Template electrodeposition to cobalt-based alloys nanotube arrays

Parallel uniform arrays of amorphous ferromagnetic Co₈₁Ni₁₉ and Co₃₇Fe₆₃ alloy nanotubes with outer diameter around 325-365 nm, wall thickness of 30-60 nm and length of over 40 μm were prepared by a direct current electrodeposition with mercury cathode using porous anodic aluminum oxide membrane as template. The morphology, structure, composition and magnetic property were studied. The results showed that mercury cathode is the key factor to form amorphous alloy nanotubes, and the as-prepared nanotube arrays exhibit obvious uniaxial magnetic anisotropy and the easy magnetization direction is perpendicular to the nanotubes axis. The mechanism of formation of Co based alloy nanotubes was also discussed.     

The metallography of a cobalt-based implant alloy after solution treatment and ageing

The ageing characteristics of a commercial Co-Cr-Mo-C alloy after solution treatment were investigated using optical and electron microscopy. An M₂₃C₆-type carbide was identified by X-ray and electron diffraction after ageing treatments between 650 and 1150° C. Nucleation and growth of this carbide took place on intrinsic stacking faults by Suzuki segregation in the cobalt matrix. High stacking-fault densities gave rise to intragranular striations which were visible after etching once precipitation had occurred. Ageing temperatures of 925° C and above increased the stability of the f c c cobalt matrix and led to precipitation on undissociated dislocations. Grain-boundary carbides were evident at all ageing temperatures.     

Improvement of creep-rupture properties of wrought cobalt-based HS-21 alloys at high temperatures

The improvement of creep-rupture properties by serrated grain boundaries is investigated using wrought cobalt-based HS-21 alloys in the temperature range 816 to 1038° C (1500 to 1900°F). Serrated grain-boundaries are produced in the early stage of the grain-boundary reaction (GBR) by a heat treatment. Specimens with serrated grain boundaries have superior creep-rupture properties compared with those with normal straight grain boundaries. The rupture lives of specimens with serrated grain boundaries are more than twice as long as those of specimens with straight grain boundaries. The rupture elongation is considerably improved by serrated grain boundaries especially at lower temperatures. A ductile grain-boundary fracture is observed in specimens with serrated grain boundaries, while brittle grain boundary facets prevail in specimens with straight grain boundaries.     

Fractal dimension of the grain boundary fracture in creep-fracture of cobalt-based heat resistant alloys

The effects of grain boundary configuration and creep conditions on the fractal dimension of the grain boundary fracture (D _f) were investigated using commercial cobalt-based heat resistant alloys, namely, HS-21 and L-605 alloys. Creep-rupture experiments were carried out under the initial creep stresses of 19.6–176 MPa in the temperature range from 1089–1422 K in air. The value of D _f was larger in specimens with serrated grain boundaries than in those with straight grain boundaries in the HS-21 alloy under the same creep condition, and the difference in the value of D _f between these specimens was large in the scale range of the analysis which was less than about one grain boundary length. However, there was almost no difference in the value of D _f between the specimens with serrated grain boundaries and those with straight grain boundaries in the L-605 alloy, because there was no obvious difference in the microstructure between these specimens. The value of D _f increased with decreasing creep stress in the scale range of the fractal analysis larger than about one grain boundary length in both HS-21 and L-605 alloys, while the stress dependence of D _f was larger in the HS-21 alloy. The stress dependence of D _f was explained by the stress dependence on the number of grain boundary microcracks linked to the fracture surface. The value of D _f estimated in the scale range smaller than about one grain boundary length showed essentially no stress dependence in both L-605 and HS-21 alloys.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Graded glass coatings for Co-Cr implant alloys

A graded glass coating for Vitallium®, a Co-Cr alloy, has been prepared using a simple enameling technique. The composition of the glasses has been tailored to match the thermal expansion of the alloys. The optimum glass composition and firing conditions (temperature and time) needed to fabricate homogeneous coatings with good adhesion to the alloy were determined. The final coating thickness ranged between 25 and 60 μm. Coatings fired under optimum conditions do not delaminate during indentation tests of adhesion. Excellent adhesion to the alloy has been achieved through the formation of 100 nm thick interfacial chromium-oxide (CrOx) layers. The graded glass (consisting of BIG and 6P50 layers) can be successfully coated to a Co-Cr alloy, and forms hydroxyapatite (HA) on the coating surface when immersed in a simulated body fluid (SBF) for 30 days.     

High-strength magnesium alloys for degradable implant applications

This article describes the design principles deployed in developing high-strength and ductile Mg-Zn-Zr-Ca-Mn(-Yb) alloys based on a concept, which aims to restrict grain growth considerably during alloy casting and forming. The efficiency of the development approach is discussed. Moreover, the microstructure and phase analysis of the alloys subjected to different thermal treatments are presented and the influence of the alloy composition, particularly the addition of Yb, on the evolution of the microstructure is discussed in connection with the mechanical properties of the materials. The newly developed alloys exhibit high strength (yield stress of up to 350 MPa) at considerable ductility (elongation to fracture of up to 19%) in the as-extruded state and reveal age hardening potential (increase in hardness of 10-15% compared to that in the recrystallization heat-treated state). Appropriate heat treatments enable tailoring of the strength-ductility relation. Thermal annealing of the material resulted in a remarkable increase in ductility (elongation to fracture of more than 20% for all heat-treated samples) while high strength is retained (yield stress ranging from 210 to 315 MPa). We attribute the attractive mechanical properties of the developed alloys to their fine-grained microstructure, where the grain boundaries and lattice defects are stabilized by second phase particles formed during casting and thermal treatments.     

Computer modelling of hot-working processes

Abstract

In this paper, the development of process modelling within the Department of Metallurgy at The University of Sheffield is traced. This was initially concerned with predicting temperature distributions generated during hot-extrusion and hot-rolling operations, but the principles of finite-difference modelling have also been applied to upset forging and compression testing. The modelling of microstructural evolution during multipass working operations from the basis of laboratory test data was developed separately. These two aspects have been combined, and the effects of microstructure, temperature, and other engineering variables on flow stress have been incorporated to produce a model of hot rolling which predicts microstructural changes at each stage of the process and takes into account their effect on rolling-load and power requirements.

MST/168     

The characteristics of TiAl-based alloys melted in graphite crucibles

ABSTRACT

In the paper, the phase composition, microstructure, and selected mechanical and operational properties at room and elevated temperature of a TiAl intermetallic alloy from the TNB group – Ti–45Al–8Nb–0.5(B, C), induction melted in special graphite crucibles, are characterised. Selected properties of this alloy were compared to those of the reference TNB-V2 alloy with similar chemical composition, prepared with technologies currently used in the world. The result of this comparison was a positive recommendation for the proposed melting technology as an alternative to this and other groups of TiAl-based alloys.

This paper is part of a thematic issue on Titanium.     

The electro-discharge machining characteristics of TiNi shape memory alloys

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the material removal rate of TiNi SMAs in the EDM process significantly relates to the electro-discharge energy mode, involving the pulse current I _P and pulse duration _P. It also has a reverse relationship to the product of the melting temperature and thermal conductivity of TiNi SMAs. In addition, a longer pulse duration _P and a lower pulse current I _P should be selected to have a precise EDM machining of TiNi SMAs. Many electro-discharge craters and re-cast materials are observed on the EDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO₂, TiNiO₃ and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration _P. The specimen's hardness near the outer surface can reach 750 Hv for EDM TiNi SMAs. This feature originates from the hardening effect of the re-cast layer. The EDM TiNi SMAs still exhibit a nearly perfect shape recovery at a normal bending strain, but a slightly reduced shape recovery at a higher bending strain due to the depression of the re-cast layer. All the Ti₄₉Ni₅₁, Ti₅₀Ni₅₀ and Ti₅₀Ni₄₀Cu₁₀ SMAs exhibit similar EDM characteristics although they have different crystal structures and mechanical properties at room temperature.     

Recent progress in porous TiNb-based alloys for biomedical implant applications

ABSTRACT

Porous TiNb-based alloys have attracted considerable attention in the medical field of orthopedic applications because of excellent biocompatibility, porous architecture, appropriate mechanical property, etc. The literature reviews the current progress of fabrication methods, porous structure, mechanical properties, corrosion resistance, bioactive hydroxyapatite surface modification, in vitro and in vivo biocompatibility of porous TiNb-based alloy. Moreover, future research directions are pointed out to expand its possible biomedical applications.     

Combustion synthesis of CoCrMo orthopedic implant alloys: microstructure and properties

Because of their excellent properties, such as corrosion resistance, fatigue strength and biocompatibility, cobalt-based alloys are widely used in total hip and knee replacements, dental devices and support structures for heart valves. In this work, CoCrMo alloys were synthesized using a novel method based on combustion synthesis (CS), an advanced technique to produce a wide variety of materials including alloys and near-net shape articles. This method possesses several advantages over conventional processes, such as low energy requirements, short processing times and simple equipment. The evaluated material properties included density and yield measurements, composition and microstructure analysis, hardness, friction and tensile tests. It was shown that microstructure of CS-material is finer and more uniform as compared to the conventional standard. It was also found that among various additives, Cr₃C₂ is the most effective one for increasing material hardness. In addition, synthesized CoCrMo alloys exhibited good friction and mechanical properties.     

Electrochemical corrosion behaviour of dental/implant alloys in saline medium

Dental alloys implanted in mouth are exposed to various aggressive conditions. Keeping this in view, corrosion behaviour of various dental alloys viz. Ni-Cr, Co-Cr, Cu-Ni-Al and commercially pure Ti (c.p. Ti) were studied in 3% NaCl medium by using Tafel polarization, cyclic polarization and electrochemical impedance spectroscopy techniques. EIS studies were carried out for different duration viz. 1 h, 1 day and 7 days to evaluate the stability of passive film and change in corrosion characteristics with time. It has been found that for Ni-Cr, Co-Cr (DRDO developed) and c.p. Ti the passive film characteristic changed with time whereas for Co-Cr (commercial) and Cu-Ni-Al alloys, the passive film characteristics remained same. From DC electrochemical studies various parameters viz. i(corr), E(corr), i(pass), E(pass) were evaluated. The corrosion rates were observed to be in the order Cu-Ni-Al > Co-Cr (commercial) > Ni-Cr > c.p. Ti > Co-Cr (DRDO).     

Corrosion evaluation of Au-Cu-Pd ternary alloys

The corrosion resistivity of single-phase Au-Cu-11 at% Pd alloys was evaluated by using a parameterQ which represented the total amount of anodic reaction in a potentiostatic polarization test. The result was compared with those for binary Au-Cu, ternary Au-Cu-11 at % Ag and some commercial alloys. The validity of usingQ as a corrosion parameter was confirmed by the good agreement between the analysed and calculated values of copper ion dissolved into the test solution. By replacing a part of the copper in Au-Cu alloys with palladium the corrosion resistivity can be greatly improved, but silver has no such significant effect. The value ofQ decreased by both treatments of homogenization and grain refinement of the alloy. One of the advantages of the alloy having a single-phase structure is that inhomogeneity in the distribution of the constituents is small even in the as-cast state, which results in a small galvanic effect.     

Microstructural evaluation of arc-melted Al-Li-Be alloys

A microstructural investigation has been carried out on a series of small castings of arc-melted Al-Li-Be alloys. The alloy composition included amounts of lithium up to 3wt% and beryllium up to 10wt%. Optical metallographic examination has revealed gross macrosegregation in the highly alloyed compositions. On the microstructural level, the structure consists of primary beryllium particles in a matrix of primary aluminium containing eutectic structure at the cell walls. A detailed Auger electron spectroscopy examination has been carried out on a section of an Al-3Li-10Be arc-cast alloy to determine the precise compositional variations. This approach is demonstrated to be a necessary prerequisite for selection of material for rapid solidification processing by techniques such as splat quenching.     

Crystallization characteristics of amorphous Fe-Si-B alloys

Crystallization of soft magnetic Fe-Si-B glasses was investigated by transmission electron microscopy and X-ray diffraction. Electron micrographs of the successive crystallization stages of some Fe-Si-B glasses were obtained and the morphology variations for different alloy compositions were determined. The compositional dependence of the crystallization mode exhibited by the Fe-Si-B glasses was analysed, and on this basis some suggestions about crystallization rules in these glasses are proposed. Two kinds of iron—silicon phases occurred, depending on the Fe-Si-B alloy composition: bcc Fe(Si) solid solution and an ordered solid solution on the structural basis of Fe₃Si iron suicide. It is suggested that the metastable Fe₃B phase (observed during crystallization of only few glasses) was not a simple consequence of boron content. Crystallization of the Fe₃B phase was related to the formation of the iron—silicon phase which was produced during the first crystallization stage.     

Microstructural characteristics of splatquenched aluminium-copper alloys

The microstructural characteristics of a range of splat-quenched aluminium-copper alloys are described. The variation of the microstructure as a function of thickness and composition has been determined, by use of conventional 100 kV and high-voltage electron microscopy, and also by ion-beam thinning techniques. With increasing thickness alloys containing < 13 at. % Cu undergo the coventional aluminium-copper precipitation sequence, but alloys of > 13 at.%Cu exhibit a range of degenerate, radial and parallel eutectic structures. Restriction of examination to standard areas, undergoing similar cooling paths, has enabled valid comparison to be made between specimens of different composition.     

Some characteristics of icosahedral aluminium alloys

Preliminary results of a study of the transformation behaviour, and those of microhardness measurements on quasi-crystalline Al-14 at.% Mn and Al-22 at.% Mn alloys are reported.