Analysis of Mechanically Induced Reactivity of Boehmite Using Kinetics of Boehmite to <Emphasis Type="Italic">γ-</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Transformation

This article focuses on the mechanically induced reactivity of boehmite prepared by thermal decomposition of gibbsite. Boehmite, which retained the morphology of gibbsite, was characterized by a specific surface area of 264 m²/g. Mechanical activation (MA) was carried out in a planetary mill up to 240 minutes. The samples were characterized in terms of morphology, characteristic particle diameters, Brunauer Emmett Teller (BET) specific surface area (SSA_BET), microcrystallite dimension (MCD), microstrain (ε) and Fourier transform infrared spectroscopy. The reactivity was construed from the kinetics of thermal transformation of boehmite into γ-Al₂O₃. The transformation observed between 600 K and 900 K (327 °C and 627 °C), manifested itself as two overlapping peaks in the differential thermogravimetric plot. These peaks correspond to two stages of dehydroxylation involving Al₂OH and AlOH groups in succession. The peaks were resolved using Gaussian deconvolution. The reactivity was assessed separately for the two stages by comparing the fraction reacted in MA samples (α) with that of nonactivated sample (α _ref). During both stages, enhanced kinetics, as revealed by α-α _ref plots, indicated an increase in reactivity with MA. The transformation mechanism conformed to n ^th order reaction (f[α] = [1 – α] ⁿ with n = 1.3–1.5 in both stages). Values of n remained similar for the activated and reference samples. Activation energies (E _a) for the first and second dehydroxylation stages were respectively 115 and 300 kJ/mol for the nonactivated sample. E _a for the second stage decreased exponentially to a value of 222 kJ/mol after 240 minutes of milling. An anomalous negative correlation between reactivity and SSA_BET was observed. Reactivity parameters were strongly correlated with MCD and ε. A plausible explanation for the observed correlations is presented.     

Kinetics of Transformation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> to MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Spinel Inclusions in Mg-Containing Steel

During ladle furnace refining, initial Al₂O₃ inclusions generally transform into MgO·Al₂O₃ spinel inclusions; these generated spinel inclusions consequently deteriorate the product quality. In this study, the transformation from Al₂O₃ to MgO·Al₂O₃ was investigated by immersing an Al₂O₃ rod into molten steel, which was in equilibrium with both MgO and MgO·Al₂O₃ spinel-saturated slag. A spinel layer, with a thickness of 4 μm, was generated on the Al₂O₃ rod surface just 10 s after its immersion at 1873 K (1600 °C). The thickness of the formed spinel layer increased with the immersion period and temperature. Moreover, the MgO content of the generated spinel layer also increased with the immersion period. In this study, the chemical reaction rate at 1873 K (1600 °C) was assumed to be sufficiently high, and only diffusion was considered as a rate-controlling step for this transformation. By evaluating the activation energy, MgO diffusion in the generated spinel layer was found to be the rate-controlling step. In addition, this estimation was confirmed by observing the Mg and Al concentration gradients in the generated spinel layer. The results of this study suggest that the MgO diffusion in the spinel inclusions plays a substantial role with regard to their formation kinetics.     

Multi-objective Optimization of Electrodeposition of Ni–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nano Composite Coating on Al6061 Substrate

Response surface methodology based Grey relation analysis has been used to optimize the coating parameters of composite coating on aluminium based alloys. This approach gives the best combination of coating parameters to get maximum coating thickness, adhesive strength, microhardness, and minimum wear rate. For each response, the effect of coating parameters at different levels have been discussed. From Grey relation grade, the optimum parameters for better composite coating performance are found to be: temperature, 34 °C; current density, 1 A/dm²; and percentage of particle loading, 1.2 g/L. At 95% significance level, the Current density shows statistical significance on overall composite coating performance.     

Effect of the milling time on the magnetic properties of powder compositions of the Sm<Subscript>2</Subscript>Fe<Subscript>17</Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compound and an Nd–Fe–B alloy

The effect of the wet milling time on the magnetic properties of powder compositions consisting of the hard magnetic Sm₂Fe₁₇N _x (x = 2.9–3.0) nitride and a rapidly quenched Nd_9.6Fe_76.3Co_4.3Zr_3.4B_6.4 (at %) alloy, which are taken in different mass proportions, is studied. The compositions containing no more than 20 wt % alloy are found to exhibit a substantial increase in the magnetic characteristics as compared to those of the nitride. It is shown that the determining effect on the coercivity is related to the degree of structural imperfection of Nd–Fe–B powders, whereas the specific remanent magnetization and the specific magnetization in a field of 2 T are determined by the corresponding characteristics of the alloy. The optimum composition and efficient treatment conditions for powder mixtures are determined.     

Synthesis of Nanocrystalline α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from Nanocrystalline Boehmite Derived from High Energy Ball Milling of Gibbiste

Dry milling of gibbsite has been carried out for 5 h in planetary ball mill to study the effect of mechanical activation on α-Al₂O₃ formation. Gibbsite undergoes phase transformation during milling and has resulted nanocrystalline boehmite after 5 h of milling. The average crystallite size and the BET surface area of the nanocrystalline boehmite resulted by 5 h milling of gibbsite are 8 nm and 140 m²/g, respectively. The nanocrystalline boehmite has shown reduction in the α-Al₂O₃ formation temperature as well as in the activation energy of α-Al₂O₃ formation. The average crystallite size of nanocrystalline boehmite derived α-Al₂O₃ is measured to be 100 nm by TEM analysis and the BET surface area of resulted nanocrystalline α-Al₂O₃ is 12 m²/g.     

Structure–Property Correlation of Al–SiC–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composites: Influence of Processing Temperatures

This paper deals with the change in the mechanical behaviour of aluminium alloy 6061 with different weight percentage of Silicon Carbide (SiC) and Alumina (Al₂O₃) ceramic powders and change in processing temperature. The crucial properties of this aluminium alloy are relatively light in weight, better corrosion resistance, wear resistance and have low production cost. These properties make them pleasant for different applications such as aerospace, defense, automotive sectors. The purpose of designing Metal Matrix Composite is to figure the desired qualities of metals and ceramics. The fabrication of the MMC was done by stir casting process. The tensile test, hardness test and impact test were performed on these composite samples to study the mechanical behaviour. The result shows that there is a significant increase in tensile strength for the samples that are processed at the temperature of 750 °C with a higher weight fraction of SiC. Also, the samples made at 850 °C exhibit better hardness and impact strength with increased content of alumina. The internal microstructure of the composites was analyzed by scanning electron microscope.     

The Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiB<Subscript>2</Subscript>/Fe Complex Reinforcement on Wear and Mechanical Properties of Al-Matrix Composites

In this study, the effect of Al₂O₃–TiB₂/Fe complex ceramic–metal reinforcement (CCMR) on wear and mechanical properties of Al-(Al₂O₃–TiB₂/Fe) composites were investigated. For this purpose, Al₂O₃–TiB₂/Fe CCMR was synthesized by mechanochemical process. The produced reinforcement powders were added to Al matrix, milled for 10 h and then hot extruded. The results showed that the metallic component (Fe rich phase) in this reinforcement acted as a pin, sticking the ceramic parts (Al₂O₃–TiB₂) to Al matrix. The best volume percentage of CCMR in Al matrix was recognized to be about 2.5 %. This composite showed a combination of wear resistance (0.005 mg/m), strength (500 MPa) and ductility (of about 6 %).     

Electrical conductivity of KAlCl<Subscript>4</Subscript>–ZrCl<Subscript>4</Subscript> molten mixtures

The electrical conductivity of commercially challenging KAlCl₄–ZrCl₄ molten mixtures has been studied as a function of temperature (in the range from 345°C to 500°C) and the ZrCl₄ concentration (0?32.5mol %) using cells of a unique design. It is found to vary in the range from 0.41 to 0.80 S cm^–1, increasing with temperature or when the mole concentration of zirconium tetrachloride in molten mixtures decreases.     

The Coexistence of Two Different Pearlites,Lamellae of (Ferrite + M<Subscript>3</Subscript>C), and Lamellae of (Ferrite + M<Subscript>23</Subscript>C<Subscript>6</Subscript>) in a Mn-Al Steel

Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel. The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M₃C and M₂₃C₆ carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously at temperatures below 923 K (650 °C). One is lamellae of ferrite and M₃C carbide, and the other is lamellae of ferrite and M₂₃C₆ carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M₂₃C₆. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately.     

Thermodynamic Modeling of Metal Desulfurization with Boron-Containing Slags of the CaO–SiO<Subscript>2</Subscript>–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript> System

In thermodynamic modeling of the desulfurization of steel by CaO–SiO₂–MgO–Al₂O₃–B₂O₃ slag on the basis of HSC 6.12 Chemistry software (Outokumpu), the influence of the temperature (1500–1700°C), the slag basicity (2–5), and the B₂O₃ content (1–4%)1 on the desulfurization is analyzed. It is found that the sulfur content is reduced with increase in the temperature from 1500 to 1700°C, within the given range of slag basicity. At 1600°C, the sulfur content in the metal is 0.0052% for slag of basicity 2; at 1650°C, by contrast, its content is 0.0048%. Increase in slag basicity from 2 to 5 improves the desulfurization, which increases from 80.7 to 98.7% at 1600°C. If the B₂O₃ content in the slag rises, desulfurization is impaired. At 1600°C, the sulfur content in the metal may be reduced to 0.0052 and 0.0098% when using slag of basicity 2 with 1 and 4% B₂O₃, respectively; in the same conditions but with slag of basicity 5, the corresponding values are 0.00036 and 0.00088%, respectively. Note that desulfurization is better for slag without B₂O₃. According to thermodynamic modeling, metal with 0.0039 and 0.00019% S is obtained at 1600°C when using slag of basicity 2 and 5, respectively, that contains no B₂O₃. The results obtained by thermodynamic modeling for the desulfurization of metal by CaO–SiO₂–MgO–Al₂O₃–B₂O₃ slag of basicity 2–5 in the range 1500–1700°C are consistent with experimental data and may be used in improving the desulfurization of steel by slag that contains boron.     

A new experimental technique for determinations of the activities of P<Subscript>2</Subscript>O<Subscript>5</Subscript> and Fe<Subscript>x</Subscript>O—the system CaO-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-Fe<Subscript>x</Subscript>O

An experimental technique was applied for simultaneous determinations of the activities of P₂O₅ and Fe_xO in slags. With this technique, phosphorus-containing liquid copper was heated inside an iron crucible at temperatures below 1600 K. When copper is melted, the iron crucible dissolves into liquid copper, to a small extent, to form ternary {Cu-Fe-P} liquid alloy, while 〈Cu-Fe-P〉 solid solutions are formed on the inner wall of the iron crucible. Slags containing tri-calcium phosphate were then brought into equilibrium with {Cu-Fe-P} liquid alloy. By measuring the equilibrium oxygen partial pressures with the aid of a zirconia electrolyte cell, activities of P₂O₅ and Fe_xO were obtainable. Between 1553 and 1593 K, the P₂O₅ activities could be expressed as

Determination of gibbs energy of formation of cuspidine (3CaO · 2SiO<Subscript>2</Subscript> · CaF<Subscript>2</Subscript>) from the electromotive force method using CaF<Subscript>2</Subscript> as the solid electrolyte

The standard Gibbs energy change for the following reaction has been directly determined by electromotive force (EMF) measurement using CaF₂ as the solid electrolyte in the temperature range from 1313 to 1329 K.

Effects of the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Content on the Viscosity of CaO-SiO<Subscript>2</Subscript>-10 Pct Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Quaternary Slag

The present study experimentally investigates the effect of Cr₂O₃ on the viscosity of molten slags. The viscosities of CaO-SiO₂-10 pct Al₂O₃-Cr₂O₃ quaternary slags with two different binary basicities (R, basic slag with R = 1.2 and acidic slag with R = 0.8) were measured by the rotating cylindrical method from 1813 K to 1953 K (1540 °C to 1680 °C). The results showed that the viscosity of both types of slag decreased as the Cr₂O₃ content increased, but the viscosity of acidic slags exhibited a greater decrease. The slags showed good Newtonian behavior at such high temperatures. Cr₂O₃ could act as a network modifier to simplify the Si-O-Si tetrahedral structure, as verified by the Raman spectral analysis, which was consistent with the decreasing trend of viscosity. The activation energy of viscous flow decreased slightly with increasing Cr₂O₃, but increasing the basicity seemed to be more effective in decreasing the viscosity than adding Cr₂O₃.     

Simultaneous Modification of Alumina and MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Inclusions by Calcium Treatment During Electroslag Remelting of Stainless Tool Steel

Calcium modification of both alumina and MgO·Al₂O₃ inclusions during protective gas electroslag remelting (P-ESR) of 8Cr17MoV stainless steel and its effect on nitrides and primary carbides were studied by analyzing the transient evolution of oxide and sulfide inclusions in the P-ESR process. The oxide inclusions that were not removed during P-ESR without calcium treatment were found to retain their original state until in as-cast ingot. Calcium treatment modified all MgO·Al₂O₃ and alumina inclusions that had not been removed in the P-ESR process to liquid/partially liquid CaO-Al₂O₃-(MgO) with uniformly distributed elements, in addition to a small proportion of partially modified inclusions of a CaO-MgO-Al₂O₃ core surrounded by a liquid CaO-Al₂O₃. The modification of low-MgO-containing MgO·Al₂O₃ inclusions involves the preferential reduction of MgO from the MgO·Al₂O₃ inclusion by calcium and the reaction of calcium with Al₂O₃ in the inclusion. It is the incomplete/complete reduction of MgO from the spinel by calcium that contributes to the modification of spinels. Alumina inclusions were liquefied by direct reaction with calcium. Calcium treatment during P-ESR refining also provided an effective approach to prevent the formation of nitrides and primary carbides in stainless steel through modifying their preferred nucleation sites (alumina and MgO·Al₂O₃ inclusions) to calcium aluminates, which made no contribution to improving the steel cleanliness.     

The effect of the aging period on the martensitic transformation and kinetic characteristic of at % Cu<Subscript>68.09</Subscript>Al<Subscript>26.1</Subscript>Ni<Subscript>1.54</Subscript>Мn<Subscript>4.27</Subscript> shape memory alloy

In this work, the effect of aging period on the characteristic transformation temperatures, thermodynamic parameters and structural variations of CuAlNiMn shape memory alloys were investigated. Aging was performed at above the austenite finish temperature of the un-aged specimen (120°C) for six different retention times, namely 1h, 2h, 3h, 4h, 5h and 6h. The changes in the transformation temperatures were examined by differential scanning calorimetry at different heating/cooling rates. The aging period was found to have an effect on the characteristic austenite and martensite transformation temperatures and thermodynamic parameters such as the enthalpy and entropy of alloys. High-temperature order-disorder phase transitions were determined using a differential thermal analysis, which showed that all the un-aged and aged specimens had an A2 → B2, B2 → L2₁ and an L2₁ → 9R, 18R transition. The structural analysis of the un-aged and aged specimens was performed through X-ray diffraction measurements at room temperature. The intensities of the diffraction peaks varied according to the aging time.     

Joint effect of scandium and zirconium on the recrystallization of aluminum Al–Mg<Subscript>2</Subscript>Si alloys

Metallographic analysis and hardness measurements are used to study the recrystallization processes in aluminum Al–Mg₂Si alloys with scandium and combined scandium and zirconium additions that occur during annealing of the cold-deformed alloys at 100–600°C. The temperature of the onset of recrystallization of the Al–Mg₂Si alloys with scandium and combined zirconium and scandium additions is shown to be 50°C higher than that of the alloys free from scandium and zirconium. It was noted that the small grain sizes of the alloyed compositions lead to weaker disordering during recovery and recrystallization.     

Formation of <Emphasis Type="Italic">β</Emphasis>-NiAl Phase During Casting of a Ni-Based Superalloy

A high-refractory Ni-based superalloy prototype was melted on a research scale while simulating industry practices. Ingots were vacuum induction melted and subjected to a computationally optimized homogenization heat treatment prior to fabrication which consisted of forging and hot rolling. Failure of one of the ingots at the early stage of the forging process was attributed to the precipitation of the β-NiAl phase during melting which stabilized the eutectic constituent.     

Wetting of the (0001) <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Sapphire Surface by Molten Aluminum: Effect of Surface Roughness

The wetting of molten aluminum on the “c”-plane (0001) of single-crystal α-Al₂O₃ (sapphire) was studied by the sessile drop technique from 800 °C (1073 K) to 1200 °C (1473 K). Systematically increasing the (0001) surface roughness by SiC abrasion increased the wetting contact angle, resulting in reduced wetting. The surface roughness factor R originally defined by Wenzel, was determined as a function of the abrasion, temperature, and time. The wetting decreases as the surface roughness increases. Rough surfaces also create time and temperature effects on wetting, changing those for a smoothly polished surface. The existence of a high-temperature surface structural transition for (0001) of α-Al₂O₃, which has been previously suggested, was confirmed. Increased roughness R accents the effect of the surface structural transition, increasing the wetting contact angle changes during the transition.     

Stabilization of the fluorite phase in the ZrO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The calculated and experimental vertical ZrO₂–Y₂O₃ sections of the Zr–Y–O system are compared to find the region of a stable fluorite structure of yttrium-stabilized zirconia (YSZ). X-ray diffraction (XRD) and Raman scattering are used to study the crystal and local structures of mixed oxide 0.82ZrO₂ · 0.18Y₂O₃ (18YSZ) powders prepared by isothermal annealing of a precursor precipitated from a salt solution. The formation of a fluorite-type fcc structure (space group \(Fm\overline 3 m\)) in the powders is detected by XRD. Raman scattering study of the local structure of the cubic 18YSZ powders revealed traces of the tetragonal phase in them.