the crystal strain in the HAp, CO₃Ap, and FAp powders was mainly accumulated without significant refinement of crystallite size. The thermal-recovery process of crystallinity and crystal structure in the milled powders was investigated. The crystallinity of HAp, CO₃Ap, and FAp powders recovered depended on annealing temperature. The novel phase of β’-TCP with higher ordering than β-TCP appeared during heat treatment from the amorphous state of α-TCP or β-TCP obtained during MG. The MG and subsequent heat treatment were, finally, concluded to be an effective process for controlling the crystallinity and changing crystal structure in calcium phosphate powders.     

Phase transformations and phase equilibria in the Fe-N system at temperatures below 573 K

The phase transformations of homogeneous Fe-N alloys of nitrogen contents from 10 to 26 at. pct were investigated by means of X-ray diffraction analysis upon aging in the temperature range from 373 to 473 K. It was found that precipitation of α″-Fe₁₆N₂ below 443 K does not only occur upon aging of supersaturated α (ferrite) and α′ (martensite), but also upon transformation of γ′-Fe₄N_1-z and ɛ-Fe₂N_1-x (<20 at. pct N). No α″ was observed to develop upon aging of γ(austenite). Therefore, it is proposed that γ′ is a stable phase at temperatures down to (at least) 373 K. Phase formation upon annealing at low temperatures is apparently governed by the (difficult) nucleation and (slow) growth of new Fe-N phases: α″ forms as a precursor for α because of slow nitrogen diffusion, and nitrogen-enriched ɛ develops as a precursor for γ′ because of a nucleation barrier.     

Carbothermic reduction of llmenite (FeTiO3) and rutile (TiO2)

The mechanically activated carbothermic reduction mechanism of ilmenite has been examined by a combination of steady-state and dynamic thermal techniques coupled with X-ray diffraction. The reaction was found to proceed via an initial, rapid reduction to elemental iron and rutile, which was followed by a slow reduction of rutile to a series of oxides of the general formula Ti _n O_2n−1 until Ti₃O₅ was formed, which was found to be relatively stable. Iron was probably incorporated into the Ti _n O_2n−1 lattice only for n>3, forming mixed oxides of uncertain composition. The formation of TiC was evident at temperatures as low as 1100 °C, but the rate of reaction was extremely slow, presumably due to a solid-state diffusion limitation. Increasing the temperature gave increasing conversion of TiO₂ to TiC until it was the only confirmed product. The effect of iron on the later stages of reduction was removed by examining the reduction of pure rutile. It was found that the reduction of Ti₃O₅ was enhanced by the presence of iron. The separation of iron from the titanium product proved to be high, with > 90 pct of iron removed after the initial reduction. The iron removal increased slowly to almost 100 pct when elemental iron and titanium carbide were the products.