Factors that influence <Emphasis Type="Italic">CSR</Emphasis> and <Emphasis Type="Italic">CRI</Emphasis>

The influence of various factors on CSR and CRI in the production of metallurgical coke at OAO MMK is evaluated.     

Analysis of the <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis> transformation in a C-Mn steel by phase-field modeling

This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.     

<Emphasis Type="Italic">In-situ</Emphasis> reactive processing of nickel aluminides by laser-engineered net shaping

Nickel aluminide intermetallics (e.g., Ni₃Al and NiAl) are considered to be attractive materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a rapid prototyping process, which involves laser processing fine metal powders into three-dimensional shapes directly from a computer-aided design (CAD) model. In this work, an attempt has been made to fabricate aluminide intermetallic compounds via reactive in-situ alloying from elemental powders using the LENS process. In-situ reactive alloying was achieved by delivering elemental Ni and Al powders from two different powder feeders, eliminating segregation observed in the samples deposited by using the premixed elemental powders. Nickel aluminides of various compositions were obtained easily by regulating the ratio of their feed rates. The aluminide deposits exhibited a high solidification and subsolidus cracking susceptibility and porosity formation. The observed porosity resulted from a water-atomized Ni powder and can be minimized or eliminated by the use of a N₂-gas-atomized Ni powder of improved quality. Cracking was due to the combined effect of the high thermal stresses generated from the LENS processing and the brittleness of the intermetallics. Crack-free deposits were fabricated by preheating the substrate to a temperature of 450 °C to 500 °C during LENS processing. Compositionally graded Ni-Al deposits with a gradient microstructure were also produced by the in-situ reactive processing.     

Synthesis of <Emphasis Type="Italic">α</Emphasis>- and <Emphasis Type="Italic">β</Emphasis>-Rhombohedral Boron Powders<Emphasis Type="Italic"> via</Emphasis> Gas Phase Thermal Dissociation of Boron Trichloride by Hydrogen

The α-rhombohedral and β-rhombohedral crystal structures of pure elemental boron powders have been synthesized via gas phase thermal dissociation of BCl₃ by H₂ on a quartz substrate. The parameters affecting the crystal structures of the final products and the process efficiency, such as BCl₃/H₂ molar ratio (1/2 and 1/4) and reaction temperature (1173 K to 1373 K [900 °C to 1100 °C]), have been examined. The experimental apparatus of original design has enabled boron powders to be obtained at temperatures lower than those in the literature. The surface/powder separation problem encountered previously with different substrate materials has been avoided. Boron powders have been synthesized with a minimum purity of 99.99 pct after repeated HF leaching. The qualitative analysis of exhaust gases has been conducted using a Fourier transform infrared spectroscope (FTIR). The synthesized powders have been characterized using an X-ray powder diffractometer (XRD) and scanning electron microscope (SEM) techniques. The results of the reactions have been compared with equilibrium predictions performed using the FactSage 6.2 (Center for Research in Computational Thermochemistry, Montreal, Canada) thermochemical software.     

<Emphasis Type="Italic">In-situ</Emphasis> formation of SiC-reinforced Al-Si alloy composites using methane gas mixtures

Theoretical and experimental studies on the in-situ formation of an Al-Si alloy composite using a methane gas mixture were carried out. An Al-Si alloy composite with in-situ formed SiC as a reinforced phase was produced by bubbling methane gas at temperatures from 1223 to 1423 K. An optical microscope, scanning electron microscope (SEM), and electron microprobe were used for the product characterization. Primary and eutectic silicon were observed in the samples taken from the top part of the crucible, and only eutectic silicon was observed in the samples taken from the bottom of crucible. The SiC formation rate increased with the decrease in the bubble size. A silicon concentration gradient existed at different vertical positions of the liquid alloy. The silicon concentration close to the top of the liquid alloy was higher than that at the bottom. The SiC concentration was very low in the bulk alloy. The bubbling of the gas mixture in the melt resulted in the formation of a layer of foam on top of the crucible. Formed SiC particles were enriched in the foam and carried out of the crucible by the overflow foam to a composite collector located under the crucible. The foam in the composite collector was broken, and composites in the foam contained up to 30 wt pct SiC. The particle size of the SiC is in the range of 1 to 10 μm. The bubbling process resulted in the unevenness of the silicon concentration and the different crystallizing processes. The SiC formation rate was found to be about 12.5 mg/(L·s). A kinetic model was developed. The model-predicted results are in very good agreement with the experimental results.     

Optimization of the johnson-mehl-avarami equation parameters for <Emphasis Type="Italic">α</Emphasis>-ferrite to <Emphasis Type="Italic">γ</Emphasis>-austenite transformation in steel welds using a genetic algorithm

A nonisothermal Johnson-Mehl-Avarami (JMA) equation with optimized JMA parameters is proposed to represent the kinetics of transformation of α-ferrite to γ-austenite during heating of 1005 steel. The procedure used to estimate the JMA parameters involved a combination of numerical heat-transfer and fluid-flow calculations, the JMA equation for nucleation and growth for nonisothermal systems, and a genetic algorithm (GA) based optimization tool that used a limited volume of experimental kinetic data. The experimental data used in the calculations consisted of phase fraction of γ-austenite measured at several different monitoring locations in the heat-affected zone (HAZ) of a gas tungsten arc (GTA) weld in 1005 steel. These data were obtained by an in-situ spatially resolved X-ray diffraction (SRXRD) technique using synchrotron radiation during welding. The thermal cycles necessary for the calculations were determined for each monitoring location from a well-tested three-dimensional heat-transfer and fluid-flow model. A parent centric recombination (PCX) based generalized generation gap (G3) GA was used to obtain the optimized values of the JMA parameters, i.e., the activation energy, pre-exponential factor, and exponent in the nonisothermal JMA equation. The GA based determination of all three JMA equation parameters resulted in better agreement between the calculated and the experimentally determined austenite phase fractions than was previously achieved.     

<Emphasis Type="Italic">In-Situ</Emphasis> Scanning Electron Microscopy/Electron Backscattered Diffraction Observation of Microstructural Evolution during <Emphasis Type="Italic">α</Emphasis> → <Emphasis Type="Italic">γ</Emphasis> Phase Transformation in Deformed Fe-Ni Alloy

This article presents in-situ observation of ferrite (α)/austenite (γ) phase transformation in an Fe-8.5 at. pct Ni alloy deformed by rolling using an automated scanning electron microscopy/energy backscattered diffraction (SEM/EBSD) system. During heating, recrystallization in α phase and α → γ phase transformation independently occurred. The γ grains nucleated in unrecrystallized α grains were most probably incorporated into the grain interior of recrystallized α grains. They did not have any specific orientation relation (OR) with recrystallized α grains and grew in an isotropic manner. On the other hand, the intragranular γ grains nucleated in recrystallized α grains had a Kurdjumov–Sachs (K-S) OR with the α grains and grew in a considerably anisotropic manner. They preferentially grew along the common direction of surface traces of {110} _α /{111} _γ . Approximately half of grain boundary (GB) allotriomorphs had either the K-S OR or the Nishiyama–Wasserman (N-W) OR with the parent α grains. The γ allotriomorphs predominantly grew into the α grain having the special OR with themselves. The GB character distribution of γ phase at high temperatures was measured. The fraction of CSL boundaries was as high as 63 pct, particularly that of Σ3 grain boundaries (GBs) was 54 pct.     

Magnetic contribution to the interdiffusion coefficients in bcc (<Emphasis Type="Italic">α</Emphasis>) and fcc (<Emphasis Type="Italic">γ</Emphasis>) Fe-Ni alloys

The interdiffusion coefficients in bcc (α) and fcc (γ) Fe-Ni alloys below their Curie temperatures have been calculated based on the magnetic contribution to the free energy for interdiffusion. The free energy for interdiffusion due to magnetic ordering in bcc Fe-Ni alloys is positive. The calculated interdiffusion coefficients in bcc Fe-Ni alloys fit the experimental data quite well. In fcc Fe-Ni alloys, the magnetic contribution to interdiffusion depends on both temperature and composition and is abnormal for Ni compositions in the Invar region. The free energy of vacancy formation is positive and the free energy of vacancy migration is negative, due to the effect of magnetic ordering. The interdiffusion coefficient in the ferromagnetic phase is lower than that extrapolated from the paramagnetic phase for Ni compositions of 50 at. pct and greater and is higher than that extrapolated from the paramagnetic phase for Ni compositions of 40 at. pct and lower.     

Effect of strain rate on the strain-induced <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis>′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10⁻⁴ and 200 s⁻¹. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.     

Study of <Emphasis Type="Italic">γ</Emphasis>/<Emphasis Type="Italic">γ</Emphasis>′ Interfaces in Nickel-Based,Single-Crystal Superalloys by Scanning Transmission Electron Microscopy

The interfaces of γ/γ′ in crept Re-bearing, single-crystal, nickel-based superalloys have been studied by scanning transmission electron microscopy. Elements Ni and Ta are congregated at the γ′ side, ahead of the Re enrichment near interfaces, which shows the retarding effect on element diffusion during creep by Re enrichment. A growth step exists at the γ/γ′ interfaces with the apex of kinks congregated with heavy atoms, which may slow down interface displacement during creep.     

Kinetics of <Emphasis Type="Italic">In-situ</Emphasis> formation of AlN in Al alloy melts by bubbling ammonia gas

In-situ processing of AlN-Al alloy composites by the gas bubbling method was investigated using ammonia as the gaseous precursor in the temperature range of 1373 to 1523 K. The products were characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray microanalysis. It was found that in-situ formation of AlN reinforcing particles was feasible by bubbling ammonia through aluminum and aluminum alloy melts. The AlN particles formed in situ were small in size and enriched in the top part of the product formed in the crucible. In comparison with the purified nitrogen bubbling gas, using ammonia as the nitrogen precursor enhanced the formation of AlN. Hydrogen gas generated from the dissociation of ammonia is an effective oxygen getter in the system, which can offset the deleterious effect of oxygen impurities and enhance the formation of AlN. The rate of formation of AlN was controlled by the diffusion of nitrogen atoms in the liquid boundary layer. A kinetic model was developed to describe the rate of formation of AlN, which was in excellent agreement with the experimental results. The influence of various processing variables on the rate of formation of AlN were also investigated. The rate of formation of AlN changed little with the content of silicon in the matrix melt, increased slightly with increase in temperature and decrease in nozzle size, and increased significantly with the increasing gas flow rate.     

Direct observations of the <Emphasis Type="Italic">α</Emphasis> → <Emphasis Type="Italic">γ</Emphasis> transformation at different input powers in the heat-affected zone of 1045 C-Mn steel arc welds observed by spatially resolved X-ray diffraction

Spatially resolved X-ray diffraction (SRXRD) experiments have been performed during gas tungstenarc (GTA) welding of AISI 1045 C-Mn steel at input powers ranging from 1000 to 3750 W. In-situ diffraction patterns taken at discreet locations across the width of the heat-affected zone (HAZ) near the peak of the heating cycle in each weld show regions containing austenite (γ), ferrite and austenite (α+γ), and ferrite (α). Changes in input power have a demonstrated effect on the resulting sizes of these regions. The largest effect is on the γ phase region, which nearly triples in width with increasing input power, while the width of the surrounding two-phase α+γ region remains relatively constant. An analysis of the diffraction patterns obtained across this range of locations allows the formation of austenite from the base-metal microstructure to be monitored. After the completion of the α → γ transformation, a splitting of the austenite peaks is observed at temperatures between approximately 860 °C and 1290 °C. This splitting in the austenite peaks results from the dissolution of cementite laths originally present in the base-metal pearlite, which remain after the completion of the α → γ transformation, and represents the formation of a second more highly alloyed austenite constituent. With increasing temperatures, carbon, originally present in the cementite laths, diffuses from the second newly formed austenite constituent to the original austenite constituent. Eventually, a homogeneous austenitic microstructure is produced at temperatures of approximately 1300 °C and above, depending on the weld input power.     

<Emphasis Type="Italic">In-situ</Emphasis> three-dimensional microstructural investigation of solidification of an Al-Cu alloy by ultrafast x-ray microtomography

This article presents the first results of a new experimental technique developed to investigate the evolution of the morphology of the solid and liquid phases during the solidification of a metallic alloy. It consists of ultrafast X-ray microtomography observations of a solidifying aluminum-copper alloy carried out at ESRF. These experiments allow investigating in-situ the formation of the casting microstructure and of the evolution of the morphology of the solid and the liquid phases. It allows also the in-situ determination of the solidification path, of the variation of the copper content in both the liquid and solid phases, and of some other characteristic parameters of the microstructure. Provided that some forthcoming technical improvements on the experimental setup are performed, more quantitative results can be obtained as well as better image quality and resolution.     

Application of electromagnetic separation of phases in alloy melt to produce <Emphasis Type="Italic">In-situ</Emphasis> surface and functionally gradient composites

The principle of electromagnetic separation of phases (primary phase) in alloy melt is that the electromagnetic force scarcely acts on the primary phases due to its low electric conductivity as compared to the melt. As a result, a repulsive force acts on the primary iron-rich phases to push them to move in the direction opposite to that of the electromagnetic force. The in-situ surface composite and the functionally gradient composite reinforced by primary Si are produced when the hypereutectic Al-Si alloy solidifies under electromagnetic force induced by static magnetic field and DC current. Similarly, the Al-Si-1.20 pct Fe-1.60 pct Mn alloy in-situ surface composite reinforced by primary iron-rich phase is produced. Based on this, a new method for production of in-situ multigradient composite with several layers, by electromagnetic separation of phases and directional solidification technique, is proposed.     

Anomalous Mixing State in Room-Temperature Ionic Liquid-Water Mixtures: <Emphasis Type="Italic">N,N</Emphasis>-diethyl-<Emphasis Type="Italic">N</Emphasis>-methyl-<Emphasis Type="Italic">N</Emphasis>-(2-methoxyethyl) Ammonium Tetrafluoroborate

The mixing states of room-temperature ionic liquid (RTIL) H₂O mixtures (x = 0.0 mol pct to 99.5 mol pct H₂O) were investigated using wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and optical absorption in an ultraviolet and visible (UV-vis) region. The RTIL is N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF₄]. In a “prepeak” region of the WAXS, the scattered intensities increased at 85 mol pct to 95 mol pct. A medium-range order (MRO) in the liquid structure as observed in network-forming materials developed markedly. In the SAXS experiments, we can detect nanoscale fluctuations relating to polar and nonpolar regions. At 65 mol pct to 85 mol pct, the SAXS intensity increased unexpectedly. Furthermore, entirely different optical absorption spectra in the UV-vis region were observed as a macroscopic property from 90 mol pct to 95 mol pct. We suppose that these anomalies relate to the MRO of the liquid structure. All anomalies probably are induced by an intrinsic property in [DEME][BF₄]-H₂O mixtures.     

<Emphasis Type="Italic">L</Emphasis><Subscript>3</Subscript>-XANES spectroscopy of the valence-unstable cerium in intermetallic compounds with 3<Emphasis Type="Italic">d</Emphasis> metals

The effective valence of cerium is studied by L ₃-XANES spectroscopy using synchrotron radiation in the following valence-unstable systems with different types of magnetic ordering: a CeNi matrix doped with neodymium, praseodymium, and gadolinium ions in the temperature range 5–300 K and the Ce₂Fe1_{7 − x} Mn _x intermetallic compound. The obtained dependences are discussed in terms of generally accepted models of states with an intermediate valence of rare-earth ions. Possible correlations between the effective valence of cerium and the magnetic properties of the substances are taken into account.     

A Derivation of a Quadratic Activity Coefficient <Emphasis Type="Italic">vs</Emphasis> Composition Relationship in a Quaternary System, <Emphasis Type="Italic">A-B-C-D</Emphasis>

In the literature, no direct derivation exists of the quadratic activity coefficient vs composition relationships for a quaternary system with high solute concentrations. Such relations for a ternary system (1-2-3) were derived by Darken by extending the results of a binary system (1-2), introducing a new concept of “hypothetical system” (2-3). To present a better scheme to find the activity coefficient-composition relations for multicomponent systems, derivations are made for a quaternary system A-B-C-D in the current work. Using a MacLaurin series expansion, the (Raoultian) activity coefficient, ln γ _i , of each component is equated with a quadratic expression of mole fractions (x), involving the activity coefficient at zero concentration ( g_i⁰ ) \left( {\gamma_{i}^{0} } \right) and nine interaction coefficients (ε). Subsequently, with the help of a Gibbs–Duhem equation, followed by a comparison of coefficients, most preceding 9 × 4, i.e., 36 interaction coefficients are eliminated, leaving behind only three self- and three ternary interaction coefficients, which are enough to express the activity coefficient vs composition relationships for the solutes B, C, and D, as well as for the solvent A. Setting the mole fraction x _D  = 0, the preceding expressions establish the same relations as proposed by Darken for the ternary system A-B-C. The derivation also clarifies how the quadratic concentration terms accompany the first-order interaction coefficients, not the second-order ones. Applications of the derived relations to determine simultaneously the activity coefficients g_i⁰ \gamma_{i}^{0} and the interaction coefficients ε in a new way in some iron- and steelmaking systems are presented. A new data on interaction coefficients in liquid iron at 1873 K (1600 °C), e_\textV^\textV = - 6. 1, \varepsilon_{\text{V}}^{\text{V}} = - 6. 1, has been generated through such an application.     

Interdiffusion behavior of Pt-modified <Emphasis Type="Italic">γ</Emphasis>-Ni + <Emphasis Type="Italic">γ</Emphasis>′-Ni<Subscript>3</Subscript>Al alloys coupled to Ni-Al-based alloys

The effect of platinum addition on the interdiffusion behavior of γ-Ni + γ′-Ni₃Al alloys was studied by using diffusion couples comprised of a Ni-Al-Pt alloy mated to a Ni-Al, Ni-Al-Cr, or Ni-based commercial alloy. The commercial alloys studied were CMSX-4 and CMSX-10. Diffusion annealing was at 1150 °C for up to 100 hours. An Al-enriched γ′-layer often formed in the interdiffusion zone of a given couple during diffusion annealing due to the uphill diffusion of Al. This uphill diffusion was ascribed to Pt addition decreasing the chemical activity of aluminum in the γ + γ′ alloys. For a given diffusion couple end member, the thickening kinetics of the γ′ layer that formed increased with increasing Pt content in the Ni-Al-Pt γ + γ′ alloy. The γ′-layer thickening kinetics in diffusion couples with Cr showed less of a dependence on Pt concentration. Inference of a negative effect of Pt and positive effect of Cr on the Al diffusion in this system enabled explanation of the observed interdiffusion behaviors. There was no or minimal formation of detrimental topologically close-packed (TCP) phases in the interdiffusion zone of the couples with CMSX-4 or CMSX-10. An overlay Pt-modified γ + γ′ coating on CMSX-4 showed excellent oxidation resistance when exposed to air for 1000 hours at 1150 °C. Moreover, the Al content in the coating was maintained at a relatively high level due to Al replenishment from the CMSX-4 substrate.     

Creep behavior of copper-chromium <Emphasis Type="Italic">in-situ</Emphasis> composite

Creep deformation and fracture behaviors were investigated on a deformation-processed Cu-Cr in-situ composite over a temperature range of 200 °C to 650 °C. It was found that the creep resistance increases significantly with the introduction of Cr fibers into Cu. The stress exponent and the activation energy for creep of the composite at high temperatures (≥400 °C) were observed to be 5.5 and 180 to 216 kJ/mol, respectively. The observation that the stress exponent and the activation energy for creep of the composite at high temperatures (≥400 °C) are close to those of pure Cu suggests that the creep deformation of the composite is dominated by the deformation of the Cu matrix. The high stress exponent at low temperatures (200 °C and 300 °C) is thought be associated with the as-swaged microstructure, which contains elongated dislocation cells and subgrains that are stable and act as strong athermal obstacles at low temperatures. The mechanism of damage was found to be similar for all the creep tests performed, but the distribution and extent of damage were found to be very sensitive to the test temperature.