Kinetic modelling of binder burnout for optimisation of slurry–powder manufacturing process of Ti/SiC composites

Abstract

A slurry–powder metallurgy method is currently under development as a low cost production route for Ti/SiC composites. One of the critical steps in the process is the complete removal of a fugitive binder used to form the powder slurry. A diffusion-controlled, shrinking-core model has been developed for the prediction of the binder burnout kinetics in a metal powder compact with relatively large particles. This model is suitable for a polymeric binder that decomposes mainly to a monomer. The model considers the degradation of the polymer and the diffusion of the monomer in the core that contains a powder structure filled with a liquid polymer–monomer solution. It is found that the dominant mechanism is the liquid diffusion of monomer and the burnout time increases significantly with the compact size. The model also predicts a longer burnout time for a larger particle volume fraction.     

Low temperature and cost-effective growth of vertically aligned carbon nanofibers using spin-coated polymer-stabilized palladium nanocatalysts

Abstract

We describe a fast and cost-effective process for the growth of carbon nanofibers (CNFs) at a temperature compatible with complementary metal oxide semiconductor technology, using highly stable polymer–Pd nanohybrid colloidal solutions of palladium catalyst nanoparticles (NPs). Two polymer–Pd nanohybrids, namely poly(lauryl methacrylate)-block-poly((2-acetoacetoxy)ethyl methacrylate)/Pd (LauMA_x-b-AEMA_y/Pd) and polyvinylpyrrolidone/Pd were prepared in organic solvents and spin-coated onto silicon substrates. Subsequently, vertically aligned CNFs were grown on these NPs by plasma enhanced chemical vapor deposition at different temperatures. The electrical properties of the grown CNFs were evaluated using an electrochemical method, commonly used for the characterization of supercapacitors. The results show that the polymer–Pd nanohybrid solutions offer the optimum size range of palladium catalyst NPs enabling the growth of CNFs at temperatures as low as 350 °C. Furthermore, the CNFs grown at such a low temperature are vertically aligned similar to the CNFs grown at 550 °C. Finally the capacitive behavior of these CNFs was similar to that of the CNFs grown at high temperature assuring the same electrical properties thus enabling their usage in different applications such as on-chip capacitors, interconnects, thermal heat sink and energy storage solutions.     

Influence of isothermal solidification on microstructural development in Ni–Si–B filler metals

Abstract

An examination is reported of the microstructure of joints between nickel substrates fabricated using Ni–Si–B filler metal (AWS classification BNi-3). This investigation was undertaken because the presence of residual boride and silicide phases after joining can affect the subsequent mechanical properties of the joint. The microstructures developed in the joints are compared with those formed in similarly heat treated filler metal. Various models of the transient liquid phase process are also considered and predicted times to complete isothermal solidification are compared with experimental values. An initial examination of the influence on filler metal microstructure of a ternary addition (chromium) to the substrate is included.

MST/1506     

Multistate resistive switching in silver nanoparticle films

Abstract

Resistive switching devices have garnered significant consideration for their potential use in nanoelectronics and non-volatile memory applications. Here we investigate the nonlinear current–voltage behavior and resistive switching properties of composite nanoparticle films comprising a large collective of metal–insulator–metal junctions. Silver nanoparticles prepared via the polyol process and coated with an insulating polymer layer of tetraethylene glycol were deposited onto silicon oxide substrates. Activation required a forming step achieved through application of a bias voltage. Once activated, the nanoparticle films exhibited controllable resistive switching between multiple discrete low resistance states that depended on operational parameters including the applied bias voltage, temperature and sweep frequency. The films’ resistance switching behavior is shown here to be the result of nanofilament formation due to formative electromigration effects. Because of their tunable and distinct resistance states, scalability and ease of fabrication, nanoparticle films have a potential place in memory technology as resistive random access memory cells.     

Manual metal arc weld modelling: Part 2 Treatment of multipass weld

Abstract

The premature failure of parts designed to operate at high temperatures often occurs in low-ductility microstructures in a weld or its heat-affected zone (HAZ). Clearly, a knowledge of how the welding process variables determine these microstructures is essential to the designer of welding procedures. In an accompanying paper relationships are established which describe how the dimensions of single manual metal arc weld beads and their associated HAZs are related to the welding process parameters. Here it is shown how these relationships can be used to optimize the metallurgical properties of multipass welds by controlling the process parameters. The factors given particular attention are dilution of the weld metal by the parent material, refinement of the HAZ using controlled deposition, and refinement of the structure of the weld metal. Data for BOC Murex Suprex B welding electrodes and a mild steel substrate are used throughout for illustration, but the methods presented are applicable to other combinations of materials provided the correct material constants are used. The present work was prompted by a need to improve metal deposition rates during manual metal arc repair of foundry defects in large Cr–Mo–V castings. The practical implementation of the results is also considered.

MST/193b     

Wetting of nickel alloys by nickel based brazes

Abstract

Sessile drop spreading tests have been carried out to evaluate the influence of braze and substrate chromium content on the wetting and flow of Ni–Cr–Si–B and Ni–Cr–P brazes on nickel and Ni–Cr substrates. The effect of filler metal chromium additions is examined in terms of fluidity processes, while the role of base metal chromium content is discussed in the context of oxide reduction and alloying phenomena.

MST/1056     

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

Abstract

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg₂Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg₂Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.

MST/871     

Development of electrosprayed artesunate-loaded core–shell nanoparticles

Objective: Artesunate (ART) is proven to have potential anti-proliferative activities, but its instability and poor aqueous solubility limit its application as an anti-cancer drug. The present study was undertaken to develop coaxial electrospraying as a novel technique for fabricating nanoscale drug delivery systems of ART as the core–shell nanostructures.

Methods: The core–shell nanoparticles (NPs) were fabricated with coaxial electrospraying and the formation mechanisms of NPs were examined. The physical solid state and drug–polymer interactions of NPs were characterized by X-ray powder diffraction (XRPD) and Fourier transform infrared (FTIR) spectroscopy. The effects of materials and electrospraying process on the particle size and surface morphology of NPs were investigated by scanning electron microscopy (SEM). The drug release from NPs was determined in vitro by a dialysis method.

Results: The ART/poly(lactic-co-glycolic) acid (PLGA) chitosan (CS) NPs exhibited the mean particle size of 303?±?93?nm and relatively high entrapment efficiency (80.5%). The release pattern showed an initial rapid release within two hours followed by very slow extended release. The release pattern approached the Korsmeyer–Peppas model.

Conclusions: The present results suggest that the core–shell NPs containing PLGA and CS have a potential as carriers in the anticancer drug therapy of ART.     

Internal oxidation

Abstract

In the classical models of internal oxidation of binary alloys comprising a more noble solvent metal and a less noble solute metal, it has been assumed that the process is controlled by diffusion of oxygen through the bulk metal lattice. However, the oxidation kinetics observed have not always been consistent with this hypothesis. In this paper, modifications to the classical theories are considered and, in particular, the importance of the morphology of the internal oxide precipitates is discussed. Internal oxidation of the Ni–Al, Ni–V, and Ni–Cr systems in the absence of an external scale is used to demonstrate that, depending on the population density of particles, enhanced diffusion of oxygen along the incoherent internal oxide/alloy interfaces and blocking effects by the particles themselves can influence considerably the rates of penetration of internal oxide. Consideration is also given to the influence of stresses generated at the internal oxide/alloy interface as the oxide particles form with an increase in volume. Under certain conditions, the rate determining step may be deformation of the matrix to accommodate this volume change, rather than oxygen diffusion through the internal oxidation zone. It has been suggested in recent research that transport of .the more noble solvent metal to the strain free surface may occur by a pipe diffusion controlled creep process and this step may become rate controlling at high temperatures and high strain rates.

MST/951     

Preparation of Ti–Ni–Fe phase by levitation and its structural characterisation

Abstract

This paper presents results concerning the electromagnetic levitation of solid metal samples. It describes the levitation system and some coils designed to melt 10 g metal samples under controlled atmosphere, as well as melting the process. The main advantage of the method is the absence of any type of crucible and owing to this, the effect of side reaction can be avoided, resulting from the influence of the active metal on the walls of the ceramic crucible in the physicochemical investigations. In many cases, the side reaction makes the correct interpretation of the experimental results sometimes difficult and/or impossible. In order to describe the crystallisation process and the change in the microstructure of the samples obtained during electromagnetic melting at the intermetallic Fe–Ni–Ti phase, a scanning electron microscopy analysis and an X-ray diffraction analysis were used.     

Repair and rejuvenation procedures for aero gas-turbine hot-section components

Abstract

The general procedures for repairing aero gas-turbine hot-section components are described. A number of examples are given in order to illustrate particular repair processes: (i) the welding of nozzle guide vanes made from high–strength, nickelbase alloy, demonstrating the effect of microstructural refinement and welding techniques; (ii) the brazing of similar nozzle guide vanes, with emphasis on the advantages of using a halide cleaning process and of modifying the braze alloy by adding a powder having a composition close to that of the parent metal; (iii) the use of an improved coating system to upgrade components; (iv) the possibility of using MCr AlY coatings applied by low–pressure plasma spraying; and (v) hot isostatic pressing as a method of restoring creep life.

MST/103     

Effect of pressure on electrical resistivity of Mg70Zn30 metallic glass

Abstract

The variation of the electrical resistivity of amorphous simple metal alloy Mg₇₀Zn₃₀ as a function of non-hydrostatic pressure has been studied. Measurements have been made in the range 0–10 kbar. The resistivity is found to increase with increasing pressure.

MST/982     

Theoretical analysis of influence of crack dimensions and geometry on mass transport in corrosion–fatigue cracks

Abstract

Theoretical models are developed to describe the concentration of dissolved species in a corrosion–fatigue crack. The models are based on two separate reaction processes: the cathodic reduction of oxygen on the crack walls, and the anodic dissolution of metal at the crack tip. The concentrations of species in a trapezoidal crack are calculated for a range of crack depths from 0·25 to 20 mm, and for different minimum/maximum load ratios, stress–intensity factors, and frequencies, assuming single–edge–notch geometry. The relative importance of diffusion and fluid flow as functions of these parameters is analysed. The influence the geometry of the specimen has on the composition of the solution is determined, and the use of parallel–sided cracks is discussed. The implications of the conclusions for corrosion–fatigue crack growth are assessed.

MST/73     

Ceramic/metal joining for structural applications

Abstract

Technological advances are extending the applications for bonded ceramic-metal components and demanding more rigorous performance characteristics. The techniques available for fabricating high–integrity joints for structural applications have been reviewed and attention is drawn to the factors that effect both direct and indirect bonding using liquid– and solid–phase materials. Two processes still largely in the developmental stage – fusion welding and diffusion bonding – have been considered, as well as the more established processes such as brazing. It is suggested that active metal brazing and indirect diffusion bonding will be among the techniques to be further developed and used more extensively in future.

MST/205     

Physicochemical characterization of spray-dried PLGA/PEG microspheres,and preliminary assessment of biological response

Context: The use of spray-drying to prepare blended PLGA:PEG microspheres with lower immune detection.

Objective: To study physical properties, polymer miscibility and alveolar macrophage response for blended PLGA:PEG microspheres prepared by a laboratory-scale spray-drying process.

Methods: Microspheres were prepared by spray-drying 0–20% w/w ratios of PLGA 65:35 and PEG 3350 in dichloromethane. Particle size and morphology was studied using scanning electron microscopy. Polymer miscibility and residual solvent levels evaluated by thermal analysis (differential scanning calorimetry – DSC and thermogravimetric analysis – TGA). Immunogenicity was assessed in vitro by response of rat alveolar macrophages (NR8383) by the MTT-based cell viability assay and reactive oxygen species (ROS) detection.

Results: The spray dried particles were spherical, with a size range of about 2–3?µm and a yield of 16–60%. Highest yield was obtained at 1% PEG concentration. Thermal analysis showed a melting peak at 59?°C (enthalpy: 170.61 J/g) and a degradation-onset of 180?°C for PEG 3350. PLGA 65:35 was amorphous, with a T_g of 43?°C. Blended PLGA:PEG microspheres showed a delayed degradation-onset of 280?°C, and PEG enthalpy-loss corresponding to 15% miscibility of PEG in PLGA. NR8383 viability studies and ROS detection upon exposure to these cells suggested that blended PLGA:PEG microspheres containing 1 and 5% PEG are optimal in controling cell proliferation and activation.

Conclusion: This research establishes the feasibility of using a spray-drying process to prepare spherical particles (2–3?µm) of molecularly-blended PLGA 65:35 and PEG 3350. A PEG concentration of 1–5% was optimal to maximize process yield, with minimal potential for immune detection.     

Physical vapour deposition coating processes for encapsulation of powder metal components before hot isostatic pressing

Abstract

Application of coatings by plasma vapour deposition involving electron beam evaporation and ion plating onto green powder metal compacts has been studied as a potential method for encapsulating powder metal products before hot isostatic pressing. The deposition of defect free coatings is essential if this concept is to provide a reliable encapsulation technique. Coating structures are therefore discussed in terms of the plasma processing conditions and surface roughness of the powder substrate. It is shown that the most promising approach is a combined coating and sinter–hot isostatic pressing cycle, which enables defects within the coating to be removed by the formation of a transient liquid phase.

MST/1455     

Effect of aging on fracture behaviour of cast stainless steel and weldments

Abstract

An investigation has been undertaken to determine the magnitude of any reduction in properties that may occur in cast duplex stainless steels and weldments during long term exposure to reactor operating conditions. Test panels were fabricated in CF3 stainless steel using a manual metal arc (MMA) process and 19.9.L consumables. The mechanical properties of the parent material and weldments were measured following accelerated aging at 375 and 400°C for up to 20 000 h. Following aging at temperatures up to 400°C, reductions in both the Charpy impact and J integral–crack growth resistance R (J–R) fracture toughness of CF3 cast austenitic steel and 19.9.L austenitic weld metal were observed. For conditions equivalent to the proposed end of life for UK pressurised water reactors, the J–R fracture toughness at 300°C of both cast steel and MMA weld metal was reduced by ~30% for crack extensions of ≥1 mm. Hence, it is important that these reductions in weld metal toughness are taken into account during the development of safety cases and structural integrity assessments for any component in the primary loop that contains MMA stainless steel weldments.

MST/1198     

Josefsson

Abstract

An investigation of 2·25Cr–1Mo–0·1C weld metal has been carried out using atom probe field ion microscopy. The weld metal had a microstructure consisting of bainitic ferrite and retained austenite, but no cementite. The carbon concentration at the austenite/ferrite interface was found to change abruptly, whereas the concentration of substitutional alloying elements was the same in both phases. No enrichment could be found at the interface.

MST/1444     

In situ polymerization monitoring of a diacrylate in an electrically conducting mesoporous nanoparticle scaffold

The properties of nanoparticle–polymer composites strongly depend on the network structure of the polymer matrix. By introducing nanoparticles into a monomer (solution) and subsequently polymerizing it, the formation of the polymer phase influences the mechanical and physicochemical properties of the composite. In this study, semi-conducting indium tin oxide (ITO) nanoparticles were prepared to form a rigid nanoparticle scaffold in which 1,6-hexanediol diacrylate (HDDA), together with an initiator for photo-polymerization, was infiltrated and subsequently polymerized by UV light. During this process, the polymerization reaction was characterized using rapid scan Kubelka–Munk FT-IR spectroscopy and compared to bulk HDDA. The conductivity change of the ITO nanoparticles was monitored and correlated with the polymerization process. It was revealed that the reaction rates of the radical initiation and chain propagation are reduced when cured inside the voids of the nanoparticle scaffold. The degree of conversion is lower for HDDA infiltrated into the mesoporous ITO nanoparticle scaffold compared to purely bulk-polymerized HDDA.

Graphical abstract

     

Rate equation for recovery,precipitation, transient creep,and sintering

Abstract

The empirical rate equations of solid state reactions such as recovery of deformed metals, clustering, and transient creep are all of the same type, referred to here as recovery kinetics. For two systems, transient creep in fcc metal crystals and the first stage of tempering of Fe–C and Fe–Ni–C martensite, the rate equations were analysed 30 years ago using the increment technique. These results can be used to deduce the common physical principles which govern the mechanisms operating in the various systems. From this basis, it can be concluded that the densification kinetics in the intermediate stage sintering of certain ceramic green pellets is also a type of recovery kinetics. Initial experimental results are shown to support this conclusion.

MST/1048