Synthesis of nanocrystalline boron carbide from boric acid–sucrose gel precursor

A novel method, based on the combustion of boric acid–sucrose xerogel was developed to synthesize nanocrystalline boron carbide powder. This xerogel was pyrolyzed at 1273 K. Boron carbide was obtained by heating this precursor at 1823 K. The yield of boron carbide was improved by the use of a novel graphite crucible designed for this purpose. The xerogel and the precursor were characterized by using Fourier transform infrared spectroscopy. The constituent phases were identified by using X-ray diffraction while their elemental composition was established with the help of chemical assay. The microstructure of the final product was examined with the help of scanning electron microscopy and transmission electron microscopy. This study demonstrates that the yield of boron carbide would be enhanced to about 48% while the free carbon content in the final product could be reduced to about 6 wt%. These are significant improvements over similar studies reported so far on the gel-based preparation of boron carbide.     

Synthesis of α-alumina fibre from modified aluminium alkoxide precursor

Polycrystalline alumina fibre was successfully synthesized by pyrolysis of a preceramic fibre formed from aluminium compounds with alkoxy and chelate ligands. A mixture of ethyl 3-oxobutanoatodiisopropoxyaluminium (EOPA) and tri-sec-butoxyaluminium (SBA) was reacted with glacial acetic acid yielding a polymeric product. The IR absorptions at 630 and 700 cm^–1 due to the Al-O bond changed from sharp to broad bands by treatment with acetic acid. The ²⁷Al resonance at 35 p.p.m. increased in intensity when EOPA-SBA (7/3) was treated with 30 mol% acetic acid. An increase in the EOPA to SBA ratio 5/5 to 9/1 also raised the intensity of the signal at 35 p.p.m. The viscosity of the polymeric product increased in intensity as the amount of acetic acid increased. The viscosity of precursor increased with increasing the ratio of EOPA to SBA, and decreased with increasing measurement temperature from 45 to 75°C. The precursor polymer pyrolysed at 500°C in air was amorphous to X-rays, and crystallized in -alumina at 840°C. The precursor fibres were pyrolysed to yield finegrained fibres of -alumina at 1200°C for 1 h.     

Precipitation of complex carbonitrides in a Nb–Ti microalloyed plate steel

Complex carbonitrides precipitated in base metal and heat-affected zone (HAZ) in Nb–Ti hot-rolled microalloyed steel plates have been identified to be Ti-rich (Nb, Ti)(C, N). As the reheating temperature is decreased from 1,200 to 1,150 °C, the average particle size in base metal is decreased from 40 to 20 nm. The morphology of complex carbonitrides in the HAZ, however, is transformed from cuboidal to rectangle shape with length of over 500 nm. Reheating at low temperature 1,150 °C may improve the toughness of HAZ by reducing the austenite size at large heat input welding.     

Synthesis and phases evolution of Si–C–Ti from polycarbosilane (PCS) and metal Ti

Si–C–Ti ceramics were synthesized by reactive pyrolysis of polycarbosilane (PCS) precursor filled with metal Ti powder. Pyrolysis of mixture with atomic ratio of Ti:Si through 3:1–3:2 was carried out in argon atmosphere at given temperature up to 1500 °C. The metal–precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy with EDX. The Ti₃SiC₂ phase was obtained firstly from reaction of PCS and Ti. Ti₃SiC₂ formation starts at 1300 °C and its amount increases significantly in a narrow temperature range between 1400 °C and 1500 °C. In addition, addition of CaF₂ can promote the formation of Ti₃SiC₂ phase.     

Synthesis and characterization of a new liquid polymer precursor for Si–B–C–N ceramics

A new liquid polyborosilazane precursor for Si–B–C–N ceramic was synthesized by co-condensation reaction of boron trichloride, organodichlorosilanes, and hexamethyldisilazane. The structure and properties of polyborosilazane were studied by means of Fourier transform-infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), rheology, and thermogravimetric analysis (TGA). The conversion of polymer to ceramic and the high-temperature behavior of the new polymer-derived ceramic were investigated by TG–MS, FT-IR, X-ray diffraction (XRD) and high-temperature TGA (HTGA). The ceramics showed good oxidative resistance and thermal stability with weight gain of 1.8 wt% at 1350 °C under air atmosphere and weight loss of 2.6% at 1900 °C under Ar atmosphere.     

Synthesis,corrosion and wear of anodic oxide coatings on Ti–6Al–4V

Electrodeposited anodic oxide coatings were produced on Ti–6Al–4V substrates using aqueous electrolytes containing dissolved calcium and phosphorus. Different coatings were produced by varying the time periods. The coatings were characterised by XRD technique and TEM. The coatings were exposed to Simulated Body Fluid (SBF). Electrochemical polarisation and ac impedance studies too were performed on the coatings in SBF. Pins were coated and run against wooden disc in pin-on-disc type of wear tests. Coatings produced from long time electrolysis showed very good resistance to the attack of SBF and less wear compared to those produced from short time exposure.     

Deformation characteristics of Ti–6Al–4V

Abstract

The deformation characteristics of Ti–6Al–4V have been established by torsion testing in the temperature range 800–1150°C. Constitutive equations are proposed for both the β-region and the α+β-region which, it is suggested, may have some practical applications. Extensive optical and electron microscopy have established that dynamic recovery is the operative deformation mode in the β-region, while dynamic recrystallisation predominates in the α+β-region.

MST/806     

Interaction of Carbon Fiber with a Ti–Al Melt during Self-Propagating High-Temperature Synthesis

Inorganic Materials - We have studied the structuring of combustion products in the Ti–Al system upon interaction with carbon fibers during self-propagating high-temperature synthesis. The...     

Corrosion behavior of Ti–8Al–1Mo–1V alloy compared to Ti–6Al–4V

The corrosion behavior of Ti–8Al–1Mo–1V alloy was investigated in 3.5% NaCl and 5% HCl solutions. Corrosion properties of Ti–6Al–4V alloy were also evaluated under the same conditions for comparison. It was found that both Ti–8Al–1Mo–1V and Ti–6Al–4V alloys exhibited spontaneous passivity and low corrosion current densities in 3.5% NaCl solution. The potentiodynamic polarization curves obtained in 5% HCl solution revealed an active–passive transition behavior and similar corrosion rates for the examined alloys. However, the results of the weight loss experiments under accelerated immersion conditions (5 M HCl at 35 °C) indicated that Ti–8Al–1Mo–1V alloy exhibited inferior corrosion behavior compared to Ti–6Al–4V alloy. These results were confirmed by scanning electron microscopy (SEM) analysis of the samples after immersion tests which revealed that the β phase was corroded preferentially for both alloys, but to a larger extent in the case of Ti–8Al–1Mo–1V alloy.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

Partial isothermal sections of Ti–Al–V ternary diagram

Abstract

The phase equilibria between the titanium aluminides, Ti₃Al and TiAl, and V have been investigated in the temperature range 1473–873 K. Microstructures were characterised using light, scanning electron, and transmission electron microscopy and the nature of the phases present was established via X-ray diffractometry, electron diffraction, and energy dispersive X-ray analysis. A series of isothermal sections have been determined and are compared with those obtained in earlier studies. Ordering of the high temperature bcc β phase to a simple cubic B2 (β₂) structure was observed: the ordering temperature was found to decrease with increasing V content, whereas the opposite effect was observed with Al. The effect of oxygen is to stabilise the α phase and to shift the α₂+β₂+γ field in the V rich direction. The lattice parameters of α₂ and β phases decrease with additions of Al and V: empirical equations have been developed to predict these changes.

MST/1881     

Microplasticity in HCF of Ti–6Al–4V

Previous research has shown that Ti–6Al–4V exhibits pronounced stress ratio effects under high cycle fatigue (HCF) loading. At high stress ratios (R>0.7), a transition of failure mode occurs from traditional surface fatigue crack initiation and growth to bulk-dominated damage initiation and coalescence of multiple microcracks consistent with a ductile tensile test. At these high stress ratios, ratchetting was shown to occur (Int. J. Fatigue 21 (1999) 679; Mech. Time-Dependent Mater. 2 (1999) 195), leading to progressive strain accumulation until final failure. This study explores the microstructural origins of this stress ratio transition in HCF using computational micromechanics. The material being studied is a two-phase Ti–6Al–4V plate forging, consisting of a duplex microstructure with a hexagonal close-packed (hcp) α-phase and lamellar grains with layers of body-centered cubic (bcc) β-phase and secondary hcp α-phase. Crystallographic slip is the dominant mode of plastic deformation in this material. A 2-D crystal plasticity model that incorporates nonlinear kinematic and isotropic hardening at the slip system level is implemented into the finite element method to simulate the cyclic plasticity behavior. The finite element model is used to qualitatively understand the distribution of microplasticity in this alloy under various loading conditions. For typical HCF stress amplitudes, it is shown that microstructure scale ratchetting becomes dominant at R=0.8, but is insignificant at R=0.1 and 0.5. Reversed cyclic microplasticity is insignificant at all three stress ratios. The effects of phase morphology and orientation distribution are shown to affect the microscale plastic strain distribution in terms of the location and magnitudes of the plastic shear bands that form within clusters or chains of primary α grains. The results of the finite element modeling are also considered in light of previous experimental results.     

Microindentation study of Ti–6Al–4V alloy

In order to study the micromechanical behavior of Ti–6Al–4V alloy, microindentation experiments were performed with five different maximum loads of 100, 150, 200, 250 and 300 mN, and with three loading speeds of 6.4560, 7.7473 and 9.6841 mN/s respectively. The experimental results revealed that loading speed has little influence on microhardness and Young’s modulus. Microindentation hardness experiments showed strong indentation size effects, i.e. increase of indentation hardness with the decrease of indentation load or depth. Then microindentation constitutive equation that described the stress as a function of the strain was proposed through dimensional analysis. And the finite element simulation results showed that the predicted computational indentation data from developed constitutive equation can track the microindentation experimental data of Ti–6Al–4V alloy.     

Synthesis of water-compatible surface-imprinted composite microspheres with core–shell structure for selective recognition of thymopentin from aqueous solution

A novel water-compatible surface-imprinted core–shell microsphere, which had multiple non–covalent interactions with template molecule, was successfully prepared by the surface grafting polymerization method in acetonitrile–water systems with thymopentin as template through ionic liquid-functionalized polyethyleneglycolmethacrylate-co-vinylimidazole microsphere as the matrix. The average diameter of matrix was 1 μm ± 20 nm and the thickness of imprinted layer was about 50 nm. The results of static adsorption experiments indicated that ionic liquid-functionalized molecularly imprinted microspheres showed the good adsorption capacity and specific recognition for template peptide. The binding-isotherm analysis showed that Langmuir isotherm models gave a good fit in the range of concentrations, suggesting that there was only one kind of binding site in imprinted layer. Measurements of the binding kinetics revealed that surface-imprinted composite microspheres reached peptide-adsorption equilibrium in 60 min and the maximum adsorption capacity for TP5 was 38.4 mg g^?1. The effects of pH, salt concentration, and temperature on the adsorption capacities were investigated. The microspheres were found to have a high specificity for TP5 with little affinity for BSA and Hb. Finally, the core–shell microspheres can be reused with only 15.6 % decrease in TP5 adsorption capacity after six times.     

Influence of the silica matrix on the formation of α-alumina in a mullite–alumina composite from a diphasic precursor

In mullite–alumina composite precursors, interaction between the silica matrix and the fine -alumina texture strongly influences the precursor phase transformation, the nucleation and the crystal geometry both of the mullite and of the -alumina. The mullite–alumina composite precursor calcined at 1000 °C has a layered structure probably derived from the layered texture of the -alumina. The phase transition of this layered texture is retarded by the presence of the silica matrix and a metastable mullite phase is formed before nucleation of -alumina. By leaching away the silica matrix, the remaining layered texture is readily transformed into very fine, thin -alumina platelets by calcination at 1000 °C. This seems to be one reason for the appearance of elongated mullite grains in a pure mullite matrix and the platelet shaped -alumina grains in the mullite–alumina composite prepared from diphasic precursors. © 1998 Chapman & Hall     

Effect of silicon on recrystallisation of V–Ti microalloyed steel

Abstract

The dynamic recrystallisation (DRX) and static recrystallisation (SRX) behaviours of three V–Ti microalloyed steels were studied by the analysis of the true stress–strain curves and the stress relaxation curves under different deformation conditions. The results of DRX showed that deformation activation energy Q_def, peak stress and peak strain increased, as a result of the solute strengthening and dragging effect due to Si. The results of SRX showed that Si increased the SRX activation energy Q_SRX. The solute retardation parameter for static recrystallisation of Si was calculated. Based on the SRX results, to quantify the drag effect of Si and V, a new model was proposed to describe the time for 50% recrystallisation (t_0·5), which was tested and verified by previously published data on similar steels. Precipitation during recrystallisation could lead to a lower value of the Avrami exponent.     

Combinatorial Synthesis and High-Throughput Characterization of Microstructure and Phase Transformation in Ni–Ti–Cu–V Quaternary Thin-Film Library

Ni–Ti–based shape memory alloys (SMAs) have found widespread use in the last 70 years, but improving their functional stability remains a key quest for more robust and advanced applications. Named for their ability to retain their processed shape as a result of a reversible martensitic transformation, SMAs are highly sensitive to compositional variations. Alloying with ternary and quaternary elements to fine-tune the lattice parameters and the thermal hysteresis of an SMA, therefore, becomes a challenge in materials exploration. Combinatorial materials science allows streamlining of the synthesis process and data management from multiple characterization techniques. In this study, a composition spread of Ni–Ti–Cu–V thin-film library was synthesized by magnetron co-sputtering on a thermally oxidized Si wafer. Composition-dependent phase transformation temperature and microstructure were investigated and determined using high-throughput wavelength dispersive spectroscopy, synchrotron X-ray diffraction, and temperature-dependent resistance measurements. Of the 177 compositions in the materials library, 32 were observed to have shape memory effect, of which five had zero or near-zero thermal hysteresis. These compositions provide flexibility in the operating temperature regimes that they can be used in. A phase map for the quaternary system and correlations of functional properties are discussed with respect to the local microstructure and composition of the thin-film library.