Hydrothermal conversion of Ti-containing minerals in system of Na_2O–Al_2O_3–SiO_2–CaO–TiO_2–H_2O

Titanium has a great effect on the digestion of bauxite in the Bayer process because it reacts readily at high temperatures in alkaline sodium aluminate solution.Under this consideration, the hydrothermal conversion of Ti-containing minerals in the system of Na_2O–Al_2O_3–Si O_2–Ca O–Ti O_2–H_2O with increased temperatures was studied based on the thermodynamic analysis and systematic experiments. The results show that anatase converts to Al_4Ti_2 SiO_(12) at low temperatures(60–120 °C), which is similar to anatase in crystal structure. As the temperature continues to rise, Al4Ti2 Si O12decomposes gradually and converts to Ca_3 Ti Si_2(Al_2Si_(0.5)Ti_(0.5)O_(14) at 200 °C. When the temperature reaches 260 °C, Ca Ti O_3 forms as the most stable titanate species for its hexagonal closest packing with O_2-and Ca_2?. The findings enhance the understanding of titanate scaling in the Bayer process and clarify the mechanism of how additive lime improves the digestion of diaspore.     

Effect of predeformation on globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr during annealing

The microstructure evolution during annealing of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy was investigated. The results show that for the alloy compressed at 810 °C and 1.0 s^?1 deformation amount (height reduction) 20% and 50% and annealed at 810 °C, thermal grooving by penetration of β phase is sufficient during the first 20 min annealing, resulting in a sharp increase in globularization fraction. The globularization fraction continuously increases with the increase of annealing time, and a height reduction of 50% leads to a near globular microstructure after annealing for 4 h. For the alloy with deformation amount of 50% by compressing at 810 °C, 0.01 s^?1 and then annealed at 810 °C, thermal grooving is limited during the first 20 min of annealing and large quantities of high-angle grain boundaries (HABs) remain. With long time annealing, the chain-like α grains are developed due to the HABs, termination migration and Ostwald ripening. The present results suggest that a higher strain rate and a larger height reduction are necessary before annealing to achieve a globular microstructure of Ti–5Al–2Sn–2Zr–4Mo–4Cr.     

Kinetics of high temperature deformation of lamellar Ti48Al–2Nb–2Cr

Based on microstructural observations, the deformation resistance of lamellar Ti48Al–2Nb–2Cr in the temperature range between 1000 and 1200 K is expressed by treating the material as a composite of regions with lamellar and globular structure deforming independently of each other. For stresses below 1000 MPa the dislocation velocity is lower in the lamellar regions compared to the globular ones due to a larger athermal hardening component caused by interfaces (lamella boundaries in the lamellar structure and (sub)grain boundaries in the globular structure). Combining deformation kinetics and structural evolution allows the modelling of the maximum deformation resistance and the subsequent softening resulting from the increase in globular volume fraction. The model is applied to predict the creep life.     

Preparation of low-oxygen-containing Ti–48Al–2Cr–2Nb alloy powder by direct reduction of oxides

A high-purity Ti–48Al–2Nb–2Cr alloy powder with an oxygen content as low as 0.0572 wt.% and a particle size of <150 μm was produced from a mixture of TiO₂, Al₂O₃, Nb₂O₅, and Cr₂O₃ powders through reduction with magnesium and deoxidation with calcium. The phase and composition of the products were analyzed. The final product mainly included γ-TiAl and minor α₂-Ti₃Al phases, and Ti, Al, Cr, and Nb were homogenously distributed in the powder with a mole ratio of 49.73:43.51:2.05:1.98. The reduction and deoxidation mechanisms were investigated by thermodynamic modeling using the HSC Chemistry software and Pandat software based on the Ti alloy database.     

Effect of heat treatment on mechanical and wear properties of Zn–40Al–2Cu–2Si alloy

In order to determine the effect of heat treatment on the mechanical and wear properties of Zn–40Al– 2Cu–2Si alloy, different heat treatments including homogenization followed by air-cooling (H1), homogenization followed by furnace-cooling (H2), stabilization (T5) and quench–aging (T6 and T7) were applied. The effects of these heat treatments on the mechanical and tribological properties of the alloy were studied by metallography and, mechanical and wear tests in comparison with SAE 65 bronze. The wear tests were performed using a block on cylinder type test apparatus. The hardness, tensile strength and compressive strength of the alloy increase by the application of H1 and T6 heat treatments, and all the heat treatments except T6, increase its elongation to fracture. H1, T5 and T6 heat treatments cause a reduction in friction coefficient and wear volume of the alloy. However, this alloy exhibits the lowest friction coefficient and wear volume after T6 heat treatment. Therefore, T6 heat treatment appears to be the best process for the lubricated tribological applications of this alloy at a pressure of 14 MPa. However, Zn–40Al–2Cu–2Si alloy in the as-cast and heat-treated conditions shows lower wear loss or higher wear resistance than the bronze.     

Evolution of lamellar structure in Ti–47Al–2Nb–2Cr–0.2W alloy sheet

The Ti–47Al–2Nb–2Cr–0.2W alloy sheets were obtained by hot pack rolling. The as-rolled sheet has an inhomogeneous duplex microstructure composed of elongated gamma grains and lamellar colonies. Heat treatments were conducted on the as-rolled sheets. The results show that the microstructures with different sizes and grain boundary morphologies were developed after different heat treatments. A coarse fully lamellar structure can be refined if the heating time, together with the cooling rate, is appropriately controlled. The grain growth exponent is found to be approximately 0.2, and the activation energy of grain boundary migration of the alloy is around 225 kJ/mol.     

Evolution of deformation mechanisms of Ti–22.4Nb–0.73Ta–2Zr–1.34O alloy during straining

The plastic deformation behavior of Ti–22.4Nb–0.73Ta–2Zr–1.34O alloy was investigated by compression testing at room temperature. The multi-peak stress oscillations of the true stress–strain curve, characterized by a stress plateau, initial strain-hardening, followed by strain-softening and a second strain-hardening stages, is observed in a titanium alloy for the first time. The experimental results show that the above four-stage plastic deformation behavior is caused by a change in the dominant deformation mechanisms. At the stress plateau stage, the alloy deforms via multiple plastic deformation mechanisms. The initial strain hardening is caused mainly by tangling of dislocations. Subsequent strain softening is due to the formation of kink bands. The second strain hardening corresponds to the formation of shear bands. The above results suggest that the dominant deformation mechanisms of Ti–Nb–Ta–Zr–O alloys are related not only to the stability of the β phase, but also to the extent of plastic deformation.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

NiTi-Al-based alloys are promising high-tem- perature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni--43Ti-4AI- 2Nb-2Hf （at%） was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope （SEM）, X-ray diffraction （XRD）, and tensile tests. Results show that the micro- structure of as-forged Ni-43Ti--4AI-2Nb-2Hf alloy con- sists of NiTi matrix, Ti2Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4A1-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4A1 alloy. At room temperature and 650℃, the yield stress of Ni--43Ti-4Al- 2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni--46Ti- 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.     

Effects of Forming Conditions and TiC–TiB_2 Contents on the Microstructures of Self-Propagating High-Temperature Synthesized NiAl–TiC–TiB_2 Composites

The NiAl–TiC–TiB2 composites were processed by self-propagating high-temperature synthesis(SHS) method using raw powders of Ni, Al, Ti, B4 C, TiC, and TiB2, and their microstructure and micro-hardness were investigated. The TiC–TiB2 in NiAl matrix, with contents from 10 to 30 wt%, emerged with the use of two methods: in situ formed and externally added. The results show that all final products are composed of three phases of NiAl, TiC, and TiB2. The microstructures of NiAl–TiC–TiB2 composites with in situ-formed TiC and TiB2 are fine, and all the three phases are distributed uniformly. The grains of NiAl matrix in the composites have been greatly refined, and the micro-hardness of NiAl increases from 381 HV100 to 779 HV100. However, the microstructures of NiAl–TiC–TiB2 composites with externally added TiC and TiB2 are coarse and inhomogeneous, with severe agglomeration of TiC and TiB2 particles. The samples containing externally added 30 wt% TiC–TiB2attain the micro-hardness of 485 HV100. The microstructure evolution and fracture mode of the two kinds of NiAl–TiC–TiB2 composites are different.     

Investigating mechanochemical behavior of Cr2O3–B2O3–Mg–C quaternary system

Fundamental aspects of reaction behavior and formation path in the Cr₂O₃–B₂O₃–Mg–C quaternary system have been studied to synthesize chromium boride–chromium carbide nanocomposite. In order to find the influence of simultaneous presence of magnesium and carbon on final products, various powder mixtures were chosen according to following reaction: B₂O₃ + Cr₂O₃ + (9 − x) Mg + x C. The value of x varied from 0 to 4. In the absence of carbon (x = 0), CrB₂ was synthesize through mechanically induced self-propagating reaction (MSR). In the presence of 8 mol Mg and 1 mol C (x = 1), the dominant boride phase was CrB while no chromium carbide was detected. By increasing C content (x = 2), the magnesiothermic reduction occurred in MSR mode; whereas, the synthesis of Cr₃C₂ initiated after combustion reaction and completed gradually during milling for 6 h. Further increase in C amount (x = 3) resulted in formation of Mg₃(BO₃)₂ as unwanted phases as well as CrB and Cr₃C₂. In the presence of 6 mol Mg and 4 mol (x = 4), no mechanical reaction was observed even after 8 h of milling. Optimum value of x for the formation of CrB–Cr₃C₂ nanocomposite was 2. Based on the morphological evolutions, it is evident that the mechanosynthesized powder is made up of nanometric particles.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Coking and Dusting of Fe–Ni Alloys in CO–H2–H2O Gas Mixtures

Metal dusting of Fe–Ni alloys was investigated in a CO–H₂–H₂O–Ar gas corresponding to a _C = 19.6 at 650 °C. Thermogravimetric analysis showed that increasing the nickel content in the alloy decreased the initial rate of carbon uptake. A uniform Fe₃C scale formed on pure iron, a layer with mixed structures of Fe₃C, γ and α-Fe developed on ferritic Fe–5Ni, and small amounts of Fe₃C developed at the surface of an austenite layer grown on two-phase (α + γ) Fe–10Ni. At nickel levels above 10%, no carbide appeared. These observations are shown to be broadly consistent with local equilibrium according to the Fe–Ni–C phase diagram. However, the failure of higher nickel austenitic alloys to form the (Fe,Ni)₃C expected at high carbon activities indicates a barrier to nucleation and growth of this phase. Graphite deposition was catalysed by (Fe,Ni)₃C on ferritics and by the metal itself on austenitics. The rates of carbon deposition on Fe–60Ni corresponded to the existence of three parallel and independent paths: the synthesis gas, the Boudouard and the carbon methanation reactions.     

Causes of surface stripe cracks of Ti–4Al–2V alloy cold-rolled sheet

The stripe cracks which formed along rolling direction on the surface of Ti–4Al–2V alloy cold-rolled sheet were observed by stereoscope, scanning electron microscope（SEM）, and energy-dispersive spectrometer（EDS）. Morphology analysis indicates that cracks are dominantly in zigzag shape, with 10–30 mm in length and less than 10 lm in depth, and there is no evidence of crack tips. Chemical composition analysis shows that crack regions feather high oxygen concentration while smooth surface is at normal oxygen level. It is obvious that the occurrence of strip cracks is mainly related to residual oxide on the surface of Ti–4Al–2V alloy cold-rolled sheet.     

Characteristics of radiation porosity and structural phase state of reactor austenitic 07C–16Cr–19Ni–2Mo–2Mn–Ti–Si–V–P–B Steel after neutron irradiation at a temperature of 440–600°C to damaging doses of 36–94 dpa

The phase composition and the characteristics of vacancy voids in cold-worked steel 07C–16Cr–19Ni–2Mo–2Mn–Ti–Si–V–P–B (CW EK164-ID) after neutron irradiation at damaging doses of 36–94 dpa and temperatures of 440–600°C are investigated. In the entire range of damaging doses and temperatures, voids with different sizes are observed in the material. The maximum void size increases with irradiation temperature up to ~550°C, whereas their concentration decreases. At higher irradiation temperatures, almost no coarse voids are observed. The concentration of fine voids (to 10 nm in size) sharply increases with temperature from 440 to 480°C. Further increases in the temperature do not result in the noticeable concentration growth. In the irradiation temperature range of 440–515°C, second phases precipitate (G phase, γ’ phase, and complex fcc carbides). At higher irradiation temperatures, there are Laves-phase particles, fine second carbides of the MC type, and needle shape precipitates identified as phosphides in the material.     

Dynamic recrystallization behavior of Fe–20Cr–30Ni–0.6Nb–2Al–Mo alloy

Single-pass compression tests of an aluminaforming austenite(AFA) alloy(Fe–20Cr–30Ni–0.6Nb–2Al–Mo) were performed using a Gleeble-3500 thermal–mechanical simulator. By combining techniques of electron back-scattered diffraction(EBSD) and transmission electron microscopy(TEM), the dynamic recrystallization(DRX) behavior of the alloy at temperatures of 950–1100 ℃ and strain rates of 0.01–1.00 s~(-1) was investigated. The regression method was adopted to determine the thermal deformation activation energy and apparent stress index and to construct a thermal deformation constitutive model. Results reveal that the flow stress is strongly dependent on temperature and strain rate and it increases with temperature decreasing and strain rate increasing. The DRX phenomenon occurs more easily at comparably higher deformation temperatures and lower strain rates. Based on the method for solving the inflection point via cubic polynomial fitting of strain hardening rate(h) versus strain(e) curves, the ratio of critical strain(ec) to peak strain(ep) during DRX was precisely predicted. The nucleation mechanisms of DRX during thermal deformation mainly include the strain-induced grain boundary(GB)migration, grain fragmentation, and subgrain coalescence.     

Microstructure evolution of eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf alloy processed by directional solidification

In this work, the near-eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf(at%) alloy was directionally solidified at 1900 ℃ with withdrawal rates of 6, 18, 36, 50 mm·min~(-1) and then heat-treated at 1450 ℃ for 12 h. The microstructure evolution was investigated. The results show that the microstructure of the directionally solidified(DS) alloy is composed of Nb_(ss)+Nb_5Si_3 eutectics within the whole withdrawal rate range, while the variation of rates makes a great difference on the solidification routes,the morphology and size of Nb_(ss)+Nb_5Si_3 eutectic cells.With the increase in withdrawal rates, the petaloid Nb_(ss)+Nb_5Si_3 eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nbssis formed gradually which isolates the Nb_5Si_3, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of Nb_5Si_3 exist in the heat-treated(HT) samples identified by crystal form and element composition, which are supposed as α-Nb_5Si_3 and γ-Nb_5Si_3, respectively. This study exhibits significant merits in guiding the optimization of Nb–Si-based alloys' mechanical properties.     

Reactive spark plasma sintering of TiB2–SiC–TiN novel composite

The effects of adding SiC as a reinforcement and TiN as an additive on TiB₂-based composites fabricated by the spark plasma sintering (SPS) technique were investigated. SPS was implemented at the sintering conditions of 1900 °C temperature, 7 min holding time and 40 MPa pressure. Adding these two secondary phases had noticeable effects on the microstructure of TiB₂-based composites. A relative densities of 99.9% was obtained for TiB₂–SiC–TiN composite. Detection of in-situ formed phases and investigation on them were done using SEM, XRD, EDS and thermodynamic assessment. These evaluations proved the formation of in-situ phases of TiC, BN nano-platelets, TiSi and B₄C in the TiB₂-based composite codoped with SiC and TiN. Formation of these in-situ phases had fascinating effects on the sinterability and ultimate microstructure of titanium diboride.     

The melting behaviour of extruded Mg–8%Al–2%Zn alloy

Microscopical techniques were used to provide the microstructural details and identify mechanisms governing phase and morphological transformations during the heating of Mg–8%Al–2%Zn pellets in solid and semisolid states. It was found that an as-extruded matrix of equiaxed α-Mg grains with a twinning substructure, was thermally unstable and experienced complete recrystallization after reheating to 200 °C. The precipitates of the Mg₁₇Al₁₂ phase and augmented concentrations of alloying elements within migrating grain boundaries and triple junctions played a key role in transformation of the equiaxed grains into thixotropic structures during partial melting. A direct link exists between sizes of equiaxed grains in the solid state and unmelted particles in the semisolid slurry. Although the morphology of primary solid particles did not change during the melting progress, the rate of particles’ coarsening at high temperatures and their internal microstructure in subsequently solidified alloy were influenced by the solid–liquid ratio. The importance of these findings for semi-solid injection molding practice is emphasized.