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 characterization

The biocompatibility of commercially pure titanium and its alloys is closely related to their surface properties, with both the composition of the protecting oxide film and the surface topography playing an important role. Surfaces of commercially pure titanium and of the two alloys Ti–6Al–7Nb and Ti–6Al–4V (wt %) have been investigated following three different pretreatments: polishing, nitric acid passivation and pickling in nitric acid–hydrogen fluoride. Nitric acid treatment is found to substantially reduce the concentration of surface contaminants present after polishing. The natural 4–6 nm thick oxide layer on commercially pure titanium is composed of titanium oxide in different oxidation states (TiO2, Ti2O3 and TiO), while for the alloys, aluminium and niobium or vanadium are additionally present in oxidized form (Al2O3, Nb2O5 or V-oxides). The concentrations of the alloying elements at the surface are shown to be strongly dependent on the pretreatment process. While pickling increases the surface roughness of both commercially pure titanium and the alloys, different mechanisms appear to be involved. In the case of commercially pure titanium, the dissolution rate depends on grain orientation, whereas in the case of the two alloys, selective -phase dissolution and enrichment of the -phase appears to occur. © 1999 Kluwer Academic Publishers     

Tribological study of lubricious DLC biocompatible coatings

DLC (diamond-like carbon) coatings have remarkable tribological properties due mainly to their good frictional behavior. These coatings can be applied in many industrial and biomedical applications, where sliding can generate wear and frictional forces on the components, such as orthopaedic metal implants. This work reports on the development and tribological characterization of functionally gradient titanium alloyed DLC coatings. A PVD-magnetron sputtering technique has been used as the deposition method. The aim of this work was to study the tribological performance of the DLC coating when metal to metal contact (cobalt chromium or titanium alloys) takes place under dry and lubricated test conditions. Prior work by the authors demonstrates that the DLC coating reduced considerably the wear of the ultra-high-molecular-weight polyethylene (UHMWPE). The DLC coating during mechanical testing exhibited a high elastic recovery (65%) compared to the values obtained from Co–Cr–Mo (15%) and Ti–6Al–4V (23%). The coating exhibited an excellent tribo-performance against the Ti–6Al–4V and Co–Cr–Mo alloys, especially under dry conditions presenting a friction value of 0.12 and almost negligible wear. This coating has passed biocompatibility tests for implant devices on tissue/bone contact according to international standards (ISO 10993).     

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.     

Modeling of cyclic stress–strain behavior and damage mechanisms under thermomechanical fatigue conditions

A multi-component model was applied to predict the cyclic stress–strain response of different alloys under thermomechanical fatigue conditions based upon isothermal hysteresis loops. A ductile AISI 304 L-type stainless steel and two high strength alloys, the near-α titanium alloy IMI 834 and the nickel-base superalloy IN 100, were chosen as test materials. These represent alloys with rather different dislocation slip modes, stress–strain characteristics and damage mechanisms. Model predictions are compared with experiments and the differences in cyclic stress–strain response and damage mechanisms under isothermal and thermomechanical fatigue conditions, respectively, are discussed based upon microstructural observations.     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

Effect of gas saturation of titanium alloys VT1-0, OT4-1, and VT-23 on their fatigue durability

Oxidation and gas saturation of titanium at high temperatures and, consequently, formation of gas-saturated layers may affect the mechanical properties of titanium alloys in different ways. The influence of the parameters of a gas-saturated layer (depth and increment of microhardness) on the cyclic durability of titanium alloys does not always fit a fixed 25% standard for the microhardness difference between the surface and the matrix of the metal. To evaluate the effect of gas saturation on the fatigue and strength properties of titanium alloys more exactly, we suggest a differentiated approach to estimating the influence of each parameter of the gas-saturated layer.Karpenko Physicomechanical Insitute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 6, pp. 110–112, November–December, 1993.     

The study of metal ion release and cytotoxicity in Co–Cr–Mo and Ti–Al–V alloy in total knee prosthesis – scanning electron microscopic observation

We surgically retrieved two cobalt(Co)–chromium(Cr)–molybdenum(Mo) and five titanium(Ti)–aluminum(Al)–vanadium(V) alloy knee prostheses from patients because of mechanical failure and pain. We examined the distribution of the small particles which were released from the Co–Cr–Mo and Ti–Al–V alloys using a backscattered scanning electron microscopy (SEM). In addition we analyzed the metals in the artificial knee joints and the tissues adjacent to them using energy dispersive X-ray spectroscopy (EDS). We demonstrated that a myriad of fine particles, produced by the abrasion of both Co–Cr–Mo and Ti–Al–V alloys, accumulated in the synovial cells. As Co–Cr–Mo alloys disintegrate easily in the cells, Co dissolves from the peripheral areas of them, although Cr remains within the cells. In contrast Ti–Al–V alloys are very stable in the synovial cells. From these findings we conclude that the Co–Cr–Mo alloys are hazardous to the body as the alloys release Co which enters the body. In contrast the Ti–Al–V alloys are very stable and are patently safer. Artificial joints, however, are still in considerable need of improvement.     

The wetting of alumina by copper alloyed with titanium and other elements

The wettability of alumina by ternary alloys of copper, titanium and aluminium, gallium gold, indium, nickel or silver has been investigated using sessile drop tests conducted in vacuum at 1050–1250° C. Substantial additions of titanium are known to induce copper to wet alumina due to the formation of a titanium rich reaction product at the alloy/ ceramic interface, but the present work has shown that the concentration of titanium can be reduced by the addition of ternary alloying elements. Additions of indium are very beneficial, of aluminium, gold or silver are moderately beneficial, and of gallium or nickel are of negligible benefit or detrimental. These observations, and previous work with copper-tin-titanium alloys [1] can be interpreted in terms of effects on the activity of titanium which it is argued will be enhanced if the ternary alloying element has a low surface energy and is readily saturated by titanium. The correlation of the experimental wetting observations with the surface energy and titanium solubility data for the ternary alloying elements provides a basis for the rational development of reactive metal brazes for joining unmetallized ceramics.     

Processing and mechanical properties of autogenous titanium implant materials

Pure titanium and some of its alloys are currently considered as the most attractive metallic materials for biomedical applications due to their excellent mechanical properties, corrosion resistance, and biocompatibility. It has been demonstrated that titanium and titanium alloys are well accepted by human tissues as compared to other metals such as SUS316L stainless steel and Co–Cr–Mo type alloy. In the present study, highly porous titanium foams with porosities 80% are produced by using a novel powder metallurgical process, which includes the adding of the selected spacers into the starting powders. The optimal process parameters are investigated. The porous titanium foams are characterized by using optical microscopy and scanning electron microscopy. The distribution of the pore size is measured by quantitative image analyses. The mechanical properties are investigated by compressive tests. This open-cellular titanium foams, with the pore size of 200–500 m are expected to be a very promising biomaterial candidates for bone implants because its porous structure permits the ingrowths of new-bone tissues and the transport of body fluids.     

Evaluation techniques of metallic biomaterials in vitro

Metals and alloys are widely used as biomedical materials and are important in medicine and they cannot be replaced with ceramics or polymers at present mainly because of their high strength and toughness. Since safety is the most important property of biomaterials, corrosion-resistant materials such as stainless steel, Co–Cr–Mo alloy, commercially pure titanium, and titanium alloys are employed as biomaterials. Evaluation techniques for corrosion with culturing cells, the characterization of reconstruction of surface oxide film, fretting fatigue, cytotoxicity, and biocompatibility are reviewed in this paper. These techniques are original and characteristics in the field of biomaterials that should contribute to the proper evaluations of biomaterials in vitro.     

Selection of Materials for High-Speed Motor Rotors

We present strength and fatigue characteristics of materials suitable for high-speed motor rotors used in centrifugal compressors. We established that laser shock peening efficiently produces deep compressive stresses and significantly decreases the fatigue crack growth rate in titanium alloys. For titanium alloys of all types considered, the microstructure of the heat-affected zone after laser treatment significantly differs from the microstructure of the original alloy. Grain coarsening occurs both in unalloyed titanium and in near-alpha, alpha–beta, near-beta, and metastable alloys. For practical applications, titanium alloys require additional heat treatment. In particular, aging and mechanical treatment are used for increasing strength characteristics.     

Corrosion-Electrochemical Behavior of Titanium and Its Alloys with Aluminum in Solutions of Hydrochloric Acid

We establish that ozone in a 10–15% hydrochloric acid solution at 20°C has no effect on the corrosion-electrochemical behavior of titanium and its alloys with aluminum but promotes passivation at 40°C. The addition of ozone to 15% HCl at 40°C and to a 20% solution at 20°C leads to the transition of alloys to the state of unstable passivity. Titanium can be used as a structural material in ozonized 5–20% HCl solutions at 20 and 40°C and in a 15% solution at 20°C. This is also true for its alloys with aluminum in 5% HCl at 20 and 40°C and in 10% HCl at 20°C.     

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.     

Estimating cyclic corrosion cracking resistance for titanium alloys

Conclusions 1. Cyclic cracking resistance tests for titanium alloys, as for steels, are affected not only by the state of stress and strain but also by the electrochemical conditions at the crack tip.2. Cyclic corrosion cracking tests require one to consider the electrochemical conditions at the crack tip.3. These methods have been tested in corrosion crack stability tests, which involved monitoring the electrochemical conditions at the surface of the specimen, which do not unambiguously define the corrosion cracking resistance for titanium alloys in a particular medium.4. It is possible to derive invariant fatigue failure diagrams for titanium alloys in a given medium only if one provides constant conditions at the crack tip during growth.5. The electrochemical conditions at the crack tip can be characterized reasonably fully (integrally) for titanium alloys, as for steels, by means of the pH and the electrode potential.6. The type of alloy governs the electrochemical characteristics at the tip of a stationary statically loaded crack and a growing one.7. To select basic fatigue-failure diagrams for a corrosive medium, one can use the methodology developed previously for steels, which is based on invariant diagrams in air and in the medium, where one examines the effects of extremal working conditions and the electrochemical conditions at the crack tip.Karpenko Physicomechanics Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 2, pp. 33–42, March–April, 1993.     

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.     

Mechanical properties and corrosion resistance of Ti–6Al–7Nb alloy dental castings

With the aim of applying a novel titanium alloy, Ti–6Al–7Nb, to a dental casting material, a comprehensive research work was carried out on its characteristics, such as castability, mechanical properties and corrosion resistance in the present study. As a result, Ti–6Al–7Nb alloy exhibited sufficient castability by a dental casting method for titanium alloys and enough mechanical properties for dental application. It is also showed excellent corrosion resistance through an immersion test in 1.0% lactic acid and an anodic polarization test in 0.9% NaCl solution. From these results, it is concluded that this Ti–6Al–7Nb alloy is applicable as a dental material in place of Ti–6Al–4V alloy, which includes cytotoxic vanadium.     

The dendrite growth kinetics of nickel-based alloys

The solidification velocity of several compositions of Ni–Ti and Ni–Sn alloys were measured as a function of undercooling by the direct imaging of levitated drops. During this investigation a plateau in the solidification velocity was observed at intermediate undercoolings as a direct result of the addition of tin and titanium to nickel. Past work has shown an additional solidification velocity plateau at high undercoolings can be attributed to residual oxygen solute. From these results, a logistic growth model for alloy solidification was developed that can describe the solidification velocity as a function of undercooling and reproduce the plateau behavior. Finally, it is shown that a logistic growth model is more accurate for describing the solidification of alloys than thermodynamic models based on the Ivanstov solution.     

Application of High-Speed Laser Polarimetry to Noncontact Detection of Phase Transformations in Metals and Alloys at High Temperatures

A high-speed laser polarimetry technique, developed recently for the measurement of normal spectral emissivity of materials at high temperatures, was used to detect solid–solid and solid–liquid phase transformations in metals and alloys in millisecond-resolution pulse-heating experiments. Experiments were performed where normal spectral emissivity at 633 nm was measured simultaneously with surface radiance temperature, resistance, and/or voltage drop across the specimen. It was observed that a phase transformation, as indicated either by an arrest in the specimen radiance temperature or changes in the resistance and/or voltage drop, generally caused a change in normal spectral emissivity. Experiments were conducted on cobalt, iron, hafnium, titanium, and zirconium to detect solid–solid phase transformations. Similar experiments were also performed on niobium, titanium, and the alloy 85titanium–15molybdenum (mass%) to detect solid–liquid phase transformations (melting).     

On the use of Ti50Be40Zr10 as a spring material

A useful figure-of-merit for spring material candidates is derived and computed for a select group of iron and titanium alloys. Two commercially available amorphous alloys are shown to have considerably more attractive figures-of-merit than do several crystalline alloys. Since springs left in a stressed condition must resist stress relaxation, tests of this type were performed on an amorphous titanium alloy. The results indicate that relaxation kinetics depend upon initial heat treatment but are independent of initial applied stress. This behaviour is shown to be consistent with hyperbolic flow in the limit of low stress. A simple least-squares analysis of the relaxation kinetics yields an activation energy of 67 kJ mol–1. When used for springs, this alloy should be heat treated.A U.S. Department of Energy Facility.