Effect of Nb coating on the sulphidation/oxidation behaviour of Ti and Ti-6Al-4V alloy

The environmental response of Nb-coated Ti and Ti-6Al-4V alloy was studied at 750 °C in an atmosphere of pS₂ ∼ 10⁻¹ Pa and pO₂ ∼ 10 ⁻¹⁸ Pa. By acting as a diffusion barrier and through the formation of a Nb_1−xS scale the Nb coating deposited enhanced the corrosion resistance of both Ti and Ti-6Al-4V alloy. The corrosion products generated on uncoated titanium in the same environment and temperature were characterized by a double layered oxide scale of TiO₂ beneath which a TiS₂ layer was formed. For the Ti-6Al-4V alloy, α-Al₂O₃ was precipitated in the external portion of the outer-layer of TiO₂ whilst a layer containing Al₂S₃, TiS₂ and vanadium sulphide (possibly V₂S₃) was idenitified underlying the inner TiO₂ layer. After prolonged exposure (168 h), the Nb coating deposited on Ti and Ti-6Al-4V alloy was consumed. A scale following the sequence of TiO₂/TiO₂+NbO₂+Nb₂O₅/Nb_1−xS/TiO₂/ TiS₂/(substrate) was observed on the surface of the Nb-coated Ti, whilst a scale with sequence of TiO₂/V₂S₃/TiO₂+NbO₂+Nb₂O₅/Nb_1−xS/TiO₂/Al₂S₃+TiS₂/(substrate) characterized the corrosion products formed on the Nb-coated Ti-6Al-4V alloy.     

Reactivity and interface characteristics of titanium-alumina composites

The reaction kinetics between -Ti alloys and single crystal sapphire, the phase composition and morphology of the reaction-zone, and the phase compatibility in the system Ti-Al-O were investigated as part of a study to determine the feasibility of fabricating useful Al₂O₃-reinforced titanium matrix composites. In the temperature range 650 to 1000° C titanium reduces Al₂O₃ to form a complex reaction layer consisting of two distinct zones; an inner zone adjacent to the Al₂O₃ of a TiO phase containing isolated particles of (Ti, Al)₂O₃, presumably, and an outer zone of a Ti₃Al phase adjacent to the Ti matrix. The isothermal growth of the reaction layer follows a parabolic rate law. The temperature dependence of the rate constants fits an Arrhenius equation yielding activation energies of 50 to 52 kcal/mol. The high Al alloys, except Ti-6Al-2Sn-4Mo-2Zr, reacted more rapidly than pure Ti indicating that Al diffusion through the reaction zone may be the rate-limiting step.     

Effects of TiO/Al ratio on microstructures and mechanical properties of TiAl base composites

Effects of TiO₂/Al ratio on the microstructures and mechanical properties of in situ Al₂O₃/TiAl based composites were investigated. The results indicate that the as-sintered products consist of grains of nearly lamellar ?₂ + ? structure with a dispersion of randomly oriented Al₂O₃ particles. A 43.9Ti-38.6Al-17.5TiO₂-nNb₂O₅ system was compared to 57.46Ti-36.78Al-5.76TiO₂-nNb₂O₅ system. The lamellar spacing of the products increases and the ?₂ phase volume decreases with decreasing TiO₂/Al ratio. For each system, as the volume of ?₂ phase increases, the average lamellar spacing decreases. Strength increases with an increasing TiO₂/Al ratio due to the amount of ?₂ phase. Al₂O₃ phase increases with increasing TiO₂/Al ratio. Toughness increases with decreasing TiO₂/Al ratio. When the Nb₂O₅ content is smaller than 6 wt.%, the lamellar spacing plays an important role in toughness than the Al₂O₃ content. When the Nb₂O₅ content is larger than 6 wt.%, the Al₂O₃ content exhibits significantly increases the values of toughness than lamellar spacing.     

Reaction pathways,activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures

In-situ aluminum matrix composites were fabricated from Al–TiO₂–graphitic C powder mixtures using exothermic dispersion method. The effects of C/TiO₂ molar ratio on the reaction processes, activation energies and mechanical properties of the resulting materials were investigated. When the C/TiO₂ molar ratio is 0, Al reacts with TiO₂ to produce fine α-Al₂O₃ particles and Ti, which then reacts with Al to form large rod-like Al₃Ti phase. By adding graphite C into the Al–TiO₂ system, the activation energy of the first reactive step increases; in addition, the resultant Ti preferentially reacts with C to form hard TiC particles. When the C/TiO₂ molar ratio increases to 1.0, the Al₃Ti phase disappears and the reinforcements consist of nano-sized α-Al₂O₃ and TiC phases. The tensile strength of the composites increases from 239.2 MPa to 351.8 MPa and the elongation increases from 4.1% to 5.6%, suggesting a marked increase in damage tolerance (i.e., toughness).     

Pyrolysis of aluminium loaded polymethylsiloxanes: the influence of Al/PMS ratio on mullite formation

Al-filler-loaded polymethylsiloxane (PMS) was pyrolysed in air atmosphere at temperatures 400–1700 °C. The effect of the Al amount added to the PMS on phase development, densification behaviour and microstructure evolution was studied by simultaneous thermal analysis, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The Al/PMS reaction route is complex producing Si, SiO₂ (amorphous and cristobalite), Al₂O₃ (γ-, ι- and α-Al₂O₃), Al₂OC, Al₄O₄C, Al₄SiC₄, and AlN, depending on the ratio of Al/PMS in the initial mixture. Increasing the Al content (high Al/PMS ratio) reduces the amount of voids and porosities after PMS degradation. The voids and porosities provide access for the oxygen atmosphere into the inner structure to oxidise the Al particles, Si or SiC and also as channels for the PMS degradation products to escape. Mullite formation was identified in sample containing >73 wt% Al at temperature as low as 1400 °C.     

Effect of segregative additives on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

The influence of B₂O₃, and Al₂O₃ as segregative additives in modifying the ρ–T characteristics has been studied in BaTiO₃ ceramics with positive temperature coefficient of resistance (PTCR). Reaction of Al₂O₃ at the grain boundary regions of BaTiO₃ ceramics leads to the segregation of the secondary phase, BaAl₆TiO₁₂ resulting in broad PTCR jump, whereas B₂O₃ addition gives rise to steeper resistivity jump. Microstructure studies by SEM reveal the formation of coherent second phase layer of barium aluminotitanate surrounding the BaTiO₃ grains. The EDX results shows varying Al to Ti ratio in the early stage of phase formation in BaAl₆TiO₁₂ resulting in electrically active layer around the BaTiO₃ grains. The TiO₂-excess melt formation results in lower resistivity for 2–4% Al₂O₃ containing n-BaTiO₃ ceramics whereas at higher alumina contents, BaAl₆TiO₁₂ phase becomes dominant leading to higher resistivity in the sample. Complex impedance analyses support the three-layer regions, corresponding to the contributions from grain interior resistance (R _g), grain boundary resistance (R _gb), and that from secondary phase (R _sec). Electron paramagnetic resonance spectroscopy (EPR) indicated barium vacancies, V _Ba ^/ as the major electron trap centers which are activated across the tetragonal-to-cubic phase transition. A charge trapping mechanism is proposed wherein the segregation of secondary phases bring carrier redistribution among the various acceptor states thereby affecting the electrical conductivity of n-BaTiO₃ ceramics. The presence of Al³⁺–O⁻–Al³⁺ or Ti⁴⁺–O⁻–Al³⁺ type hole centers at the grain boundary layer (GBL) regions results in charge redistribution across the modified phase transition temperature due to symmetry-related vibronic interactions resulting in broad PTCR characteristics extending to higher temperatures.     

Effect of ribbon-wheel contact on the microstructure of melt-spun Al-Cu eutectic alloy ribbons

The microstructure of melt-spun Al-17.3 at% Cu eutectic alloy ribbons with different thicknesses have been revealed by means of electron microscopy and X-ray techniques. Thick ribbons (>20μm) consist of finely-dispersed α-Al and Al₂Cu equilibrium phases showing lamellar or irregular morphology. In thin ribbons a considerable fraction of supersaturated α-Al solid solution and the metastable Al₄Cu₉ phase have been detected. The experimental facts on the identity of the phases, their morphology and particle size for given processing conditions are discussed within the framework of a heat flow model including the solidification process as well as post-solidification transformations. The microstructural features are considered to be determined by local and time variations of the ribbon-wheel contact.     

Spray processing and wear characteristics of Al-Cu-Al2O3-Pb based composites

The spray deposition process has been employed in synthesis of Al-4.5Cu-10Al₂O₃ and Al-4.5Cu-10Al₂O₃-10Pb based composites. The microstructure and wear characteristics of composites were investigated. The rapid solidification inherent in spray deposition processing resulted in a uniform dispersion of Al₂O₃ and Pb particles co-existing in the matrix of the- primary α-phase. The grain size of the Al-4.5Cu-10Al₂O₃-Pb composite was observed to be higher than that of the Al-4.5Cu-10Al₂O₃ composite in various sections of the spray deposit. The wear rate of composite materials decreased with addition of Pb phase. This behavior is discussed in the light of the microstructural modification induced by spray deposition and the morphology of debris particles on the wear track surfaces. The wear characteristics of the composites are compared with that of the liquid immiscible Al-4.5Cu-10Pb alloy.     

Effect of Nb and Nb2O5 additives on mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research, TiAl matrix nano-composite with Al₂O₃ reinforcement was obtained by mechanical activation of TiO₂ and Al powder mixture and its subsequent heat treatment. Effect of Nb and/or Nb₂O₅ additions on the process was investigated. Structural changes and thermal behavior of the samples were evaluated by X-ray diffraction and differential thermal analysis, respectively. Moreover, the microstructure was characterized by transmission electron microscopy. The results confirmed the partial dissolution of Nb in Al during the milling stage in the Nb-added samples. The reaction mechanism during heat treatment in the sample without any additives was a two-stage process that was quite similar to the sample with Nb addition. However, Nb₂O₅ addition led to the progress of reaction through a single stage and with a higher rate. Both additives promoted formation of the Ti₃Al phase in the final products. The results confirmed the formation of nano-sized Al₂O₃ particles in a nano-crystalline Ti–Al matrix with a mean crystallite size of 30 nm.     

Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing

In situ composites of TiAl reinforced with Al₂O₃ particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb₂O₅ by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al₂O₃, NbAl₃, as well as small amounts of the Ti₃Al phase. The in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al₂O₃ distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb₂O₅ content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m^1/2, respectively, when the Nb₂O₅ content reaches 6.62 wt.%. The strengthening mechanism was also discussed.     

High-pressure high-temperature transitions in nanocrystallineγ Al2O3,γ Fe2O3 and TiO2

We report first observation of new polymorphs of Al₂O₃ and Fe₂O₃ in specimens of xerogelγ Al₂O₃ andγ Fe₂O₃ quenched from high pressures and temperatures. At about 5 GPa and 1400°C, xerogel gamma alumina (XGA) transformed into a polymorphic mixture of phasesα Al₂O₃, B Al₂O₃ and C Al₂O₃, while XGA containing 1 wt% Cr₂O₃ transformed into a mixture of phasesαAl₂O₃, H Al₂O₃ and k′ Al₂O₃. The phases B Al₂O₃, C Al₂O₃ and H Al₂O₃ have the monoclinic-, cubic- and hexagonal-rare earth sequioxide (Ln₂O₃) type structure, respectively. At 5·2 GPa and 1450°C, XGA yielded a mixture ofα Al₂O₃ and hexagonalμ Al₂O₃. At STP, the phaseμ Al₂O₃ was found to transform to another hexagonal phaseλAl₂O₃ over a 10 week period. At 5·2 GPa and 900°C,γ Fe₂O₃ showed transition to a new phase H Fe₂O₃ which probably has an 8 layer close packed structure. In nanocrystalline TiO₂, only the anatase to rutile transition was found. The results are discussed using the free energy vs temperature diagram for xerogel and nanocrystalline materials.     

Oxidation behavior of in situ TiB short fibre reinforced Ti-6Al-1.2B alloy in air

The oxidation behavior of an in situ TiB short fibre reinforced Ti-6Al-1.2B alloy was investigated in a flowing air over the temperature range 873–1223 K. The alloy exhibited a parabolic oxidation behavior with the activation energy of 250 kJ/mol, which was supposed to be mainly controlled by the inward diffusion of oxygen. The oxide scales consisted mainly of TiO₂, a small amount of Al₂O₃ and amorphous B₂O₃. The morphology of TiO₂ changed from fine needle-shape at 1023 K to fine sphere at 1073 K and then well-developed block at a higher temperature. The microstructure of the subsurface showed that B₂O₃ pores and crack appear at the Ti/TiB interface above 1023 K as a result of the oxidation of TiB and evaporation B₂O₃. Combined with the thermodynamic analysis, it was suggested that the presence of TiB and formation of B₂O₃ could not act as an oxidation resistance. Finally, the diffusion coefficient of oxygen in Ti-6Al-1.2B exposure to 1073 K in air was determined to be 4.22 × 10^–11 cm²/s.     

Compositional tailoring of the thermal expansion coefficient of tantalum (V) oxide

The effect of Al₂O₃ and Nb₂O₅ additions on the microstructure and thermal expansion behavior of tantalum (V) oxide has been studied. Both singly doped and co-doped compositions were examined. Compositions with 3 wt% Al₂O₃ (or greater), contained AlTaO₄ as a second phase. Complete solubility was observed for Nb₂O₅ contents in the range 1–5 wt%. It was found that doping with Al₂O₃ results in a decrease of the coefficient of thermal expansion (CTE), such that increasing amounts of Al₂O₃ (1–5 wt%) cause a successive decrease in the CTE value. In the case of Nb₂O₅ additions, the result is an increase in the CTE of the tantalum (V) oxide. However, in the range 1–5 wt% Nb₂O₅, the CTE value is relatively insensitive to the amount of Nb₂O₅ added. Due to the countervailing influences of these two additive oxides, it was demonstrated that co-doping with Al₂O₃ and Nb₂O₅ is an effective strategy for tailoring the CTE of tantalum (V) oxide.     

Partial thermodynamic functions of aluminum in sodium aluminum vanadium Β-bronzes

The partial thermodynamic functions of Al in the Β-bronzes Na_2-yAl_yV₁₂O₃₀ (0 <y ≤ 0.7), Na₂Al_yV₁₂O₃₀ (0 <y ≤1.4), and Na_1.3Al_yV₁₂O₃₀ (0 <y≤ 2.1) were determined as a function of composition in the temperature range 294–348 K. The δG (Al), δ-H (Al), and δS (Al) of the bronzes were found to vary in a complex manner with Al content, passing through extrema neary = 1 as a result of the structural ordering of Al³⁺ ions. The temperature dependences of δG (Al) show changes in slope between 303 and 318 K, attributable to redistribution of the Al³⁺ ions over the possible crystallographic sites in the Β-bronze structure.     

Metal distribution in alumina/aluminium composites synthesized by directed metal oxidation

The directed oxidation of molten aluminium alloys by vapour phase oxidants can be used to produce Al₂O₃/Al ceramic matrix composites. The toughness of these composites is determined by the amount and the nature of metal distribution in the composite. This paper addresses the problem of understanding the metal distribution in Al₂O₃/Al composites and its dependence on growth temperature. Electrical conductivities and microstructures of Al₂O₃/Al composites synthesized by directed oxidation of Al-5056 alloy are investigated. The high conductivity of the Al₂O₃/Al composite compared to sintered Al₂O₃-4 wt% MgO is shown as a proof of the presence of some continuous metal channels in the composite. The activation energy forthe diffusion of the dominant charge carrier in the oxide matrix is found to be 1.36 eV from the analysis of the conductivity data. Both the amount of metal in the composite and the extent of interconnection of the metal channels decrease with increasing growth temperature. The observed changes in microstructure with temperature can be explained by considering temperature variations of grain boundary energies in alumina and the alumina/aluminium interfacial energy. The metal content of the Al₂O₃/Al composites, prepared by directed oxidation of Al-5056 alloys, can be tailored by the choice of the growth temperature.     

Phase,microstructural characterization and dielectric properties of Ca-substituted Sr<Subscript>5</Subscript>Nb<Subscript>4</Subscript>TiO<Subscript>17</Subscript> ceramics

The effect of Ca substitution for Sr on the phase, microstructure and microwave dielectric properties of the Sr_5−x Ca _x Nb₄TiO₁₇ composition series was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), an LCR meter, and vector network analyzer. Below 1450 °C, Sr_5−x Ca _x Nb₄TiO₁₇ (x = 1, 2, 3, or 4) compositions formed single-phase Sr₄CaNb₄TiO₁₇, Sr₃Ca₂Nb₄TiO₁₇, Sr₂Ca₃Nb₄TiO₁₇, and SrCa₄Nb₄TiO₁₇ ceramics, respectively. At x = 0 and 5, Sr₅Nb₄TiO₁₇ and Ca₅Nb₄TiO₁₇ formed, but along with Sr₂Nb₂O₇ (at x = 0) and CaNbO₃ and CaNb₂O₆ (at x = 5) secondary phases. Above 1450 °C, all the compositions formed two-phase ceramics. At low frequencies, a phase transition was observed in the composition Sr₅Nb₄TiO₁₇. The substitution of Ca for Sr enabled processing of highly dense Sr₂Ca₃Nb₄TiO₁₇, with ε_r ~ 53.4, τ_f ~ −6.5 ppm/°C and Q _u  × f _o  ~ 1166 GHz. Further investigations are required to improve the quality factor of these ceramics for possible microwave applications.     

High-temperature oxidation process of intermetallic compound Ti-42 at% Al

The oxidation process of two-phase (Ti₃Al and TiAl) intermetallic compound, Ti-42 at% Al, in air at 1073 and 1273 K has been investigated. The oxidation at 1273 K is much faster than that at 1073 K; however, the oxidation kinetics are similar. During heating up, TiO₂ scale forms initially on the compound surface at about 973 K, and then Al₂O₃ scale forms at about 1273 K. For the isothermal heating, TiO₂ scale slowly grows up at 1073 K, while at 1273 K both TiO₂ and Al₂O₃ scales grow up drastically. The outer oxide scale consists of TiO₂ and the inner one consists of a mixture of TiO₂ and Al₂O₃. The volume of Al₂O₃, which forms after TiO₂ formation at the initial stage of oxidation, is larger at an area adjacent to the oxide-compound interface.     

Evolution and stability of texture during thermomechanical processing of Ti-24Al-11Nb alloy

The evolution of basal texture during thermomechanical processing of Ti-24Al-11Nb alloy has been studied as a function of different processing variables like hot rolling temperature, amount of deformation, cooling conditions etc. The stability of the deformation texture during post-rolling annealing and during theα ₂→β→α ₂ phase transformation cycle was also investigated. Unrestricted rolling of primaryα ₂ to maximum thickness reduction at the lowest rolling temperature has been found to be most favourable for obtaining a good basal texture. Texture of transformed (secondary)α ₂ is generally non basal when the transformation takes place from deformedβ. Rolling texture does not seem to change during annealing leading to recrystallization. Theα ₂→β→α ₂ phase transformation cycle does not change the starting basal texture and a starting non basal texture also does not give rise to basal texture due to this treatment.     

Evolution of microstructures and phases of Al–Mg alloy under 4 GPa high pressure

Optical microscope (OM), energy dispersive X-ray (EDX) analysis, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscope (TEM) were applied to investigate the solidification microstructures and phases of Al–9.6 wt.% Mg alloy which solidified under 4 GPa high pressure with the melting temperature 1,153 K. Fine dendritic microstructures were obtained, and the second dendritic arm spacing reduced. Area fraction of the primary α-Al phase increased and that of the second phase decreased. In addition, the solid solubility of Mg in α-Al phase increased. The lattice constant of α-Al phase increased. Specially, the new double phase regions (α-Al′ + Al _x Mg _y ) formed besides a small amount of Al₃Mg₂ phases under high pressure. The Al _x Mg _y phase presented a mean size of about 20 nm, and had the hexagonal structure with the lattice constant of a = 0.288 nm, c = 0.8165 nm probably. Wherein the lattice constant of α-Al′ phase differed from that of α-Al phase greatly. Moreover, evolution mechanism of microstructures and phases under 4 GPa high pressure was discussed.     

Structure,properties and response to heat treatment of melt-spun Al-Y and Al-La alloys

Al-Y and Al-La binary alloys containing 0.7–18 wt% (0.2–6.3 at%) Y and 0.9–18 wt% (0.2–4.2 at%) La, were rapidly solidified by chill-block melt-spinning to produce ribbons between 35 and 70 m thick. Microstructures were of the classical zone A/zone B type with a notable increase in Al lattice parameter for the Al-6.3 at% Y composition, which exhibited a Knoop hardness of 430±30 kg mm^–2 as-spun. Isochronal ageing for 2 h at 200–500 °C gave significant hardening at 200 and/or 300 °C for all of the more concentrated alloys, the largest responses being produced by Al-6.3 at % Y and Al-4.2 at % La at 200 °C. X-ray diffraction asspun indicated the presence of only Al and equilibrium Al₁₁La₃ in the Al-La alloy ribbons and Al and a non-equilibrium Al₄Y/Al₁₁Y₃ in the Al-Y ribbons. This non-equilibrium Al-Y phase was identified by X-ray diffraction as isomorphous with orthorhombic or tetragonal Al₁₁ La₃ with lattice parameters determined as a _o = 0.42 ± 0.02 nm (b _o = 1.26 ± 0.06 nm) and c _o = 0.97 ± 0.05 nm. TEM showed that it was present as an intercellular network with Energy dispersive spectroscopic analysis indicating an average composition Al-46 wt% Y consistent with the Al₄Y/Al₁₁Y₃ stoichiometry and diffraction patterns consistent with an orthorhombic or tetragonal cell with these lattice parameters. While no significant change in phase constitution of the Al-La ribbons was detected by X-ray diffraction as a result of heat treatment, the Al₁₁Y₃ in Al-Y ribbons was seen to be replaced by AI₃Y on heat treatment at 400 and 500 °C.