Cora Vasilescu

Synthesis and characterisation of a new superelastic Ti–25Ta–25Nb biomedical alloy

In this study, a new Ti-25Ta-25Nb (mass%) beta alloy was synthesised by cold crucible semi-levitation melting. This technique made it possible to obtain homogeneous ingots although the elements used have very different melting points. After melting, a thermo-mechanical treatment was applied in order to obtain a perfectly recrystallised beta microstructure. For this alloy composition, the tensile tests showed a very low Youngs modulus associated with an important super-elastic behaviour, which contributes to decrease the elastic modulus under stress and to increase the recoverable strain. On the other hand, the corrosion tests, which were carried out in a neutral Ringer solution, indicated a corrosion resistance higher than that of the commercially pure CP Ti alloy. These results show that this new alloy possesses all the characteristics necessary for its long-term use in medical implants.

Mechanical and corrosion resistance of a new nanostructured Ti–Zr–Ta–Nb alloy

In this work, a multi-elementary Ti-10Zr-5Nb-5Ta alloy, with non-toxic alloying elements, was used to develop an accumulative roll bonding, ARB-type procedure in order to improve its structural and mechanical properties. The alloy was obtained by cold crucible semi-levitation melting technique and then was ARB deformed following a special route. After three ARB cycles, the total deformation degree per layer is about 86%; the calculated medium layer thickness is about 13 μm. The ARB processed alloy has a low Youngs modulus of 46 GPa, a value very close to the value of the natural cortical bone (about 20 GPa). Data concerning ultimate tensile strength obtained for ARB processed alloy is rather high, suitable to be used as a material for bone substitute. Hardness of the ARB processed alloy is higher than that of the as-cast alloy, ensuring a better behaviour as a implant material. The tensile curve for the as-cast alloy shows an elastoplastic behaviour with a quite linear elastic behaviour and the tensile curve for the ARB processed alloy is quite similar with a strain-hardening elastoplastic body. Corrosion behaviour of the studied alloy revealed the improvement of the main electrochemical parameters, as a result of the positive influence of ARB processing. Lower corrosion and ion release rates for the ARB processed alloy than for the as-cast alloy, due to the favourable effect of ARB thermo-mechanical processing were obtained.

In vitro biocompatibility and corrosion resistance of a new implant titanium base alloy

One objective of this work was to study the corrosion resistance of the new implant Ti–10Zr–5Ta–5Nb alloy in physiological fluids of different pH values, simulating the extreme functional conditions. Another objective was in vitro biocompatibility evaluation of the new alloy using human fetal osteoblast cell line hFOB 1.19. Cytocompatibility was assessed by determination of possible material cytotoxic effects, cell morphology and cell adhesion. The thermo-mechanical processing of the new implant alloy consisted in plastic deformation (almost 90%) performed by hot rolling accompanied by an initial and final heat treatment. The new Ti–10Zr–5Ta–5Nb alloy presented self-passivation, with a large passive potential range and low passive current densities, namely, a very good anticorrosive resistance in Ringer solution of acid, neutral and alkaline pH values. Cell viability was not affected by the alloy substrate presence and a very good compatibility was noticed.

Microstructure, mechanical, and anticorrosive properties of a new Ti-20Nb-10Zr-5Ta alloy based on nontoxic and nonallergenic elements

For an alloy to be suitable for use as an implant material, it must have a low specific weight and Young’s modulus, good mechanical properties that are similar to those of bone, and very good corrosion resistance and biocompatibility. In this study, we have developed a novel Ti-20Nb-10Zr-5Ta alloy that is composed of nontoxic, nonallergenic, corrosion-resistant elements. This alloy has low specific weight and Young’s modulus and good mechanical properties. It has a fine microstructure with a matrix that is mainly composed of the β phase and some α phase due to recrystallization during cooling. It shows elastoplastic behavior with a fairly linear elastic behavior and low Young’s modulus (59 GPa). In addition, its ultimate tensile strength, 0.2% yield strength, and hardness are higher than those of CP Ti, commercial Ti-6Al-4V, and similar β-type alloys. It exhibited a very stable passive state and its electrochemical parameters and corrosion and ion release rates were better than those of CP Ti in Ringer’s solutions of different pH values that simulate the severe functional conditions of an implant; this is attributable to the beneficial influence of the alloying elements and to the better protective properties of the coated passive film.

Design of a nitrogen-implanted titanium-based superelastic alloy with optimized properties for biomedical applications.

In this study, a superelastic Ni-free Ti-based biomedical alloy was treated in surface by the implantation of nitrogen ions for the first time. The N-implanted surface was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and secondary ion mass spectroscopy, and the superficial mechanical properties were evaluated by nano-indentation and by ball-on-disk tribological tests. To investigate the biocompatibility, the corrosion resistance of the N-implanted Ti alloy was evaluated in simulated body fluids (SBF) complemented by in-vitro cytocompatibility tests on human fetal osteoblasts. After implantation, surface analysis methods revealed the formation of a titanium-based nitride on the substrate surface. Consequently, an increase in superficial hardness and a significant reduction of friction coefficient were observed compared to the non-implanted sample. Also, a better corrosion resistance and a significant decrease in ion release rates have been obtained. Cell culture experiments indicated that the cytocompatibility of the N-implanted Ti alloy was superior to that of the corresponding non-treated sample. Thus, this new functional N-implanted titanium-based superelastic alloy presents the optimized properties that are required for various medical devices: superelasticity, high superficial mechanical properties, high corrosion resistance and excellent cytocompatibility.

Surface characterization and biocompatibility of titanium alloys implanted with nitrogen by Hardion+ technology

In this study, the new Hardion+ micro-implanter technology was used to modify surface properties of biomedical pure titanium (CP-Ti) and Ti–6Al–4V ELI alloy by implantation of nitrogen ions. This process is based on the use of an electron cyclotron resonance ion source to produce a multienergetic ion beam from multicharged ions. After implantation, surface analysis methods revealed the formation of titanium nitride (TiN) on the substrate surfaces. An increase in superficial hardness and a significant reduction of friction coefficient were observed for both materials when compared to non-implanted samples. Better corrosion resistance and a significant decrease in ion release rates were observed for N-implanted biomaterials due to the formation of the protective TiN layer on their surfaces. In vitro tests performed on human fetal osteoblasts indicated that the cytocompatibility of N-implanted CP-Ti and Ti–6Al–4V alloy was enhanced in comparison to that of the corresponding non treated samples. Consequently, Hardion+ implantation technique can provide titanium alloys with better qualities in terms of corrosion resistance, cell proliferation, adhesion and viability.

Corrosion resistance improvement of titanium base alloys

The corrosion resistance of the new Ti-6Al-4V-1Zr alloy in comparison with ternary Ti-6Al-4V alloy in Ringer-Brown solution and artificial Carter-Brugirard saliva of different pH values was studied. In Ringer-Brown solution, the new alloy presented an improvement of all electrochemical parameters due to the alloying with Zr; also, impedance spectra revealed better protective properties of its passive layer. In Carter-Brugirard artificial saliva, an increase of the passive film thickness was proved. Fluoride ions had a slight negative influence on the corrosion and ion release rates, without to affect the very good stability of the new Ti-6Al-4V-1Zr alloy.

Anodic passivity of some titanium base alloys in aggressive environments

The passivity of titanium, binary Ti-15Mo and ternary Ti-15Mo-5Al alloys in hydrochloric acid solutions was studied by potentiostatic, potentiodynamic, linear polarization and electrochemical impedance spectroscopy (EIS) techniques. The anodic passivity of binary Ti-15Mo and ternary Ti-15Mo-5Al titanium alloys differs from that of the base metal in hydrochloric acid solutions. The corrosion potentials of both alloys are nobler than of the titanium because the beneficial effect of molybdenum. The critical passivation current density for binary Ti-15Mo alloy is higher than of titanium; this fact can be explained by the instability of the constituent phases in hydrochloric acid solutions. Ternary Ti-15Mo-5Al alloy exhibits two critical passivation current densities (icr1 and icr2) with higher values than of the base metal and two critical passivation potentials (Ecr1 and Ecr2); at the first critical passivation potential (Ecr1) the porous titanium trioxide (Ti3O5) is formed and at the second critical passivation potential (Ecr2) this oxide is converted to a still higher valence oxide, the compact and protective titanium dioxide (TiO2). The dissolution current densities in the passive range of alloys are higher than of the base metal due the dissolution of the alloying elements in this potential range. The alloys are more resistant than titanium presenting lower corrosion rates. A three time constants equivalent circuit was fitted: one time constant is for the double layer capacity (Cdl) and for the passive film (Rp); another time constant is for the charge transfer reactions visualised by a constant phase element (CPE) and a resistance (R1); the third time constant is for diffusion processes through the passive film represented by a resistance (R2) and a Warburg element (W). Anodische Passivitat von verschiedenen Titanbasislegierungen in aggressiven Umgebungen Die Passivitat von Titan sowie der binaren Legierung Ti-15Mo und der ternaren Legierung Ti-15Mo-5Al wurde in Salzsaurelosungen mittels potentiostatischer, potentiodynamischer und linearer Polarisation sowie elektrochemischer Impedanzspektroskopie (EIS) untersucht. Die anodische Passivitat der binaren Ti-15Mo und der ternaren Ti-15Mo-5Al Legierungen unterscheidet sich in Salzsaurelosungen von dem reinen Titan. Die Korrosionspotentiale beider Legierungen sind edler als das des Titans aufgrund des positiven Einflusses des Molybdans. Die kritische Passivierungsstromdichte fur die binare Ti-15Mo-Legierung ist hoher als die von Titan, was durch die Instabilitat der Bestandteile in den Salzsaurelosungen erklart werden kann. Die ternare Ti-15Mo-5Al-Legierung weist zwei kritische Passivierungsstromdichten (icr1 und icr2) auf, die hoher liegen als fur das reine Metall, und zwei kritische Passivierungspotentiale (Ecr1 und Ecr2); beim ersten kritischen Passivierungspotential (Ecr1) wird das porose Titantrioxid (Ti3O5) gebildet und beim zweiten kritischen Passivierungspotential (Ecr2) wird dieses Oxid in ein Oxid mit noch hoherer Wertigkeit, das kompakte und schutzende Titandioxid (TiO2), umgewandelt. Die Auflosungsstromdichten im Passivbereich der Legierungen sind hoher als beim reinen Metall aufgrund der Auflosung der Legierungselemente in diesem Potentialbereich. Die Legierungen sind widerstandsfahiger als das Titan, was sich in niedrigeren Korrosionsgeschwindigkeiten bemerkbar macht. Ein elektrischer Aquivalentschaltkreis wurde angepasst: eine Zeitkonstante steht fur die Doppelschichtkapazitat (Cdl) und fur den Passivfilm (Rp); eine weitere Zeitkonstante steht fur die Ladungsubergangsreaktionen, die durch ein Konstantphasenelement (CPE) und einen Widerstand (R1) dargestellt werden; die dritte Zeitkonstante steht fur den Diffusionsprozess durch den Passivfilm, was durch einen Widerstand (R2) und ein Warburgelement (W) reprasentiert wird.

Microstructure, mechanical properties, and corrosion resistance of Ti-20Zr alloy in undoped and NaF doped artificial saliva

The corrosion behavior of a new, advanced Ti-20Zr alloy with α+β microstructure (determined by optical microscopy, XRD, and SEM) and very good mechanical properties (obtained from the stress-strain curve) is studied in this paper. The composition of the alloy native passive film was determined from a XPS analysis and the long-term corrosion resistance in undoped and doped states with 0.05M NaF artificial Carter-Brugirard saliva of different pH values, simulating the severe functional conditions of a dental implant, was analyzed by electrochemical methods. This alloy possesses an advantageous balance between good mechanical resistance and plasticity and Young’s modulus and exhibits more favorable electrochemical parameters and corrosion resistance than CP Ti due to its more resistant passive layer containing Ti2O3, TiO2, and ZrO2 protective oxides. After 1000 h of immersion in saliva, the protective properties of the alloy were enhanced due to the deposited surface layer that incorporated protective phosphates (shown by SEM and XPS).

CORROSION BEHAVIOR OF SOME TITANIUM BASE ALLOYS IN ACID SOLUTIONS

Abstract Corrosion and passivation behavior of new, ternary titanium alloys Ti-10Mo-10Al and Ti-15Mo-5Al in nitric acid solutions of different concentrations (20% and 60%) and temperatures (25○, 50○, and 75○C) is presented in this paper. Electrochemical impedance spectroscopy (EIS) data were obtained at different potentials as Bode plots. The spectra show some deviations from the ideal behavior, which demonstrate the necessity to consider a complex equivalent circuit with two time constants. This equivalent circuit is composed of the uncompensated ohmic resistance of the solution (R Ω ), the double layer capacity (C dl ) in parallel with the polarization resistance (R p ) for a passive film, and a Warburg element (W) associated with a resistance (R 1 ) for the diffusion processes. The anodic polarization curves present the self-passivation of these alloys. Both titanium and its ternary alloys directly pass into a passive state, due to the fact that, in strong oxidizing nitric acid solutions, the titanium directly oxidizes to Ti 4+ ions and spontaneously forms titanium dioxide TiO 2 . The passivation of the titanium alloys is due to the compact and protective film containing titanium dioxide, TiO 2 , and some insoluble molybdenum oxide, MoO 3 .