The influence of ageing on the stability of β-γ/γ′ derived microstructures in Ni-Al-Cr-(Co) alloys

Multiphase Ni-Al-(Fe)-(Cr)-(Co)-based intermetallics with (B2)- (A1)/(L1₂), - or - microstructures can exhibit significant room-temperature tensile ductility. In the case of Ni-Al-Cr-based alloys, microstructural development is complicated by the precipitation of -Cr, which can supplant the -phase during ageing of three-phase -/ microstructures. An investigation of the stability, during ageing, of cast Ni-Al-Cr-(Co) alloys with microstructures derived from -/ is reported. In the as-cast condition, the materials investigated consisted of a dendritic matrix containing L1₀ type martensite and interdendritic /. Extensive intra- and interdendritic -Cr precipitation was also observed. The stability during ageing of the interdendritic / microstructure is also considered and transformation of the L1₀ martensite is examined.     

Chromium substitution and crystallinity changes in γ-FeOOH

Cr-substitution in-FeOOH takes place with changes in the unit cell parameters and a marked decrease in crystallinity. The-FeOOH structure is maintained up to 5.0 atomic % with a progressive diminution of crystallite size, as shown by X-ray line broadening analysis and electron microscopy. The dehydroxilation of the Cr-substituted oxyhydroxide to-sesquioxides and the phase transition sesquioxide take place at lower temperatures than in-FeOOH, due to the lower crystallinity.     

γ ? α2 Phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%)

The ₂ phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%) alloy has been studied by analytical electron microscopy. ₂ and phases were found at grain boundaries. ₂ layers that suspended in layers and interfacial ledge higher than 2d ₍₁₁₁₎ at /₂ interfaces were observed in the lamellar grains. These facts indicated that ₂ phase transformation and dynamic recrystallization have occurred during high temperature stress rupture deformation. It can be concluded that deformation induced ₂ phase transformation and dynamic recrystallization resulted in the presence of particles at grain boundaries. A structural and compositional transition area between deformation-induced ₂(or ) and its adjacently original (or ₂) phases was found by HREM and EDS and is suggested as a way to transform between and ₂ phase during high temperature stress rupture deformation. The transition area was formed by slide of partial dislocations on close-packed planes and diffusion of atoms.     

Performance improvement of carbon fiber/polyethylene fiber hybrid composites

Carbon fiber/polyethylene (PE) fiber hybrid composites were fabricated by open leaky mold method. The positional effect of PE fiber was investigated concerning the mechanical performance improvement of carbon fiber/PE fiber hybrid composites. The influence of adhesion level of PE fiber on the hybrid properties was studied using oxygen plasma, -MPS, and -MPS-modified polybutadiene (PB/-MPS) as surface modifiers. In case of carbon fiber/vinylester composite, -MPS and PB/-MPS acted as the bridge to bond carbon fiber to vinylester resin through the chemical bonding. In case of PE fiber/vinylester composite, plasma treatment of PE fiber introduced the etching and micro-pitting rather than the functional group on fiber surface. Therefore, the plasma treated composite exhibited a large increase in flexural strength compared with untreated composite. In the case of carbon fiber/PE fiber hybrid composites, the mechanical properties of hybrid composite strongly depended on the reinforcing fiber position. When carbon fiber was at the outermost layer, the hybrid composite exhibited the highest flexural strength among other hybrid composites. This was attributed to the fact that the compressive and tensile stress had maximum magnitudes in the outermost layer. The surface treatment of PE fiber at outermost layer had a significant effect on the flexural strength of hybrid composite.     

Study of the reaction of γ – methacryloxypropyltrimethoxysilane (γ – MPS) with slate surfaces

Thermal Analysis (TA) and Fourier Transform Infrared Spectroscopy (FT–IR); were used in order to study the reaction of -methacryloxypropyltrimethoxysilane (-MPS) with slate surfaces at different temperature conditions (25, 35 and 50°C), concentrations (0.5–5% weight of silane) and reaction times (1, 3, 5 and 10 min). By these conditions, a typical physical adsorption of the -MPS silane on the slate surface has been observed. A monolayer of -MPS is firstly adsorbed on the slate surface by hydrogen bonding through the carbonyl group of the silane and after that, silane multilayers are formed where free carbonyl groups are predominant. A concentration of 1% is considered as appropriate for silanization of the slate particles with -MPS.     

Hydroxyapatite as a filler for dental composite materials: mechanical properties and in vitro bioactivity of composites

Hydroxyapatite (HAp) powder was treated with -methacryloxypropyltrimethoxy-silane (-MPS) using standard techniques in both non-polar and polar systems. Infrared spectra (DRIFT) and thermogravimetric analysis (TGA) confirmed the presence of -MPS on the surface of the HAp filler particles. Series of experimental composites consisting of bisphenol-a-glycidyl methacrylate (BisGMA) based resin and untreated or treated HAp filler were produced to determine the mechanical properties and in vitro bioactivity. The incorporation of HAp filler into the BisGMA base resin had an enhancing effect on the flexural strength and Young's modulus of the base resin, the latter being increased by a factor of three. The mechanical properties of the filled resin were not affected by the surface treatment of the HAp, but filler loading was found to have a significant effect on Young's modulus. Higher proportions of silane-treated HAp of smaller particle size could be incorporated in the monomer phase giving rise to composites of higher stiffness. Examination of the fracture surfaces showed that the silanized HAp particles maintained better contact with the polymer matrix. In vitro study revealed that the composites incorporating silanized HAp formed a compact and continuous calcium phosphate layer on their surface after 4 weeks immersion in a simulated body fluid (SBF). © 2001 Kluwer Academic Publishers     

Effect of silane-coupling agent on natural rubber filled with silica generated in situ

The effect of silane coupling agent was investigated for the novel in situ silica loading to the natural rubber (NR) matrix. The silica was generated in situ by the sol-gel reaction of tetraethoxysilane in the NR matrix before its crosslinking. -mercaptopropyltrimethoxysilane (-MPS) significantly prevented the delay of sulfur curing and increased the wettability of NR onto in situ silica, which resulted in the increase of reinforcement effect for the NR vulcanizate. -MPS decreased the interaction between the in situ silica particles followed by dispersing the in situ silica particles homogeneously and decreasing the hardness, compression set, hysteresis loss and storage modulus at the rubbery state of in situ silica-filled NR vulcanizate. The NR/in situ silica composite with -MPS is a promising material for a high performance rubber product.     

First principles study of effect of lattice misfit on the bonding strength of Ni/Ni3Al interface

By using a discrete variational X (DV-X) method, the electronic structures and bonding strengths of Ni/Ni₃Al (or /) interface with different lattice misfits () were calculated in the framework of the nonrelativistic first-principles theory. In order to describe the effect of on the interfacial binding strength and the structural stability of coherent / interface, we calculated the interfacial binding covalent bond density (CBD) and the local environmental total bond overlap population (LTBOP). Very obvious effects of lattice misfits on the electronic structures of coherent / interface were found. On one hand, less than –0.6% negative lattice misfit can increase the binding strength of the /' interface. On the other hand, the local environmental total bonding strength of the /' interface decreases with increasing magnitude of . Therefore, the magnitude and sign of lattice misfit must be carefully controlled to balance the high-temperature creep strength of Ni-base single crystal superalloy and the structural stability of the /' interface when one designs new alloys.     

Effect of elastic interaction energy on coarsening of γ′ precipitates in a mechanically alloyed ODS Ni-base superalloy

The coarsening behavior of precipitates with a uniform size distribution and with a bimodal size distribution in a mechanically alloyed ODS Ni-base superalloy were investigated to clarify the effect of elastic interaction energy on the coarsening behavior of precipitates. The coarsening rate decreased with increasing size of precipitates with a uniform size distribution, contrary to the classical LSW theory, and the coarsening behavior of precipitates with a bimodal size distribution exhibited Ostwald ripening in which the larger precipitates grow at the expense of smaller precipitates. The driving force for coarsening of precipitates was analyzed based on the two-particle model, considering the effect of elastic interaction energy in addition to the effect of interfacial energy. The contribution of elastic interaction energy on the total energy was found to increase with increasing size of precipitates, and the decelerated coarsening of precipitates was attributed to the decrease in the driving force for coarsening with increasing size of precipitates.     

Direct current electrical conductivity of gamma ferric oxide

The study of the direct current electrical conductivity, , of freshly prepared -Fe₂O₃ and that of a sample stored for seven days in static air suggests that -Fe₂O₃ adsorbs oxygen and water from the atmosphere. From infra-red spectra it is deduced that the absorbed water in -Fe₂O₃ is present as the physically adsorbed water and as lattice water. The adsorbed oxygen and physically adsorbed water are removed by heating to 100 C, while the lattice water remains in -Fe₂O₃ even up to 280 C. The removal of lattice water is associated with a decomposition during which some of the hydrogen formed occupies the vacancy sites. This suggested formation of the hydrogen ferrite phase is based on the kink in the log against T ^–1 curve observed at 177 C. This kink is very well resolved for a sample equilibrated at 100 C in normal atmosphere, and the measurements of above 100 C of this sample are done in an N₂ atmosphere. The suggestion that the hydrogen ferrite phase is formed has been substantiated by comparison of the X-ray diffraction patterns of -Fe₂O₃ heated under the different atmospheres. From the log against T ^–1 plot for a sample heated under a nitrogen atmosphere the activation energy is small (< 0.05 eV) up to 215 C, and it is comparatively large (0.95 eV) above 215 C. These results suggest a hopping mechanism for the direct current electrical conductivity of -Fe₂O₃. This suggestion has been substantiated by data of the temperature variation of Seebeck voltage.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

γ′ Formation and morphology evolution in the undercooled structure of DD3 Single crystal superalloy

An investigation of the formation and its morphology evolution in the bulk undercooled DD3 single crystal superalloy is performed. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210 K. The microstructure formation has been respectively discussed with respect to undercooling, nucleation and solute segregation during solidification and post-solidification, by employing the classical nucleation theory. Furthermore, TEM and SEM technique are adopted to reveal the microscopy and to illustrate other factors correlated to the precipitation. It is found that the size, fraction, and distribution of precipitated in as-solidified DD3 single crystal superalloy are all influenced by melt undercooling.     

Strengthening in a DS casting Ni3Al base alloy IC6

A unidirectionally solidified casting base alloy IC6, with the chemical composition (at.%) 16.3–17.6% Al, 8.0–8.3% Mo, 0.16–0.78% B, Ni balance has been developed for advanced gas turbine blades and vanes. The experimental results show that this alloy has high yield strengths from room temperature to 1100°C, excellent creep resistance at temperatures up to 1100°C. The microstructural observations and analysis indicate that the superior mechanical properties of this alloy may be attributed to solid solution hardening by the large molybdenum addition, second phase strengthening by phase and other minor phases that precipitate in various temperature ranges, the formation of a raft structure during creep, and to the existence of high density misfit dislocation networks at / interface areas due to a high value of / misfit.Abbreviations BSE back scattered electron - DS unidirectionally solidified - EDS energy dispersive spectroscopy     

Influence of sterilization upon a range of properties of experimental bone cements

Bone cement, used to fix prostheses into the bone, must be sterilized prior to implantation. Two sterilization techniques, and radiation, were investigated, examining the influence upon molecular weight, static and dynamic mechanical characteristics and rheological properties. A number of experimental cements were studied prepared from methylmethacrylate (MMA) co-polymers, either single powders or powder blends, mixed with MMA monomer. It was found that with both and radiation, there was a decrease in molecular weight of all powders, including a MMA/styrene co-polymer, in relation to the radiation dose. This fall in molecular weight resulted in a drop in tensile strength, Youngs modulus and strain to failure of all cements tested. However, the deterioration in mechanical strength was highlighted by the dynamic testing. Fatigue lives of cements after testing in tension–tension, at 2 Hz under load control and irradiated with 25 kGy radiation, displayed significant decreases. This result indicated the utmost importance of conducting such tests upon experimental bone cements prior to in vivo use. The rheological time profiles of curing cements were also found to be influenced by 25 kGy radiation, with a reduction of complex viscosity after sterilization.     

Flow-stress recovery of nickel-aluminium alloys

Flow-stress recovery measurements along with structural observations using electron microscopy have been carried out on room-temperature prestrained alloys of Ni-11.9, 13.1 and 13.5 at.% Al aged at 735° C. These alloys contained respectively 0, 5, and 8 vol% Ni₃Al) in a fine dispersion. Samples were recovered at and below the ageing temperature for times ranging from 0.1 to 100 h. The influence of volume fraction of , distribution, amount of prestrain and recovery temperature on recovery kinetics was investigated.Results for samples recovered at 735° C showed a large fractional recovery (about 30%) following the first 0.1 h anneal for all samples. About 50 to 70% of the flow stress is recovered at the end of 100 h recovery anneals. The changes in dislocation structure agreed quantitatively with the changes in flow stress.Interpretation of the data in terms of a network growth model of recovery show the solid solution alloy to agree with theory for long recovery times (t>10 h) whereas the -strengthened alloys deviate considerably from the simple model.     

ESR study of degraded bovine bone

A comparison is made between the electron spin resonance (ESR) spectra obtained from bovine bone samples after (i) mechanical degradation, (ii) -irradiation, and (iii) mechanical degradation followed by -irradiation. Mechanical degradation is achieved by filing in either liquid nitrogen or air at room temperature. -irradiation is performed at 77 K. The stability of the radical species produced at 77 K is studied as a function of temperature. It is concluded that the spectra obtained in all instances arise from radicals formed both in the collagen and mineral phase of the bone, those from the collagen predominating. The relative radical concentrations from the collagen and mineral phase vary with the degradation method employed. It is observed that additional hydrogen atom radicals are produced only in the case of the -irradiated samples.On leave of absence from C.N.R. Centre for Macromolecular Physics, University of Bologna, Italy.     

Effect of zinc on the magnetic properties of acicular cobalt-modified γ-Fe2O3 particles

Acicular -FeOOH particles with a particle length of about 0.35 m and an axial ratio of about 7 were synthesized by the coprecipitation method using the reaction of FeCl₂-NaOH. The (Co, Zn)-modified -F₂O₃ particles were produced by absorbing Co²⁺ and Zn²⁺ ions on the surfaces of -FeOOH particles followed by dehydration, reduction and oxidation. The saturation magnetization and thermal stability of the coercivity of (Co, Zn)--Fe₂O₃ particles were all higher than those of Co--F₂O₃ particles. For the same (Co+Zn) content, the saturation magnetization of (Co, Zn)--Fe₂O₃ particles increased with increasing zinc content but the coercivity decreased.     

Microstructures of titanium-aluminides produced by laser surface alloying

The microstructures of Ti-Al layers (from 43–80 at %Al) produced by laser surface alloying of titanium substrate with a powder feed technique have been investigated. The laser processing parameters were; 1.8 kW laser power, 3 mm beam diameter, 7 mm s^–1 traverse speed, and values of powder flow rates of aluminium ranging from 0.07–0.11 g s^–1. The microstructures were dendrites of ₂ and interdendritic regions of ₂+ in the Ti-43 at %Al alloy; dendrites of either ₂ or ₂+ with interdendritic in the Ti-50 at %Al alloy; dendrites of ₂+ with interdendritic in the Ti-55 at %Al alloy; single phase in the Ti-60 at %Al alloy and TiAl₃ dendrites and Al solid solution in the interdendritic regions in the Ti-80 at %Al alloy. The microstructures were fine and comparable to those produced by other methods of rapid solidification processing. The microstructures of the Ti-50 and Ti-55 at %Al alloys were in agreement with the existence of the peritectic reactions:L + andL + , in the Ti-Al system.     

Phase transformations in permanent-mould-cast aluminium bronze

The phases obtained in aluminium bronze (Cu-10Al-4Fe) cast into a permanent mould were investigated. The parameters examined were the pre-heating temperature of the mould and the graphite coating thickness. The phases and ₂ were detected as well as the metastable phases and . The intermetallics of the system Fe-Al were obtained in various stoichiometric compositions. The different cooling rates of the casting resulted in two mechanisms of transformation to grains out of the unstable phase, one being nucleation and growth producing needle-shaped grains, the other exhibiting a massive transformation to spherical grains. These two mechanisms determine the changes in the size of the a grains as result of changes in the cooling rate in its various ranges.     

Microstructural study of ε-iron-carbonitrides formed by a glow discharge technique

The microstructure and phase transformations occurring in-iron-carbonitrides have been studied by means of X-ray and electron diffraction, electron microscopy and Mössbauer spectrometry. Ordering of the interstitial atoms, N or C, results in a hexagonal unit cell for Fe₃(C, N) with parametersa=a3 andc=c wherea andc are the lattice parameters of the hexagonal close-packed (h c p) iron unit cell. Stacking faults on (0001) planes and partial dislocations with Burgers vector b=1/31 0 ¯1 0 are observed in quenched-Fe₄ (C, N). After quench-aging, the carbonitrides show a structural hardening due to the precipitation of a metastable phase. Slow cooling of-carbonitrides with less than 25 at% interstitials leads to the precipitation of-carbonitride and ferrite in-phase grains which allows the orientation relationships between the-,- and-phases to be defined and a model of the-phase--phase transformation to be proposed.