Microstructure and mechanical properties of nanocrystalline high strength Al–Mg–Si (AA6061) alloy by high energy ball milling and spark plasma sintering

In the present paper, the microstructure and mechanical properties of nanostructured Al–Mg–Si based AA6061 alloy obtained by high energy ball milling and spark plasma sintering were reported. Gas atomized microcrystalline powder of AA6061 alloy was ball milled under wet condition at room temperature to obtain nanocrystalline powder with grain size of 30 nm. The nanocrystalline powder was consolidated to fully dense compacts by spark plasma sintering (SPS) at 500 °C. The grain size after SPS consolidation was found to be 85 nm. The resultant SPS compacts exhibited microhardness of 190–200 HV_100 g, compressive strength of 800 MPa and strain to fracture of 15%.     

Thin films deposition with ECR planar plasma source

The results of film deposition of pure tungsten as well as intermetallic compound of NdFeB type on various substrates using planar ECR plasma source (with multipole magnetic field) developed in our laboratory are presented. The frequency of 2.45 GHz was generated within the magnetic system by two-slot antenna. The ions of ECR argon plasma are used for target sputtering. The main plasma parameters are density 10¹⁰ cm⁻³, T_e15 eV, ions energy 20 eV, ion current density 3.5 mA/cm² at the ultimate magnetron power. Under sputtering of Nd₈Fe₈₆B₆ target the amorphous films with high adherence and thickness of 5 μm were formed on the substrate. The deposition rate of tungsten films (target biasing 900 V) was 0.59 nm/s. The fine-grained films with high adhesion were obtained. They were tested against heat loads up to 100 J/cm² produced under irradiation of coatings with plasma streams.     

Synthesis, characterization and dielectric properties of nanocrystalline strontium zirconate prepared through a modified reverse micellar route

Nanocrystalline strontium zirconate has been synthesized using a modified reverse micellar route (avoiding Sr-alkoxide). Powder X-ray diffraction studies show the monophasic nature of the powder of SrZrO₃ after heating at 800 °C. The particle size has been obtained from X-ray line broadening studies (62 nm) and transmission electron microscopic studies (72 nm). The average grain size of 85 nm was found to be reasonably stable to sintering till 1000 °C. A dielectric constant of 19.8 is observed for nanocrystalline SrZrO₃ sintered at 1000 °C. Detailed dielectric studies have been carried out with respect to frequency and temperature.     

The influence of Yb doping on the upconversion luminescence of Pr in aluminum oxide based powders prepared by combustion synthesis

The effect of ytterbium (Yb³⁺) doping on the upconversion (UC) emission of praseodymium (Pr³⁺) doped in aluminum oxide based powders prepared by combustion synthesis is reported for near-infrared excitation (λ = 980 nm). Our experimental results show that the crystalline structure and the UC emission changes with the Yb³⁺ concentration. The sample containing only Pr³⁺ (1.0 wt.%) did not show any UC signal and the UC emission profiles of the samples containing Pr³⁺ (1.0 wt.%) and Yb³⁺ (0.5, 2.0 wt.%) are quite different. The sample containing 0.5 wt.% of Yb³⁺ has five emission lines in the visible range associated to Pr³⁺ 4f–4f transitions, ³P₀ → ³H₄ (497 nm), ³P₀ → ³H₅ (525 and 550 nm), ³P₀ → ³H₆ (620 nm) and ³P₀ → ³F₂ (650 nm). We believe that the UC process has its origin in energy transfer from Yb³⁺ ions to Pr³⁺ ions in Pr_0.83Al_11.83O₁₉ phase. The sample containing 2.0 wt.% of Yb³⁺ has only one emission line in the visible range peaked at 507 nm which we believe has its origin in cooperative UC emission due to excited Yb³⁺ pairs in YbAlO₃ phase. The samples containing Yb³⁺ also present UC emission lines in the near-infrared which are assigned to intrinsic lattice defects.     

Low-temperature fabrication of 0.65 PMN–0.35 PT by a mixed oxide method

Single-phase perovskite 0.65 PMN–0.35 PT was achieved at low temperature by a conventional mixed oxide method. It was prepared by ball-milling a mixture of PbO(orthorhombic), TiO₂, Nb₂O₅ and (MgCO₃)₄Mg(OH)₂·5H₂O instead of MgO and heat treatment at 800 °C for 2 h. The formation was studied by means of DSC, FT-IR, Coupled TG-Mass, XRD, and SEM. It proceeded via formation of PbO(tetragonal) and Pb₂Nb₂O₇(P₂N) intermediates to form perovskite phase. The pure perovskite PMN-PT powder was obtained in particle size of 0.5–0.8 μm, agglomerate-free, and pseudo-cube. The powder calcined at 600 °C was sintered to 97% T.D. at 900–1000 °C for 2 h and showed room temperature dielectric constant of 3200, loss of 1–2%, and specific resistance of 5 × 10¹¹ Ω cm.     

Dielectric and electrostrictive properties of PMNT near MPB

The dielectric constant (ε_r), dielectric loss (tan δ) and strain induced by electric field in lead magnesium niobate–lead titanate (PMN-PT/PMNT) solid solutions in the morphotropic phase boundary (MPB) region at different sintering temperatures have been studied. ε_r and tan δ increase, whereas Curie phase transition range decreases with the increase in sintering temperature. Strain levels in the range of 0.07–0.2% were obtained. A high saturated strain% 0.19, a high d₃₃ coefficient 320 pm/V and a low strain hysteresis% 3.5 in PMNT 68/32 composition sintered at 1200 °C indicate its suitability for actuator applications.     

Nano-crystalline LaFeO3 powders synthesized by the citrate–gel method

A novel sol–gel process was developed for preparing nano-sized, perovskite-type LaFeO₃ powder by the thermal decomposition of the gel-complex of LaFe–(C₆H₈O₇·H₂O). The structural evolution has been systematically investigated by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Perovskite powder of  25 nm size could be obtained at a temperature of  600 °C without formation of any secondary phases of La₂O₃ and Fe₂O₃ single oxides and no requirements of high temperature/vacuum/pH control etc. Analysis of the X-ray powder diffraction data showed a decrease in the value of lattice strains with increasing decomposition temperature, whereas the particle size increases with increasing decomposition temperature.     

Influence of additives on the impact toughness of Al–10.8% Si near-eutectic cast alloys

This article investigates the effects of melt treatment and addition of alloying elements on the impact toughness of as-cast and heat-treated Al–10.8% Si near-eutectic alloys. Increasingly precise impact behaviors are discussed in the context of differentiating between initiation and propagation energies, including the ductility index, which is the ratio of the propagation to initiation energies; total energy as a useful measure is also discussed. Details concerning the evaluation of tensile properties are reported in a separate article [Mohamed AMA, Samuel FH, Samuel AM, Doty HW. Influence of additives on the microstructure and tensile properties of near-eutectic Al–10.8%Si cast alloy. Mater Des, in press]. The concentration of elements in the alloys was changed to the following range: Fe 0.5–1 wt%, Mn 0.5–1 wt%, Cu 2.25–3.25 wt%, and Mg 0.3–0.5 wt%, while the impact toughness upon artificial aging in a temperature range of 155–240 °C for 5 h was also investigated. The results indicate that the morphology of fibrous Si in Sr-modified alloys enhances toughness because of its profound effect on crack initiation and crack propagation resistance. The combined addition of modifier and grain refiner leads to a 33% increase in the impact strength compared to the untreated alloy. In alloys containing high levels of iron, such as the RF2 (1% Fe, 1% Mn) and RF4 (1% Fe, 0.5% Mn) alloys, the addition of iron leads to an increased precipitation of sludge or β-Fe platelets, respectively; these particles also act as crack initiation sites and reduce the impact properties noticeably. In alloys already containing high levels of copper, such as the RC2 (3.25% Cu, 0.3% Mg) and RC5(0.3.25% Cu, 0.5% Mg) alloys, increasing the copper level lowers the impact properties significantly, in view of the fact that the fracture behavior is now predominantly influenced by the Al₂Cu phase rather than by the Si particles. The average crack propagation speed of impact-tested samples shows a good inverse relationship to impact energy. Crack propagation speed can thus provide a qualitative estimation of the impact energy expected for special alloy conditions.     

Effect of Zr (IV) doping on the optical properties of sol–gel based nanostructured indium oxide films on glass

Zr (IV) doped indium oxide thin films (55 nm) were deposited onto pure silica glass by the sol–gel dip coating technique utilizing the precursors of 6 wt% equivalent oxide content. Three different Zr (IV) oxide (ZrO₂) dopant concentrations (5.0, 10.0 and 15.0 wt% w.r.t. total oxides) were chosen. XRD patterns suggested the films were of distinct cubic symmetry of In₂O₃. Nanostructured surface feature was revealed by FESEM images. Average cluster size decreased with increasing dopant concentration as evidenced from TEM study. Blue shift of band gap and UV cut off wavelength (lambda-50) occurred with increase in dopant concentration. The refractive index gradually increased with doping. Baking atmosphere plays an important role in tailoring the refractive index (RI) of the films and relatively high RI was obtained in the case of baking in pure oxygen. Presence of both free and bound excitons was detected by the photoluminescence (PL) study. The 5 wt% doped film exhibited relatively high PL intensity at 380 nm responsible for free exciton. The PL emissions gradually quenched with increase in dopant concentration. Similar behaviour was also observed when the film was baked in pure oxygen atmosphere.     

Mechanical and thermal properties of AlN–BN–SiC ceramics

Mechanical and thermal properties were characterized for two AlN:BN:SiC composite ceramics produced from BN with different particle sizes. The ceramics were hot pressed at temperatures from 1950 to 2100 °C to 97% relative density. For both materials, the matrix (90:10 vol% SiC:AlN) had a grain size of 0.4 μm, and the BN grains (10 vol%) were crystallographically aligned. Microhardness values were between 20 and 22 GPa, while fracture toughness values were between 2.5 and 3.1 MPa m^1/2. Other properties were found to be dependent on testing direction. Elastic moduli were between 260 and 300 GPa and strengths were 630 MPa for small particle BN additions. Thermal conductivity was calculated to be between 25 and 37 W/m K at room temperature and 17 and 25 W/m K at 900 °C. The low values compared to traditional SiC ceramics were attributed to AlN–SiC solid solution formation and sub-micron matrix grain sizes.     

Studies on optical absorption and photoluminescence of thioglycerol-stabilized ZnS nanoparticles

ZnS quantum dots of size 3 nm are prepared at 303 K using ZnSO₄ and Na₂S₂O₃ precursors with thioglycerol as stabilizing agent. Cd²⁺ doped ZnS were prepared by varying doping concentration from 1 to 8 wt.%. ZnS quantum dots were mixed with CdS quantum dots of size 4 nm in the 3:1, 2:1, 1:1, 1:2, 1:3 and 1:4 M ratio. The nanoparticles were characterized by UV–vis, photoluminescence (PL), XRD and high-resolution TEM measurements. The XRD pattern, high-resolution TEM image and SAED pattern reveal that the nanoparticles are in well-crystallized cubic phase. The band gap of ZnS has increased from the bulk value 3.7 to 4.11 eV showing quantum size effect. Excitonic transition is observed at 274 nm in UV absorption and PL emission at 411 nm. Doping with Cd²⁺ red-shifts both UV and PL spectral bands and enhances the PL band of ZnS nanoparticles. Mixing CdS and ZnS quantum dots in different molar ratios shows red-shift of the band edge in the CdS/ZnS hybrid system. In the 1:1 hybrid system of CdS/ZnS nanoparticles, PL band is red-shifted and the intensity is almost doubled with respect to that of CdS nanoparticles.     

Silver cofirable Bi1.5Zn0.92Nb1.5O6.92 microwave ceramics containing CuO-based dopants

Bi₂O₃–ZnO–Nb₂O₅ system has emerged as a good low sintering (1050 °C) microwave material because it exhibits high dielectric constant and low temperature coefficient of resonance frequency (τ_f). We have lowered the sintering temperature of Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) below 900 °C by using 3 wt.% of CuO-based dopants, such as 0.21BaCO₃–0.79CuO (BC) and 0.81MoO₃–0.19CuO (MC). The doped BZN exhibits high microwave dielectric constant at 2.3 GHz (k  120). The interfacial behavior between BZN and silver was investigated by using X-ray diffractometer, scanning electronic microscope, and electronic probe microanalyzer. The extent of silver migration of MC and BC dopants is reduced at least by one order of magnitude as compared with V₂O₅ dopant when the samples was prepared at 900 °C for 4 h. Thus, CuO-based dopants can replace V₂O₅ to lower the sintering temperature of BZN and to be cofired with silver.     

From iron(III) precursor to magnetite and vice versa

The syntheses of nanosize magnetite particles by wet-chemical oxidation of Fe²⁺ have been extensively investigated. In the present investigation the nanosize magnetite particles were synthesised without using the Fe(II) precursor. This was achieved by γ-irradiation of water-in-oil microemulsion containing only the Fe(III) precursor. The corresponding phase transformations were monitored. Microemulsions (pH  12.5) were γ-irradiated at a relatively high dose rate of 22 kGy/h. Upon 1 h of γ-irradiation the XRD pattern of the precipitate showed goethite and unidentified low-intensity peaks. Upon 6 h of γ-irradiation, reductive conditions were achieved and substoichiometric magnetite (Fe_2.71O₄) particles with insignificant amount of goethite particles found in the precipitate. Hydrated electrons , organic radicals and hydrogen gas as radiolytic products were responsible for the reductive dissolution of iron oxide in the microemulsion and the reduction Fe³⁺ → Fe²⁺. Upon 18 h of γ-irradiation the precipitate exhibited dual behaviour, it was a more oxidised product than the precipitate obtained after 6 h of γ-irradiation, but it contained magnetite particles in a more reduced form (Fe_2.93O₄). It was presumed that the reduction and oxidation processes existed as concurrent competitive processes in the microemulsion. After 18 h of γ-irradiation the pH of the medium shifted from the alkaline to the acidic range. The high dose rate of 22 kGy/h was directly responsible for this shift to the acidic range. At a slightly acidic pH a further reduction of Fe³⁺ → Fe²⁺ resulted in the formation of more stoichiometric magnetite particles, whereas the oxidation conditions in the acidic medium permitted the oxidation Fe²⁺ → Fe³⁺. The Fe³⁺ was much less soluble in the acidic medium and it hydrolysed and recrystallised as goethite. The γ-irradiation of the microemulsion for 25 h at a lower dose rate of 16 kGy/h produced pure substoichiometric nanosize magnetite particles of about 25 nm in size and with the stoichiometry of Fe_2.83O₄.     

Superplastic flow in a nanostructured aluminum alloy produced using high-pressure torsion

Samples of a spray-cast Al-7034 alloy were processed by high-pressure torsion (HPT) at temperatures of 293 or 473 K using an imposed pressure of 4 GPa and torsional straining through five revolutions. Processing by HPT produced significant grain refinement with grain sizes of 60 and 85 nm at the edges of the disks for the two processing temperatures. In tensile testing at room temperature, the alloy processed by HPT exhibited higher strength and lower ductility than the unprocessed material. Good superplastic properties were achieved in tensile testing at elevated temperatures with a maximum elongation of 750% for the sample processed at 473 K and tested in tension at 703 K under an initial strain rate of 1.0 × 10⁻² s⁻¹. The measured superplastic elongations are lower than in samples prepared by equal-channel angular pressing because of the use of very thin disks in the HPT processing.     

Visualization of emitting sites on carbon fiber cathode by surface treatment in high-current vacuum discharge process

In this paper, a carbon fiber cathode, having robust, easily shaped, and epoxy-free properties, is constructed by squeeze casting technique that can overcome some disadvantages of conventional methods. Carbon fiber emitters on the cathode surface had a high distribution density, thus ensuring sufficient emission centers or emission uniformity. The fabricated cathode was tested in a diode powered by a 350 kV, 40 Ω, 400 ns high-voltage pulse generator. The turn-on electric field was estimated to be 50 kV/cm, and the field enhancement factor was (1.2–2.0) × 10³. It was found that the electron emission of carbon fiber cathode is initiated from the individual bright spots at a current density of up to 400 A/cm². Most notably, the X-ray images of electron beam on anode foil demonstrate the development of bright spots on the cathode surface. As a whole, this class of cathodes can endure high-current pulsed emission, and has a positive application prospect.     

Effect of environment on fatigue crack growth in ultrafine grain Al–Mg

The fatigue crack growth rates, obtained in high vacuum and in ambient air, of ultrafine grain (UFG) Al–7.5Mg (grain size  250 nm) at various load ratios were compared to those of powder-metallurgy (P/M) Al–7Mg (grain size  2 μm) and ingot-metallurgy (I/M) Al–7Mg (grain size  100 μm). In both vacuum and ambient air, fatigue crack growth rates at all stress ratios decrease with increasing grain size. The fatigue crack growth threshold (ΔK_th) follows the reverse order, increasing with increasing grain size. These trends are interpreted in terms of fracture surface roughness effects that are correlated with grain size. In vacuum, the thresholds of all three materials exhibit no load ratio dependency at load ratios from 0.1 to 0.5. In air, the threshold of UFG Al–7.5Mg exhibits weak load ratio dependency, while P/M and I/M Al–7Mg exhibit modest load ratio dependency. The environmental effect on the fatigue crack growth rates is assessed by determining the difference in crack growth driving force (ΔK) between air and vacuum. It was found that the environmental contribution to the driving force of all three materials is similar, nearly independent of grain size.     

Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 °C, cooled at rates of 0.3, 1.2 and 3.0 °C s⁻¹ to temperatures in the range 750–1050 °C, and then strained to failure at initial strain rates of 1 × 10⁻⁴ or 1 × 10⁻³ s⁻¹. It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 °C s⁻¹ and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from 900 °C (near the Ae₃ temperature) to 1000 or 1050 °C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.     

Dynamic recrystallization during high temperature deformation of magnesium

As a consequence of the high critical stresses required for the activation of non-basal slip systems, dynamic recrystallization plays a vital role in the deformation of magnesium, particularly at a deformation temperature of 200 °C, where a transition from brittle to ductile behavior is observed. Uniaxial compression tests were performed on an extruded commercial magnesium alloy AZ31 at different temperatures and strain rates to examine the influence of deformation conditions on the dynamic recrystallization (DRX) behavior and texture evolution. Furthermore, the role of the starting texture in the development of the final DRX grain size was investigated. The recrystallized grain size, measured at large strains (  −1.4) seemed to be more dependent on the deformation conditions than on the starting texture. In contrast to pure magnesium, AZ31 does not undergo grain growth at elevated deformation temperatures, i.e. 400 °C, even at a low strain rate of 10⁻⁴ s⁻¹. Certain deformation conditions gave rise to a desired fully recrystallized microstructure with an average grain size of 18 μm and an almost random crystallographic texture. For samples deformed at 200 °C/10⁻² s⁻¹, optical microscopy revealed DRX inside of deformation twins, which was further investigated by EBSD.     

The effect of Li on the tensile properties of cast Al–Mg2Si metal matrix composite

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in situ prepared Al–15%Mg₂Si composite were investigated. It was found that the addition of 0.3%Li reduces the average size of Mg₂Si primary particles from 30 to 6 μm. The effect of cooling rate was investigated by the use of a mold with different section test bars. The results showed an increase in both UTS and elongation values with reduction in section thicknesses corresponding to increasing cooling rates. Adding Li also raised the tensile strength and elongation values and reduced the number of decohered particles observed in fracture surfaces thereby increasing the alloy's ductility. Data scatter and unexpected low tensile values of 3 mm sections were attributed to casting defects observed in fracture surfaces. Large clusters of Mg₂Si particles and eutectic cell boundaries were found to be potential crack propagation paths in this alloy.     

Reformation of phase composition in decomposed aluminium titanate

The ability of decomposed Al₂TiO₅ to undergo reformation in phase composition has been characterised by neutron diffraction at elevated temperature and verified by differential thermal analysis. It is shown that the process of phase decomposition in metastable Al₂TiO₅ is reversible and that reformation occurs readily when decomposed Al₂TiO₅ is re-heated above 1300 °C. Subsequent decomposition of reformed Al₂TiO₅ during cooling below 1200 °C is governed by the temperature-dependent atomic diffusion rates.