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.     

The influence of BaO additives on the reaction of Al2O3–SiO2 ceramics with molten Al and Al–Si alloys

BaO considerably affects the composition and the microstructure of the reaction zone formed between BaO-doped aluminosilicate ceramics and molten aluminium alloys under vacuum. The reduced Ba and Si form AlBaSi precipitates, found adhered to the interface and dispersed in the metal Al-matrix, whose formation apparently controls the reaction kinetics.     

Phase identification of Al–B4C ceramic composites synthesized by reaction hot-press sintering

A series of boron carbide (B₄C) matrix composites with different contents of Al, were synthesized by reaction hot-press sintering with milled B₄C and pure metallic Al powder at 1600 °C for 1 h. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to identify the phase constituent of the milled powders and the composites. The results have shown that parts of B₄C and Al particles were oxidized to boron oxide (B₂O₃) and alumina (Al₂O₃) during the milling. Thermit reaction occurred and B₂O₃ was reverted during hot-press sintering. A ternary phase of Al boron carbide (Al₈B₄C₇) was found in the composites, and the B₄C transformed to a rich boron phase (B_6.5C) because of the superfluous boron in the system.     

Development of in situ NiAl–Al2O3 nanocomposite by reactive milling and spark plasma sintering

NiAl–10 vol.% Al₂O₃ in situ nanocomposite has been synthesized by reactive milling and subsequent spark plasma sintering. The synthesized nanocomposites have ～96% of theoretical density after sintering at 1000 °C for 5 min. Microstructural analysis of consolidated samples using TEM has revealed the presence of α-Al₂O₃ particles of 10–12 nm size in NiAl matrix of submicron grain size. Consolidated NiAl–10 vol.% Al₂O₃ nanocomposite has shown very high hardness of 772 HV_0.3 and compressive strength of 2456 MPa with ～14% plastic strain. The high hardness and compressive yield strength are attributed to the presence of nanocrystalline α-Al₂O₃ particles and the appreciable plastic strain is attributed to the submicron grains of NiAl.     

Phase composition and structure of grain boundary of oversintered Y3Al5O12 ceramics

Phase composition and microstructures of grain boundary of oversintered yttrium aluminum garnet （Y3Al5O12, YAG） ceramics by vacuum sintering at 1 850 ℃ were investigated. For synthesizing YAG, grain boundary is a key factor for YAG ceramics. The morphology of grain boundary was observed by SEM, TEM and its composition was analyzed by EDS. It is identified that the grain boundary is composed of a-AI2O3 and yttrium aluminum perovskite （YAP, YAlO3） eutectics. At the edge of YAG crystal grain, YAG phase is decomposed into perovskite YAP and α-Al2O3 during high temperature sintering. Due to refractive indexes of YAP and α-Al2O3 phases in wide grain boundary are different from those of YAG, the transmittance of oversintered YAG ceramics is lower than that of YAG ceramics sintered at 1 750 ℃.     

Synthesis and sintering behavior of a nanocrystalline α-alumina powder

Nanocrystalline α-alumina powders with a primary mean particle diameter of 10 nm were synthesized from aluminum nitrate and ammonia solution using a precipitation method. The presence of ammonium nitrate (a by-product of the precipitation reaction) in the Al(OH)₃ dry gel can reduce the formation temperatures of γ-, δ-, θ-, and α-Al₂O₃ during heating. The combined effect of 5 wt% α-alumina seed crystals, 100 nm in diameter, and 44% ammonium nitrate can reduce the θ-Al₂O₃→α-Al₂O₃ transformation temperature from 1200 to 900°C. The α-Al₂O₃ powder milled in anhydrous alcohol has an agglomeration strength of 76 MPa (soft agglomerated), while the one milled in deionized water has an agglomeration strength of 234 MPa (hard agglomerated). For both the soft and the hard agglomerated powders initial stage sintering is controlled by grain boundary diffusion, with activation energies of 365 and 492 kJ/mol, respectively. The alumina ceramic produced by sintering the soft agglomerated powder at 1400°C for 2 h has a mean grain size of 0.93 μm, a mean flexural strength of 700 MPa, and a fracture toughness of 4.75 MPa m^1/2.     

Fabrication of finegrained Al2O3 ceramic at low sintering temperature

A research on fabrication of finegrained Al2O3 ceramic at lower sintering temperature was carried out.Al2O3 powder with 50 nm in diameter is compounded with 11.24%Al and 4.75% Fe(mass fraction) by high-energy ball-milling. AI is got from Al powder which is a component of the materials being milled and Fe from steel milling balls and milling jar during the milling. In this way, nearly no impurity is brought into the composite powder during milling. With hot pressing of the composite powder and pure Al2O3 powder, it is proved that Al2O3 powder can be densified at lower sintering temperature when the powder is compounded in this way. Al2OC and AlFe form during sintering process of the composite powder. With the reactive sintering and multiphase sintering mechanisms, finegrained Al2O3 ceramic is fabricated at low sintering temperature.     

Synthesis and reaction behavior of calcium silicate hydrate in basic system

1　INTRODUCTIONIt skeytoseparatealuminafromsilicainaluminaproductionbyBayerorsinteringprocess .Whethersilicachangestosodiumaluminosilicatehydrateorhydro garnet(bothtitleddesilicationproduct,DSP) ,bothcontainso daoralumina .IfDSPisdischargedoutdirectly ,alargeamountofsodaandaluminawillbewasted ,meanwhiledischargedDSPwillresultinheavyenvironmentalpollu tion .InpresentitcostsalottorecoveraluminaandsodafromDSPbysinteringprocess[1] .Soitisurgenttodevel opnewhydro processtechnologytorecove…     

Phase relations and hydrogenation behavior of （La1-xMgx）3Al alloys

The crystal structures and hydrogenation behavior of （La1-xMgx）3Al （x = 0.1, 0.2, 0.3, and 0.4） alloys were investigated. It was found that the alloys with x = 0.1 and 0.2 consist of La（Mg,Al）, La, and a novel phase. The novel phase was determined as La2Al. It is shown that the amount of La2Al decreases as the Mg content increases. When x increases to 0.3, only La（Mg,Al） and a small amount of La2Al exist. When x is 0.4, La2Al phase disappears and the alloy contains both La（Mg,Al） and La（Al,Mg）2 Laves phase. The （Lao.9MgoA）3Al and （La0.TMg0.3）3Al alloys can be decomposed into LaH3, MgH2, and La2Al5 by the absorption of hydrogen at 473 K.     

Phase equilibria and structural investigations in the system Al–Fe–Si

The Al–Fe–Si system was studied for an isothermal section at 800 °C in the Al-rich part and at 900 °C in the Fe-rich part, and for half a dozen vertical sections at 27, 35, 40, 50 and 60 at.% Fe and 5 at.% Al. Optical microscopy and powder X-ray diffraction (XRD) was used for initial sample characterization, and Electron Probe Microanalysis (EPMA) and Scanning Electron Microscopy (SEM) of the annealed samples was used to determine the exact phase compositions. Thermal reactions were studied by Differential Thermal Analysis (DTA). Our experimental results are generally in good agreement with the most recent phase diagram versions of the system Al–Fe–Si. A new ternary high-temperature phase τ₁₂ (cF96, NiTi₂-type) with the composition Al₄₈Fe₃₆Si₁₆ was discovered and was structurally characterized by means of single-crystal and powder XRD. The variation of the lattice parameters of the triclinic phase τ₁ with the composition Al_2+xFe₃Si_3?x (?0.3 < x < 1.3) was studied in detail. For the binary phase FeSi₂ only small solubility of Al was found in the low-temperature modification LT-FeSi₂ (ζ_β) but significant solubility in the high-temperature modification HT-FeSi₂ (ζ_α) (8.5 at.% Al). It was found that the high-temperature modification of FeSi₂ is stabilized down to much lower temperature in the ternary, confirming earlier literature suggestions on this issue. DTA results in four selected vertical sections were compared with calculated sections based on a recent CALPHAD assessment. The deviations of liquidus values are significant suggesting the need for improvement of the thermodynamic models.     

A Comparative Study of High Temperature Corrosion of Al2O3, SiO2 and Al2O3–SiO2 Forming Alloys in a Na2SO4–NaCl Atmosphere

The present paper is designed to provide a summary of our study on the high temperature corrosion of Al₂O₃, SiO₂ and Al₂O₃–SiO₂ forming alloys in the gas phase and liquid phase of Na₂SO₄–NaCl system by comparing their corrosion resistance at 1000 °C. The obtained results show that the alumina-forming alloy experiences severe internal corrosion in the gas phase compared to in the liquid phase due to oxide cracking. This results in an increase in the inward diffusion and/or penetration of constituents of the salts and oxygen to form internal Al-oxide and Cr-sulfides. In the liquid phase, however, the formation of yttrium sulfide beneath a continuous double oxides layer of Al₅Y₃O₁₂ and Al₂O₃ may be related to the high affinity of yttrium for sulfur. On the other hand, it is apparent from the cross-sectional observations that a SiO₂ and Al₂O₃–SiO₂ forming alloys form a continuous and dense oxides layer, and demonstrate a high resistance against internal oxidation and corrosion in both corrosive environments.     

The mechanical behavior of a cryomilled Al–10Ti–2Cu alloy

The mechanical behavior of a cryomilled Al–10Ti–2Cu (wt.%) alloy has been studied by performing uniaxial tension tests at temperatures ranging from room temperature to 525°C. Elastic–nearly perfectly plastic stress–strain behavior is observed at all temperatures. Tension–compression asymmetry of the room temperature yield stress is also observed. These characteristics are in agreement with those recently reported in the literature for single-phase NC materials. The flow stress (700 MPa at room temperature) decreases dramatically with increasing temperature. Testing of material following thermal exposures suggests that microstructural coarsening alone cannot account for the decrease in strength with increasing temperature. From a coarsening standpoint, this material appears to be very thermally stable. The ductility is influenced by several factors. Low levels of internal porosity along with the presence of fine oxide and carbide dispersoids contribute to lower ductility. The absence of work hardening exhibited by the Al–10Ti–2Cu also leads to reduced strain to failure. The features observed on fracture surfaces suggest that fracture occurs by the nucleation and growth of voids at particle–matrix interfaces. Evidence of fracture along prior powder particle boundaries is present as well. The microstructure consists primarily of regions containing grains measuring in the range 30–70 nm. Large grained regions consisting of nominally pure Al ranging in size from 300 to 500 nm are also present. No evidence of dislocation activity within either the fine or large grained regions can be found in the as extruded material. Specimens deformed at room temperature and 93°C reveal evidence of dislocation activity within the large grain regions. Dislocation configurations suggest an Orowan bypass mechanism. No dislocations are found within the 30–70 nm size grains following tensile deformation.     

Effect of Mg addition on mechanical and thermoelectrical properties of Al–Al2O3 nanocomposite

The effects of Mg addition on mechanical thermo-electrical properties of Al–Mg/5%Al₂O₃ nanocomposite with different Mg contents (0, 5%, 10% and 20%) produced by mechanical alloying were studied. Scanning electron microscopy analysis (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) were used to characterize the produced powder. The results show that addition of Mg forms a predominant phase (Al–Mg solid solution). By increasing the mass fraction of Mg, the crystallite size decreases and the lattice strain increases which results from the atomic penetration of Mg atoms into the substitutional sites of Al lattice. The microhardness of the composite increases with the increase of the Mg content. The thermal and electrical conductivities increase linearly with the temperature increase in the inspected temperature range. Moreover, the thermal conductivity increases with the increase of Mg content.     

β Phase decomposition in Ti–5Al–5Mo–5V–3Cr

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ～500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ～90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ～1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties.     

Friction and wear behaviors of Al2O3–13 wt%TiO2 coatings

Nanostructured and conventional Al2O3-13 wt%TiO2 coatings were manufactured by air plasma spray. Friction and wear behaviors of coatings were investigated at room and elevated temperatures using an SRV wear test machine. The nanostructured coating has "two regions" microstructure, while the conventional coating has typical layered microstructure with obvious interfaces among splats. The coefficient of friction decreases with rising of temperature because of the formation of tribo-layer at elevated temperatures. The wear resistance of the nanostructured coatings is higher than that of the conventional coating, and the wear threshold of applied load is 30 N for conventional coating and 40 N for nanostructure coating. The wear resistance difference is related to the "two regions" microstructure of nanostructure coating, which could blunt or branch the cracks propagation. In our test ranges, the wear rates rising are more sensitive with the applied wear load rising than with the temperature rising.     

Phase equilibria in the Ti–Al binary system

New experimental results on the phase equilibria between the β-Ti (A2), α-Ti (A3), α₂-Ti₃Al (D0₁₉) and the γ-TiAl (L1₀) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α/γ and the α₂/γ equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the α/β equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the β phase field extends to higher aluminum contents and that the width of the α+β two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order–disorder transition in the β phase is also confirmed by a combination of differential scanning calorimetric (DSC) analysis and extrapolation of ordering phase boundaries from the Ti–Al–X (X=Cr, Fe) ternary systems. A thermodynamic analysis has been carried out taking into account the ordering configurations in the β-Ti (A2)/β₂-TiAl (B2), f.c.c.-Al (A1)/γ-TiAl (L1₀) and α-Ti (A3)/α₂-Ti₃Al (D0₁₉) equilibria. It is proposed that the anomalous α/β equilibrium is due to the A2/B2 ordering reaction.     

Hot deformation behavior and microstructure evolution of TiC–Al_2O_3/Al composites

Hot compression behavior of TiC–Al2O3/Al composites was studied using the Gleeble-1500 system at a temperature range of 300–550 °C and at strain rate range of 0.01–10.00 s-1. The associated structural changes were studied by TEM observations. The results show that stress level decreases with deformation temperature increasing and strain rate decreasing, which can be represented by a Zener–Hollomon parameter in an exponent-type equation with hot deformation activation energy Q of 172.56 kJ·mol-1.Dynamic recovery occurs easily when strain rates are less than 10.00 s-1. Dynamic recrystallization can occur at strain rate of 10.00 s-1.     

Effect of the magnesium content on the mechanochemical behavior in ternary system Mg–B2O3–C

The influence of Mg content in Mg–B₂O₃–C mixture on the MgO–B₄C composite formation and mechanism of reactions during ball milling process was investigated. In keeping with this aim, a mixture of boron oxide powder along with different amounts of Mg (4–10 mol) and C (1–3 mol) was activated in a ball mill. Thermodynamic calculations and differential thermal analysis (DTA) results revealed that Mg value played a main role, thereby; overall reaction enthalpy and adiabatic temperature (T_ad) changed by variation of magnesium content. T_ad enhanced with the increase in the Mg content and found maximum value (2852 °C) at stoichiometric ratio (Mg = 6 mol). DTA results showed that increasing the activation time to 5.5 h could decrease the temperature of combustion reaction to 530°C before magnesium melting. According to experimental findings, in the mixture of powder with 4 mol Mg, magnesiothermic reaction occurred in MSR mode and no carbothermal reaction took place. However, when the Mg content reached within a range of 9/2–6 mol, the magnesiothermic reaction occurred in MSR mode and activated the carbothermal reaction. Further enhancement in Mg content (10 mol Mg), as a diluent agent, led to MSR magnesiothermic reaction and no carbothermal reduction occurred.