High-Pressure Processing and Characterization of Fe–High-C/N Alloys

A new process has been developed that results in (i) enhanced nitrogen addition to ferritic iron–carbon alloys and (ii) melt-casting in a single operation. This new processing technique enables Fe–C alloys to retain high nitrogen interstitial concentrations and to reduce significantly, and possibly eliminate, carbide formation. In this study two commercial-grade, steel alloys were cast under elevated nitrogen pressures, resulting in solid solution (austenite, ferrite, and martensite) high-carbon and high-nitrogen iron alloys that were, within detection limits, carbide- and nitride-free. These alloys were subsequently thermally processed to transform part of the retained austenite to martensite. The microstructure and mechanical properties of the alloys were studied as a function of carbon and nitrogen composition and as a function of thermal processing. The retain high nitrogen concentrations in these cast and processed iron–carbon alloys resulted in a substantial improvement in compression strengths.     

Wetting behavior of liquid Fe–C–Ti alloys on sapphire

Wetting behavior in the (Fe–C–Ti)/sapphire system was studied at 1823 K. The wetting angle between sapphire and Fe–C alloys is higher than 90° (93° and 105° for the alloys with 1.4 and 3.6 at.% C, respectively). The presence of Ti improves the wetting of the iron–carbon alloys, especially for the alloys with carbon content of 3.6 at.%. The addition of 5 at.% Ti to Fe–3.6 at.% C provides a contact angle of about 30°, while the same addition to Fe–1.4 at.% C decreases the wetting angle to 70° only. It was established that the wetting in the systems is controlled by the formation of a titanium oxicarbide layer at the interface, which composition and thickness depend on C and Ti contents in the melt. The experimental observations are well accounted for by a thermodynamic analysis of the Fe–Ti–Al–O–C system.     

High carbon–nitrogen iron alloys

A new processing technique produces high carbon–high nitrogen iron alloys by melting iron-carbon steels in a hot isostatic pressing (HIP) furnace with nitrogen as the pressurizing gas. Furnace cooling O-1 tool steel with enhanced nitrogen concentrations resulted in the retention of the austenite phase without formation of carbide and nitride precipitates. The duplex austenite/ferrite structure has enhanced hardness, strength, and wear resistance.     

Testing the cytotoxicity of metal alloys used as magnetic prosthetic devices

Technical magnetic materials are increasingly used for the development of magnetic retained dental prosthetic and orofacial epithetic devices. Since most of the magnets based on rare earth metals, such as samarium–cobalt based alloys have a high tendency for corrosion they were first coated by tin and then encapsulated by titanium. However, the high mechanical load particularly on dental devices may cause a rupture of the titanium capsule and the alloys contact directly biological fluids. Hence, it is important to know the cytotoxicity of these magnets to assess their potential effects on the surrounding tissue. In this study, the cytotoxicity of neodymium–iron–boron and samarium–cobalt (plain, tin and titanium coated) magnets was tested. First, magnets were incubated up to 7 days in culture medium to prepare extracts for cytotoxicity measurements. Changes in the surface morphology due to corrosion were visualized by scanning electron microscopy and analysis of the elemental composition. 3T3 mouse fibroblasts were cultured in the presence of extracts and their viability measured by neutral red and metabolic assays. To learn more about a possible toxic activity of the main components of magnets, salt solutions of different concentrations resembling those elements, which are main constituents of the magnets, were used. 3T3 fibroblasts were also cultured in direct contact with the materials and material induced effects on cell morphology and growth monitored by microscopy. As a result of this study it was found that samarium–cobalt magnets have a strong tendency for corrosion and exert a considerable cytotoxicity. Neodymium–iron–boron magnets have a lesser tendency for corrosion and are only moderate cytotoxic. Coating of samarium–cobalt magnets with tin or titanium makes the materials non-toxic. Application of salt solutions shows that cobalt has a tendency to be cytotoxic at higher concentrations, but enhances cell metabolism and proliferation at lower concentrations while the other magnet constituents had a lower or negligible cytotoxic potential.     

A new resource-saving, high manganese and nitrogen super duplex stainless steel 25Cr–2Ni–3Mo–xMn–N

A new family of resource-saving, high manganese and nitrogen super duplex stainless steels (DSSs), with a composition of 25 wt.%Cr, 2 wt.%Ni, 3 wt.%Mo, 8–12 wt.%Mn, and 0.45–0.55 wt.%N, have been developed by examining the effect of Mn and N on the microstructure, mechanical properties and corrosion properties. The results show that these alloys have a balanced ferrite–austenite relation. The ferrite content increases with the solution treatment temperature, but it decreases with an increase in Mn and N. The element Mn accelerates σ phase precipitations. The increases in manganese and nitrogen, especially nitrogen, enhance the ultimate tensile strength (UTS) and ductility of the material. The pitting corrosion potential increases first and then decreases with an increase in the amount of Mn, which is due primarily to the presence of a small amount of σ phase when the amount of Mn is 12 wt.%. Among the designed DSS alloys, 25Cr–2Ni–3Mo–10Mn–0.5N is found to be an optimum alloy with proper phase proportion, a better combination of UTS and elongation, and higher pitting corrosion resistance compared with those of the other alloys. The mechanical strength and corrosion resistance and lower production cost of the materials are better than those of SAF2507.     

The Surface Tension of Undercooled Binary Iron and Nickel Alloys and the Effect of Oxygen on the Surface Tension of Fe and Ni

The surface tension of the undercooled binary alloys, iron–nickel and nickel–cobalt, was measured as a function of composition and temperature using the oscillating droplet method combined with electromagnetic levitation. The measurements cover a wide temperature range, including the overheated as well as the undercooled regime. In addition, the effect of oxygen on the surface tension of the elements, iron and nickel, has been determined. When exposed to an oxygen/helium gas flow of constant composition and flux, the surface tension decreases exponentially with increasing exposure time. The oxygen content in the gas mixture was monitored by a gas analyzer using a zirconia transducer. In addition, the amount of oxygen in the sample was determined by X-ray photo-electron-spectroscopy (XPS) before the experiment as well as for processed samples.     

High-Temperature Compatibility of Al–Rare Earth Alloys with Titanium Carbide

The wetting of titanium carbide by molten Al–rare earth alloys was studied. The physicochemical properties of the alloys were correlated with the contact angle and the carbon and oxygen affinities of the substrate and alloy. The properties of the alloys were investigated as a function of composition. The phase relations in the Al-rich corners of the Al–Y and Al–Nd phase diagrams were refined using viscosity data. The x-ray microanalysis and Auger electron spectroscopy data were used to assess the melt structure, adsorption behavior of the melt components at the liquid–gas and liquid–solid interfaces, interfacial area, and composition of the transition layer. The introduction of rare-earth additions into Al was found to reduce the temperature at which good adhesion to TiC can be achieved by 100–200 K.     

Surface Tension and Viscosity of Quasicrystal-Forming Ti–Zr–Ni Alloys

Recent X-ray scattering measurements show that icosahedral short-range order in Ti–Zr–Ni alloys is responsible for a change in phase selection from the stable C14 Laves phase to the quasicrystalline icosahedral phase, and that icosahedral short-range order increases at deeper undercoolings. This change in short-range order should be reflected in changes in the thermophysical properties of the melt. The surface tension and viscosity of quasicrystal-forming Ti–Zr–Ni alloys were measured over a range of temperature, including both stable and undercooled liquids, by an electrostatic levitation (ESL) technique. ESL is a containerless technique which allows processing of samples without contact, greatly reducing contamination and increasing access to the metastable undercooled liquid. The measured viscosity is typical of glass-forming alloys of similar composition to the quasicrystal-forming alloys studied here; however, the surface tension shows an anomaly at deep undercoolings.     

Interface interaction in the B4C/(Fe–B–C) system

The wetting behavior in the B₄C/(Fe–C–B) system was investigated in order to clarify the role of Fe additions on the sinterability of B₄C. Iron and its alloys with C and B react with the boron carbide substrate and form a reaction zone consisting of a fine mixture of FeB and graphite. The apparent contact angles are relatively low for the alloys with a moderate concentration of the boron and carbon and allow liquid phase sintering to occur in the B₄C–Fe mixtures. A dilatometric study of the sintering kinetics confirms that liquid phase sintering actually takes place and leads to improved mass transfer. A thermodynamic analysis of the ternary Fe–B–C system allows accounting for the experimental observations.     

Microstructure and texture of rolled and annealed β titanium alloy Ti–10V–4.5Fe–1.5Al

This work describes the evolution of texture during cold rolling and annealing of a hot rolled and solution treated sheet of a low cost β titanium alloy Ti–10V–4.5Fe–1.5Al. The alloy was cold rolled up to 60% reductions and then annealed in β phase field at different temperatures to study the re-crystallisation textures. The rolling and re-crystallisation textures obtained in this study are compared with those of other β titanium alloys and bcc metals and alloys such as tantalum and low carbon steel.     

Grain growth behaviour and consolidation of ball-milled nanocrystalline Fe–10Cr alloy

Nanocrystalline iron–chromium alloys may provide considerable corrosion resistance, even at low chromium contents. However, processing of such alloys could be a challenge. This paper describes successful synthesis of nanocrystalline Fe–10%Cr alloy by ball-milling route. In the absence of suitable hot compaction facility, the alloy powder could be successfully compacted close to the desired density, by employing a step of prior annealing of the powder. Grain growth behaviour of Fe–10%Cr nanocrystalline alloy was investigated at 500, 600 and 700 °C. At 500 °C, no appreciable grain growth was observed, after the initial grain growth. However, sudden and rapid grain growth was observed after 90 min at 600 °C, and 30 min at 700 °C.     

Effect of rare-earth elements on the microstructural characterization in rapidly quenched thermally strengthened aluminium alloys

The influence of rare-earth elements on the microstructural features of rapidly solidified Al93.3-xFe4.3V0.7Si1.7Mmx(x=0, 0.5, 1.0, 3.0, 6.0) alloy was systematically studied by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and energy dispersive X-ray analysis. Experimental results show that there are different type of phase transformation depending on mischmetal (Mm) concentration. For Al87.3Fe4.3V0.7Si1.7Mm6.0 metallic glass, a shoulder was observed on the high-angle side of the main peak in the X-ray diffraction patterns due to quenched-in aluminium nuclei and a prepeak resulting from Mm–Mm pairs. By means of particle extraction analysis, it has been proved that the -Al13(Fe, V)3Si phase existing in as-cast Al–Fe–V–Si alloy is wholly or partly inhibited for Al93.3-xFe4.3V0.7Si1.7Mmx (x=0.5, 1.0, 3.0) crystalline alloys. In addition, a new phenomenon has been reported that the lattice parameter of as-quenched Al–Fe–V–Si–Mm alloys decrease with increasing Mm content; the "cell lessening effect". This effect is presumably due to the results of composite interactions between rare-earth elements and alloy elements.     

Production of Functional Nanocrystalline Materials in Hydrogen

On the example of KS25 and KS37 samarium–cobalt-base commercial alloys and LaNi_4.5Al_0.5 alloy, we show the possibility, in principle, of obtaining functional materials in the nanocrystalline state with the help of a planetary mill in hydrogen medium. Milling with a rotational speed of 600 rpm during 24 h leads to the disproportionation of KS25 and KS37 alloys into samarium hydride and iron–cobalt (cobalt) and of LaNi_4.5Al_0.5 into Ni₃Al and amorphous products. After vacuum annealing up to 1181 K, the main phases of samarium–cobalt materials are recombined. The crystallite sizes after annealing are 58–72 and 70 nm for KS25 and KS37, respectively. We established that LaNi_4.5Al_0.5 alloy is not recombined in vacuum, and the nanocrystalline state in it can be reached by milling up to 30 min. The crystallite sizes constitute 45–78 nm.     

Hydrogen and nitrogen effects on optical and structural properties of amorphous carbon

Two series of amorphous carbon alloys were deposited by reactive sputtering using a graphite target and argon as a sputtering gas. The effect of hydrogen or nitrogen on the structure of amorphous carbon was investigated using photothermal deflection spectroscopy (PDS), UV–Vis–near infrared spectroscopy, Fourier Transform Infrared (FT-IR), Raman and Photoluminescence (PL) techniques. The change in the structure of hydrogenated amorphous carbon (a-C:H) is due to the fact that H incorporation favours the formation of sp³ sites. In fact, the hydrogen incorporation relaxes the structure enough to improve electronic properties by increasing the number of terminal bonds. In the amorphous carbon nitride (a-CN) films, the lone pairs belonging to the nitrogen atoms are important in determining the optical properties of the films. The nitrogen alters the structure of carbon and creates cavities to be responsible for hydroxyl (OH) inclusions.     

Incipient melting of Al5Mg8Si6Cu2 and Al2Cu intermetallics in unmodified and strontium-modified Al–Si–Cu–Mg (319) alloys during solution heat treatment

The present work was performed on seven alloys containing in common Al–6.5 wt%Si–3.5 wt%Cu, with magnesium in the range 0.04–0.45 wt%, and strontium in the range 0–300 p.p.m. The alloys were cast in the form of tensile test bars, solution heat treated in the temperature range 480–540°C for times up to 24 h. Two types of solution heat treatment were applied: (i) single-stage, where the test bars were solution treated at a certain temperature for 12 h prior to quenching in hot water (60°C); (ii) two-stage, where the test bars were solution treated for 12 h/510°C+12 h/T°C (T=510, 520, 530, 540°C), followed by quenching in hot water. In the low-magnesium alloys (i.e. with Mg0.04 wt%), melting of the Al2Cu phase commenced at 540°C. Increasing the magnesium content to 0.5 wt% reduced the incipient melting temperature of the Al5Mg8Si6Cu2 phase to 505°C. The mechanism of incipient melting and its effect on the tensile properties have been discussed in detail. © 1998 Chapman & Hall     

Amorphous alloys resistant to corrosion in artificial saliva solution

The tailoring of new corrosion-resistant alloys with specific properties has recently been performed mostly by the sputter deposition technique. The aim of this work was to investigate corrosion resistance of aluminum–tungsten (Al–W) amorphous alloys in artificial saliva solution, pH=5.5, based on the electrochemical methods of cyclic voltammetry and linear polarization. Thin alloy films were prepared on a sapphire substrate by magnetron codeposition. Completely amorphous films were obtained in the Al₈₀W₂₀–Al₆₇W₃₃ composition range. Amorphous Al–W alloys exhibit very high corrosion resistance due to their homogeneous single-phase nature. The passive films spontaneously formed at their surface are uniform with characteristics of an insulator film and prevent corrosion progression in the bulk in a very demanding oral environment. The mechanism of increasing resistivity of Al–W alloys to pitting corrosion and generalized corrosion has been discussed in the view of increasing tungsten content in the alloy. Considering these exceptional corrosion properties and microhardness which falls in the range 7.5±1.6 Pa, Al–W alloys represent promising materials for dental applications.     

Effects of thermo-mechanical processing and trace amount of carbon addition on tensile properties of Cu–2.5Fe–0.1P alloys

The effects of thermo-mechanical processing, including intermediate aging treatment and/or solution heat treatment, and a trace amount of carbon (C) addition were studied on tensile behavior of Cu–2.5Fe–0.1P alloys. In this study, Cu–2.5Fe–0.1P alloy sheets without and with a carbon content of 0.05 wt.% were cast and subsequently rolled and thermo-mechanically treated following various processing routes. The introduction of intermediate aging treatment between cold rolling improved the tensile strength of Cu–2.5Fe–0.1P alloys. Solution heat treatment prior to aging was proved to be detrimental on the tensile strength, probably due to recovery and recrystallization causing the complete loss of work hardening during previous cold rolling. The present study also suggested that two-step aging is more effective in improving the strength of Cu–2.5Fe–0.1P alloys than one-step aging. The effect of C addition on improving the tensile strength of Cu–2.5Fe–0.1P alloys was real but marginal, probably due to the limited solubility of C in Cu–2.5Fe matrix. The effects of intermediate heat treatments between cold-rolling processes on tensile properties of Cu–2.5Fe–0.1P specimens with and without C addition are discussed based on optical, scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs, and SEM fractographs.     

Superplasticity of Fine-Grained Fe–C Alloys Prepared by Ingot-And Powder-Processing Routes

Tensile elongation behavior of fine-grained Fe–C alloys has been investigated as a function of cementite volume fraction, degree of microstructural refinement, and the Zener-Hollomon parameter. The strain rate–stress relationships and creep strengths of Fe–C alloys with carbon contents from 1.3 to 5.25 wt. % C are found to be similar when grain size is similar. Superplastic ductility of ingot-processed alloys initially increases with carbon content but starts to decrease after 2.1% C. The increase of tensile ductility with carbon content below 2.1% C is attributed to a reduction in the case of dynamic grain growth associated with an increase in the number of fine cementite particles, whereas the decrease of tensile ductility above 2.1% C is due to an increase in the number of coarse cementite particles and an increase in the area of cementite/cementite grain boundaries. Superplastic ductility of Fe–C alloys with carbon contents higher than 2.1% C can be significantly enhanced when powder-processing routes are utilized instead of ingot-processing routes. Tensile elongation behavior of cementite-based alloys is revealed to be different from that of iron-based alloys when compared as a function of the Zener-Hollomon parameter.     

Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties

Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 μm s⁻¹. Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young's modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen.