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  • Powder Technology 11/2014; 266:7–15.
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    ABSTRACT: The high-temperature mechanical properties of precipitate-strengthened advanced alloys can be heavily influenced by adjusting chemical composition. The widely-accepted argument within the community is that, under certain temperature and loading conditions, plasticity occurs only in the matrix, and dislocations have to rely on thermally-activated climb mechanisms to overcome the barriers to glide posed by the hard precipitates. This is the case for γ′γ′-strengthened Ni-based superalloys. The presence of dilute amounts of slow-diffusing solute atoms, such as Re and W, in the softer matrix phase is thought to reduce plasticity by retarding the climb of dislocations at the interface with the hard precipitate phase. One hypothesis is that the presence of these solutes must hinder the flow of vacancies, which are essential to the climb process. In this work, density functional theory calculations are used to inform two analytical models to describe the effect of solute atoms on the diffusion of vacancies. Results suggest that slow-diffusing solute atoms are not effective at reducing the diffusion of vacancies in these systems.
    Materials Science and Engineering A 11/2014;
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    ABSTRACT: Co–Cr–Mo alloys are one of the most widely used metallic biomaterials for metal-on-metal joint prostheses. However, concerns over increased revision rates mainly due to nano-sized wear debris have been raised. This study was aimed at enhancing the friction, wear and load-bearing properties of Co–Cr–Mo alloys by developing a new duplex surface system combining super hard and wear-resistant S-phase layer with self-lubricating, low-friction carbon-based coating. To this end, ASTM F1537 Co–Cr–Mo alloy surface was plasma carburised (PC) at 450 °C and then coated with a carbon-based GiC coating. The microstructures of the duplex and single treated (PC or GiC coating) Co–Cr–Mo surface systems were characterized and their mechanical, tribological and corrosion properties were evaluated. The results showed that the new duplex surface system exhibited a high load bearing capacity, a low friction coefficient, excellent wear resistance and good corrosion behaviour.
    Applied Surface Science 02/2014; 292:336–344.
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    ABSTRACT: Pd–Cu alloys are of potential interest for use as hydrogen purification membranes, but have relatively low permeability compared to the commercially used alloys such as Pd–Ag. In this work, the effects of partial Ag substitution on the hydrogen diffusivity, solubility and the permeability of Pd–Cu membranes with a bcc structure have been investigated. With the addition of 2.3 and 3.9 at% Ag to Pd–Cu, lattice expansions of 0.11% and 0.35% were observed. Structural analyses by in-situ XRD showed that the bcc structure of the 2.3 at% Ag alloy is retained upon heating to 600 °C, whereas an fcc phase forms in the 3.9 at% Ag alloy resulting in a mixed (bcc+fcc) structure. Whilst the diffusion coefficients between 350 and 400 °C for both Pd–Cu–Ag ternary samples were shown to be lower than their binary alloys (which had similar structures), higher solubility values were obtained. The lower diffusion coefficients of the ternary alloys are related to an increase in the diffusion activation barrier in the presence of Ag, and the higher solubility values may be attributed to the lattice expansion and high Ag–H chemical interaction. Hydrogen permeation measurements showed that an enhancement in the hydrogen solubility of the bcc phase Pd45.8Cu51.9Ag2.3, does not have a substantial effect on the permeability of the membrane. In contrast, for the Pd45.1Cu51Ag3.9 sample with a mixed (bcc+fcc) phase, higher hydrogen solubility can lead to a remarkable improvement in permeability. Hence, it is suggested that the hydrogen permeability in the bcc phase is mainly controlled by hydrogen diffusion, and the solubility enhancement can only significantly improve the hydrogen permeability when the fcc phase is present.
    Journal of Membrane Science 02/2014; 451:216–225.
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    ABSTRACT: Poly (ethylene terephthalate), PET, crystallized isothermally at 234 °C has been annealed above the crystallization temperature in regimes II, below the onset of melting, and in regime III, in the range of observable melting [1] and [2]. The mechanism has been analyzed by differential scanning calorimetry and FTIR spectroscopy and shown to be one of the limited recrystallization nucleated by crystalline regions which survived melting followed by a process of lamellae thickening.Annealing above the crystallization temperature in regime II where melting and recrystallization did not occur involved lamellae thickening by growth of the lamellae along the chain direction. This was a diffusion controlled mechanism involving either diffusion of non-crystallizable impurities or chain entanglements. The process of stem thickening was the same as that of secondary crystallization previously observed in isothermal crystallization [3] and which is responsible for the observed melting point of the polymer being dependent on the annealing time as well as the temperature.In regime III melting of the polymer occurred initially followed by recrystallization on the residual lamellae acting as seeds by one dimensional growth along the original growth direction. This was later accompanied by thickening of the lamellae with the same characteristic time dependence of diffusion control as observed in regime II and with secondary crystallization. This involves reptation of the molecular chain from the entangled melt onto the growth face.
    European Polymer Journal 01/2014; 50:235–242.
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    ABSTRACT: The hydrogenation disproportionation desorption recombination (HDDR) process has been investigated as a possible means of producing bonded magnets from used NdFeB-type sintered magnets with compositions, Nd13.4Dy0.8Al0.7Nb0.3Fe78.5B6.3 and Nd12.5Dy1.8Al0.9Nb0.6Co5.0Fe72.8B6.4 (atomic%). It has been shown that by increasing the processing temperature, an increase in the equilibrium pressure for disproportionation and in the overall reaction time was observed. The magnetic properties of the lower Dy content magnet were affected significantly by the change in processing temperature with a peak in properties observed at 880 °C producing magnetic powder with a remanence of 1.08 (±0.02) T, a coercivity of 840 (±17) kA m-1, and a maximum energy product of 175 (±2.5) kJ m-3. Further work on magnets with a significantly higher Dy content has shown that simultaneous processing of sintered magnets with varying compositions can be achieved by increasing the hydrogen pressure, however a range of magnetic properties are produced depending on the initial compositions of the samples in the input feed.
    Journal of Magnetism and Magnetic Materials 01/2014;
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    ABSTRACT: Al–Mn–(Mg) particles in the size range of 20–200 nm have been found in a sand cast AZ91 magnesium alloy. Their morphologies, chemical composition and structures were investigated using transmission electron microscopy. It was found that these particles have a chemical composition of (Al6.52Mn)1−xMgx (x < 0.13) and a decagonal quasi-crystalline structure. These particles were stable during a solution treatment at 420 °C for 72 h and acted as preferential nucleation sites for Mg17Al12 precipitates during the subsequent ageing. The potential effect of these Al–Mn–(Mg) particles on the strengthening of AZ91 magnesium alloys is discussed.
    Journal of Alloys and Compounds 12/2013; 579:34–38.
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    ABSTRACT: Cobalt‐chromium (Co‐Cr) alloys are one of the best materials for biomedical applications owing to their fairly good wear resistance, high mechanical properties, adequate corrosion resistance and acceptable bio-compatibility. However, recently, there are concerns over the wear behaviour of metal-on-metal artificial hip joints and the formation of numerous nano-sized wear debris from the articulating Co‐Cr surfaces. This is mainly because such extremely fine wear debris could release chromium and cobalt ions into the host body, which can cause potential toxicity. Therefore, how to improve the wear resistance of Co‐Cr alloys is a scientifically interesting and clinically important research topic. This study aimed to improve the hardness, wear, corrosion and corrosion–wear properties of medical grade Co‐Cr by plasma surface alloying with nitrogen and with both nitrogen and carbon at very low temperatures (between 300 and 400 °C). The metallurgy, mechanical, chemical and wear properties of the surface alloyed layers were characterised and the results demonstrate that the optimised treatment conditions have produced very promising surface layers on Co‐Cr alloy for biomedical applications.
    Surface and Coatings Technology 10/2013; 232:906–911.
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    ABSTRACT: Montmorillonite nanoclays (PGV and PGN) were dispersed in poly(acrylic acid) (PAA) for utilization as reinforcing filler in glass ionomer cements (GICs). Chemical and physical interaction of PAA and nanoclay (PGV and PGN) was studied. PAA-PGV and PAA-PGN solutions were prepared in different weight percent loadings of PGV and PGN nanoclay (0.5-8.0 wt%) via exfoliation-adsorption method. Characterization was carried out by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and fourier transform infrared (FTIR) spectroscopy. XRD results of PAA-PGN demonstrated that the interlayer space expanded from 12.83 to 16.03 Å indicating intercalation whereas the absence of the peak at d 001 in PAA-PGV indicated exfoliation. XPS scans of PGV and PGN nanoclays depicted the main peak of O 1s photoelectron due to Si-O-M (M = Mg, Al, Fe) whereas, Si-O-Al linkages were identified by Si 2p or Si 2s and Al 2p or Al 2s peaks. The disappearance of the Na peak confirmed that PAA molecules exchanged sodium ions present on surface of silicate layers and significantly reduced the electrostatic van-der-Waals forces between silicate plates resulting in intercalation or exfoliation. FTIR spectra of PAA-nanoclay suspensions demonstrated the presence of a new peak at 1,019 cm(-1) associated with Si-O- stretching vibrations which increased with increasing nanoclays concentration. Information concerning the dispersion of nanoclay in PAA aqueous solutions, chemical reaction and increase interlayer space in montmorillonite nanoclay is particularly useful regarding dispersion and reinforcement of nanoclay in PAA.
    Journal of Materials Science Materials in Medicine 09/2013;
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    ABSTRACT: Export Date: 23 August 2013, Source: Scopus, CODEN: MCHPD, :doi 10.1016/j.matchemphys.2013.05.010, Language of Original Document: English, Correspondence Address: Attallah, M.M.; School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom; email: M.M.Attallah@Bham.ac.uk, References: Frenzel, J., George, E.P., Dlouhy, A., Somsen, C., Wagner, M.F.-X., Eggeler, G., (2010) Acta Mater., 58, pp. 3444-3458;
    Materials Chemistry and Physics 08/2013; 141(1):272-277.
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