Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

In situ fabrication of titanium carbide reinforced copper MMC

Abstract

The in situ fabrication of titanium carbide reinforced copper and aluminium bronze (AB2) composites by carbothermal reduction of titanium in an induction furnace has been investigated. An inert atmosphere was maintained with carbon monoxide created as a byproduct from the heat of reaction between the induction field, graphite crucible and graphite lid. Titanium carbide particles of the order 1–3 μm were formed in aluminium bronze at approximately 1250°C and, in copper, particles of order 1–6 μm were produced at approximately 1330°C. Dispersion concentrations of titanium carbide of 20% and 6.5% were obtained for copper and aluminium bronze respectively. In addition, evidence is presented indicating that iron could be used as a dispersion medium for titanium carbide particulates in aluminium bronze alloys.     

Effect of Hf ion implantation on grain growth in Ni nanocrystalline electrodeposits

Abstract

The microstructural stability of Ni nanocrystalline electrodeposits was investigated to verify general principles underlying the suppression of grain growth by microalloying with elements of very low solid solubility. Hf ions at 300 keV energy were implanted in Ni nanocrystalline foils at low (5·8 × 10¹⁵ ions cm^?2) and high (3·0 × 10¹⁶ ions cm^?2) doses. Their effects on grain growth at 550°C were studied in situ by transmission electron microscopy at 1·25 MeV and by selected area electron diffraction. Grains roughly doublled in size during implantation, but grain growth during subsequent heat treatment was dose dependent and significantly less than in specimens without implantation. Observation on implanted Ni single crystals revealed clustering and the formation of fine Ni₅Hf precipitates. A possible mechanism of grain growth suppression is discussed.     

Influence of ion energies on the structure,composition, and properties of multilayer Ti–Al–Si–N ion-plasma-deposited coatings

It is established that the energy of deposited particles influences the structure, composition, and properties of multilayer nitride coatings consisting of alternating layers of nanocrystalline TiN and amorphous Si₃N₄ phases with inclusions of nanocrystalline hexagonal AlN formed at energies of titanium, aluminum, and silicon ions exceeding ~317 × 10^–19, 267 × 10^–19, and 230 × 10^–19 J, respectively. As the energy of titanium ions bombarding the substrate increases above ~512 × 10^–19 J, the phase transition from disordered TiN _x to Ti₃N₂ and the appearance of 2- to 3-nm-thick sublayers in 15-nm-thick nanocrystalline TiN _x layers take place in the coating. The maximum hardness of such coatings reaches a level of ~54 GPa.

     

Preparation of nano-TiN powders by Ni-catalysed carbothermal reduction nitridation

TiN nanoparticles were prepared at 900–1100 °C by Ni-catalysed carbothermal reduction nitridation from sol-gel using tetrabutyl titanate, citric acid monohydrate and nickel chloride hexahydrate as starting materials. The catalytic effects of nickel on the carbothermal reduction nitridation of xerogels were investigated. Ni has a crucial promoting effect on the carbothermal reduction reaction of the xerogels and visibly enhanced carbothermal reduction and nitridation reactions. The dry gel with 5% NiCl₂ was added to obtain nano-TiN at 900 °C. The transmission electron microscopy analysis and Materials Studio simulation results showed that the facilitating effect of Ni on carbothermal reduction nitridation reaction was ascribed to the provision of heterogeneous nucleation sites for amorphous titania, promoting crystallisation and adsorption of N₂, resulting in its smooth dissociation into highly active N atoms and consequently enhancing the carbothermal reduction nitridation.     

Synthesis and growth mechanism of NbC–SiC micro/nanowire by carbothermal reduction

Abstract

NbC–SiC micro/nanowires (MNWs) with NbC content varying from 5 to 20 mol.-% were synthesised at 1600–1800°C via carbothermal reduction utilising silica sol, niobium pentoxide powder and carbon black as starting materials. The synthesis process and growth mechanism of NbC–SiC system were investigated. Results show that the morphology of the synthesised products mainly appears as curve shaped microwires or nanowires. The crystalline consists of both SiC and NbC phases which doped with each other by substitution and interstitial reactions in solid solution. NbC–SiC MNWs were developed by vapour–liquid–solid mechanism according to the existence of liquid droplet phase in the tip at reaction temperature. β-SiC twin crystal growing along [112] direction was formed in the stem, and NbC polycrystal was dissociated from Nb–Si liquid phase. The varied concentration of Nb and Si in the Nb–Si liquid phase could be a significant reason for the curved growth of NbC–SiC MNWs.     

Effect of coating layers on nano-TiO<Subscript>2</Subscript> particles on the preparation of nanocrystalline λ-Ti<Subscript>3</Subscript>O<Subscript>5</Subscript> by carbonthermal reduction

Three different kinds of rutile TiO₂ nanoparticles with different surface coating conditions were prepared by precipitation method for synthesizing λ-Ti₃O₅. The effect of surface layers on nano rutile TiO₂ particles on the preparation of λ-Ti₃O₅ was firstly investigated. X-ray diffraction, scanning electron microscope, energy dispersive spectrometer and transmission electron microscope were used to characterize these powders. The nano rutile TiO₂ powders were mixed with carbon black separately and then annealed at an isothermal heat condition. The phase structures of the products demonstrate that high purity nanocrystalline λ-Ti₃O₅ powders can be synthesized by carbothermal reduction of the Al₂O₃–SiO₂ dual-coated TiO₂ powders, but not by the uncoated and SiO₂-coated TiO₂ powders. Thus, the coating layers of the TiO₂ nanoparticles played a decisive role in the formation of nanocrystalline λ-Ti₃O₅. It proposed that both of the coating layers and metal ion (i.e. Al³⁺) make a contribution to stabilize the metastable phase λ-Ti₃O₅ to ambient temperature in the carbothermal reduction of the TiO₂/C system. This study provided considerable insights into the preparation of nanocrystalline λ-Ti₃O₅.     

Preparation and optical properties of GaN nanocrystalline powders

Abstract

GaN nanocrystalline powders were synthesised by decomposition of gallium nitrate, followed by nitrogenising with ammonia under different temperature. X-ray diffraction (XRD) and the transmission electron microscopy (TEM) indicated that the crystallinity of the powder is improved and the average size of the GaN nanocrystallites increases from 4·8 to 23·9 nm as the temperature increases from 850 to 1050°C. The Raman spectra displayed four broadened peaks corresponding to A₁ (LO), A₁ (TO), E₁ (TO) and E₂ (high) modes of würtzite GaN respectively. Two additional modes at 252 and 421 cm^–1 attributed to boundary phonons activated by the finite size effects and octahedral Ga–N₆ bonds were observed respectively. A strong blue photoluminescence (～353 nm) was detected for room temperature measurement, indicating that the GaN nanocrystalline powders have few defects and high quality.     

Role of Al additions in wear control of nanocrystalline Mo(Si1−xAlx)2 coatings prepared by double cathode glow discharge technique

Abstract

Four kinds of nanocrystalline Mo(Si_1?xAl_x)₂ coatings with differing Al contents are prepared onto a Ti–6Al–4V substrates by a double cathode glow discharge apparatus. The microstructural features of the deposited coatings were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These coatings are composed of the equiaxed C40–MoSi₂ grains with the average grain size of ～5 nm. Nano-indentation measurements indicated that the hardness H, elastic modulus E and the H/E or H³/E² ratio of the nanocrystalline Mo(Si_1?xAl_x)₂ coatings slightly increase with the increase in Al content. The tribological behaviour of the nanocrystalline Mo(Si_1?xAl_x)₂ coating sliding against a ZrO₂ ceramic ball at room temperature has been compared using a ball-on-disc type tribometer under unlubricated conditions. Experimental results showed that the specific wear rates of the nanocrystalline Mo(Si_1?xAl_x)₂ coatings decrease with increasing Al content and are dramatically reduced by more than 1–2 orders of magnitude over the uncoated Ti–6A1–4V. The enhancement of wear resistance of the nanocrystalline Mo(Si_1?xAl_x)₂ coatings by Al additions is correlated with the increased mechanical properties and the forming oxide layer by tribochemical reactions.     

Thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys

Abstract

The thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys has been studied using TEM, X-ray diffraction, and atom probe analysis. All of the as deposited materials were purely fcc and had grain sizes of 10–20 nm. Heat treatment of nanocrystalline nickel in the range 190–320°C resulted in abnormal grain growth with an activation energy of 122±15 kJ mol^-1. Abnormal grain growth in Fe–50 at.-%Ni was only observed at 400°C but not at 220 or 300°C, where grain growth was very slow. In Fe–33 at.-%Ni, room temperature aging resulted in the formation of large grained areas (～1 µm), some of which transformed to bcc. In heat treated nickel specimens, some evidence of sulphur and carbon enrichment was found at grain boundaries.     

Magnéli phase Ti4O7 powder from carbothermal reduction method: formation, conductivity and optical properties

Magnéli phase Ti₄O₇ was prepared by carbothermal reduction of TiO₂ in vacuum. The products synthesized in different carbon black content and heated temperature were investigated by X-ray diffraction, scanning electron microscopy, and ultraviolet–visible (UV–Vis) spectra of products were tested also. The results show that the pure Ti₄O₇ powder is obtained with carbon black content of 4 wt% at 1,025 °C for 2 h in vacuum, their average particle size and BET surface area are about 6.10 μm and 4.107 m² g^?1, respectively. The resistivity of the samples is up to 0.11 Ω cm, and UV–Vis spectra shows the optical absorption bands cover the full range of visible-light wave lengths and extend into near-infrared region.     

Superplastic behaviour of as received superplastic forming grade IN718 superalloy

Abstract

Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures T to investigate the stress σ versus strain rate ? · behaviour, the nature of the stress versus strain ? curves, ductility, and microstructure upon failure. The log σ–log ? · plot for the ? · range ～5 × 10^-6–3 × 10^-2 s^-1 at T = 1173–1248 K exhibited a strain rate sensitivity index m = 0·62 at low strain rates and m = 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol^-1, respectively. All the σ–? curves, obtained at ? · = 1 × 10^-4 s^-1 for the temperature range 1173–1273 K, and at T = 1198 K for the strain rate range 1 × 10^-4–1 × 10^-2 s^-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III.     

Formulation design and optimization of novel soft glycerosomes for enhanced topical delivery of celecoxib and cupferron by Box–Behnken statistical design

Background: The present study describes glycerosomes (vesicles composed of phospholipids, glycerol and water) as a novel drug delivery system for topical application of celecoxib (CLX) and cupferron (CUP) compound.

Aim: The goal of this research was to design topical soft innovative vesicles loaded with CLX or CUP for enhancing the efficacy and avoiding systemic toxicity of CLX and CUP.

Methods: CLX and CUP loaded glycerosomes were prepared by hydrating phospholipid-cholesterol films with glycerol aqueous solutions (20–40%, v/v). Box–Behnken design, using Design-Expert® software, was the optimum choice to statistically optimize formulation variables. Three independent variables were evaluated: phospholipid concentration (X₁), glycerol percent (X₂) and tween 80 concentration (X₃). The glycerosomes particle size (Y₁), encapsulation efficiency percent (Y₂: EE %) and drug release (Y₃) were selected as dependent variables. The anti-inflammatory effect of CLX and CUP glycerosomal gel was evaluated by carrageenan-induced rat paw edema method followed by histopathological studies.

Results: The optimized formulations (CLX2* and CUP1*) showed spherical morphology under transmission electron microscopy, optimum particle size of 195.4?±?3.67?nm, 301.2?±?1.75?nm, high EE of 89.66?±?1.73%, 93.56?±?2.87%, high drug release of 47.08?±?3.37%, 37.60?±?1.89% and high cumulative amount of drug permeated in 8?h of 900.18?±?50.24, 527.99?±?34.90?µg.cm^?2 through hairless rat skin, respectively. They also achieved significant remarkable paw edema inhibition in comparison with the control group (p? Conclusion: Finally, the administration of CLX2* and CUP1* loaded glycerosomal gel onto the skin resulted in marked reduction of edema, congestive capillary and inflammatory cells and this approach may be of value in the treatment of different inflammatory disorders.     

Structure refinement for tantalum nitrides nanocrystals with various morphologies

Tantalum nitrides (TaN_x) nanocrystals with different phase and morphology have been synthesized through homogenous sodium reduction under low temperature with the subsequent annealing process under high vacuum. The crystal structures of tantalum nitrides were determined by Rietveld refinement based on the X-ray diffraction data. The morphologies of various tantalum nitrides nanocrystals in high quality were analyzed through the electron microcopies examinations. The spherical TaN nanoparticles, cuboidal TaN_0.83 and TaN_0.5 nanocrystals have been selectively prepared at different annealing temperatures. In addition, the specific surface areas of the tantalum nitrides nanocrystals measured by BET method were around 9.87–11.64 m² g^?1, indicating that such nano-sized tantalum nitrides could be suitable for capacitor with high specific capacitance.     

Co Nanoparticles Confined in 3D Nitrogen‐Doped Porous Carbon Foams as Bifunctional Electrocatalysts for Long‐Life Rechargeable Zn–Air Batteries

Proper design and simple preparation of nonnoble bifunctional electrocatalysts with high cost performance and strong durability for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly demanded but still full of enormous challenges. In this work, a spontaneous gas‐foaming strategy is presented to synthesize cobalt nanoparticles confined in 3D nitrogen‐doped porous carbon foams (CoNCF) by simply carbonizing the mixture of citric acid, NH₄Cl, and Co(NO₃)₂·6H₂O. Thanks to its particular 3D porous foam architecture, ultrahigh specific surface area (1641 m² g^?1), and homogeneous distribution of active sites (C–N, Co–N_x, and Co–O moieties), the optimized CoNCF‐1000‐80 (carbonized at 1000 °C, containing 80 mg Co(NO₃)₂·6H₂O in precursors) catalyst exhibits a remarkable bifunctional activity and long‐term durability toward both ORR and OER. Its bifunctional activity parameter (ΔE) is as low as 0.84 V, which is much smaller than that of noble metal catalyst and comparable to state‐of‐the‐art bifunctional catalysts. When worked as an air electrode catalyst in rechargeable Zn–air batteries, a high energy density (797 Wh kg^?1), a low charge/discharge voltage gap (0.75 V), and a long‐term cycle stability (over 166 h) are achieved at 10 mA cm^?2.     

The strengthening mechanism of a nickel-based alloy after laser shock processing at high temperatures

Abstract

We investigated the strengthening mechanism of laser shock processing (LSP) at high temperatures in the K417 nickel-based alloy. Using a laser-induced shock wave, residual compressive stresses and nanocrystals with a length of 30–200 nm and a thickness of 1 μm are produced on the surface of the nickel-based alloy K417. When the K417 alloy is subjected to heat treatment at 900 °C after LSP, most of the residual compressive stress relaxes while the microhardness retains good thermal stability; the nanocrystalline surface has not obviously grown after the 900 °C per 10 h heat treatment, which shows a comparatively good thermal stability. There are several reasons for the good thermal stability of the nanocrystalline surface, such as the low value of cold hardening of LSP, extreme high-density defects and the grain boundary pinning of an impure element. The results of the vibration fatigue experiments show that the fatigue strength of K417 alloy is enhanced and improved from 110 to 285 MPa after LSP. After the 900 °C per 10 h heat treatment, the fatigue strength is 225 MPa; the heat treatment has not significantly reduced the reinforcement effect. The feature of the LSP strengthening mechanism of nickel-based alloy at a high temperature is the co-working effect of the nanocrystalline surface and the residual compressive stress after thermal relaxation.     

Preparation of high tap-density LiFePO4/C composite cathode materials by carbothermal reduction method using two kinds of Fe precursors

LiFePO₄ belongs to a new generation cathode material for lithium ion batteries. The improvement of the material's tap density is considered as an important research direction. LiFePO₄/C composite was synthesized by a solid-state carbothermal reduction (CTR) method. The iron resource was obtained by the addition of (i) Fe₂O₃ and citrate ferric or (ii) single Fe₂O₃ as the Fe³⁺ precursors during synthesis. The LiFePO₄/C composite synthesized with two kinds of Fe³⁺ precursors exhibited trimodal distribution and consisted of nanometer-sized and micrometer-sized particles, whereas the LiFePO₄/C composite prepared with single Fe³⁺ precursor demonstrated unimodal distribution and was composed mainly of micrometer-sized particles. Because of the nanometer-sized particles filling in the space between the micrometer-sized particles, the composite synthesized with two kinds of Fe³⁺ precursors exhibited less vacancy than that prepared with single Fe³⁺ precursor and led to high tap density. The composite synthesized with two kinds of Fe³⁺ precursors had smaller grain size and resulted in superior discharge capacities at the rates of 0.1–1.0 C to that prepared with single Fe³⁺ precursor. The two kinds Fe³⁺ precursors method provides a simple and effective route to rapidly prepare high tap-density LiFePO₄ product with excellent electrochemical performance, so it will achieve a wide-range of applications to the production of LiFePO₄.     

Preparation of aluminum nitride granules by a two-step heat treatment method

The sintered aluminum nitride granules were prepared in a continuous process through a two-step heat treatment at 1600?°C for 5?h and 1850?°C for 5?h using the spherical γ-alumina/phenol-resin precursor obtained by spray granulation as the starting material. In the first step of heat treatment, the alumina powders were nitrided to form porous aluminum nitride granules via a carbothermal reduction reaction and then were continuously heated in the second step of heat treatment to produce dense spherical aluminum nitride granules without using sintering aids. The obtained porous and sintered aluminum nitride granules were characterized by XRD, SEM and BET measurements and exhibited an average grain size of about 1 and 3?µm, respectively. Furthermore, the specific surface area of the obtained porous and sintered aluminum nitride granules dramatically decreased from 2.49 to 0.16?m²/g showing that the low-porosity, dense, polycrystalline aluminum nitride granules were successfully prepared.     

Synthesis of beta silicon carbide powders from biomass gasification residue

The synthesis of beta silicon carbide (β-SiC) powders by carbothermal reduction of silica with carbon in a high temperature tube furnace was investigated. As carbon source, fine carbon-containing char from biomass gasification was used, in order to verify the feasibility of producing a high added value material starting from waste residue, and to promote the gasification processes as alternative route of energy production by the proper by-products exploitation. Starting mixture was prepared by mechanically mixing silica and char in the weight ratio of SiO₂:C of 1.514; the mixture then reacted in a tube furnace at temperature of 1,550 °C at different residence times (1, 1.5, 2 and 2.5 h), at a constant flow rate of 0.8 dm³ min⁻¹ of argon. The reaction products were characterised with XRD, FTIR spectroscopy and transmission electron microscopy (TEM). The process turned out to be capable of producing high quality SiC powders, suitable for making ceramic materials and composites; the product shows an uniform spherical shape, a very fine particle size (within the range of 30–100 nm) and a purity degree varying from 70% to 95%, depending on the residence time.     

Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance

Abstract

The joining of macroscopic films of vertically aligned multiwalled carbon nanotubes (CNTs) to titanium substrates is demonstrated by active vacuum brazing at 820 °C with a Ag–Cu–Ti alloy and at 880 °C with a Cu–Sn–Ti–Zr alloy. The brazing methodology was elaborated in order to enable the production of highly electrically and thermally conductive CNT/metal substrate contacts. The interfacial electrical resistances of the joints were measured to be as low as 0.35 Ω. The improved interfacial transport properties in the brazed films lead to superior electron field-emission properties when compared to the as-grown films. An emission current of 150 μA was drawn from the brazed nanotubes at an applied electric field of 0.6 V μm^?1. The improvement in electron field-emission is mainly attributed to the reduction of the contact resistance between the nanotubes and the substrate. The joints have high re-melting temperatures up to the solidus temperatures of the alloys; far greater than what is achievable with standard solders, thus expanding the application potential of CNT films to high-current and high-power applications where substantial frictional or resistive heating is expected.