Elsevier

Electrochimica Acta

Volume 55, Issue 3, 1 January 2010, Pages 759-770
Electrochimica Acta

Electrochemical behavior of centrifuged cast and heat treated Ti–Cu alloys for medical applications

https://doi.org/10.1016/j.electacta.2009.09.016Get rights and content

Abstract

The aim of this study was to evaluate the general electrochemical corrosion resistance of Ti–5, 7.1 and 15 wt.% Cu alloys with a view to medical applications. A centrifuged casting set-up and a solution heat treatment at 900 °C for 2 h were used to prepare the samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.15 M NaCl solution at 25 °C. An equivalent circuit analyses was also conducted. It was found that the corrosion rate increased with increasing Cu content. The results have shown that the addition of Cu has not stabilized the β phase. Martensite and Ti2Cu intermetallic particles provided by casting and heat treatment processes, respectively, have important roles on the resulting impedance parameters and passive current densities of the Ti–Cu alloys.

Introduction

Due to their outstanding corrosion resistance as well as high specific strength, commercial application of Ti and Ti alloys is continuously growing in the last few decades. Titanium is extensively applied in several markets including automotive, power generation, marine and offshore, architecture, chemical, petrochemical, sport and medicine. Commercially pure Ti does not hold optimized mechanical behavior, which restricts its use in a number of fields, especially when high mechanical strength is necessary. Nevertheless, its mechanical behavior may be notably enhanced by alloying titanium with a number of other elements. Addition of alloying elements allows one to manipulate the balance of stable and metastable phases in the final microstructure and hence, titanium alloy mechanical properties, including mechanical strength, ductility and elastic modulus.

In dentistry, titanium alloys are applied in the manufacture of a number of dental devices such as bridges and partial denture frameworks [1], [2], [3], [4]. When in use, these devices are frequently subjected to relatively high mechanical stresses. Consequently, in order to reduce elastic deformation, they must be manufactured with materials having a high modulus of elasticity. On the other hand, the most utilized route in the making of dental devices is the precision casting process. Casting of Ti alloys is not a simple task as it always comprises several complexities, including high melting temperature, affinity with interstitial elements and considerable activity with mould materials. An alternative method employed to minimize these difficulties as well as to improve the mechanical behavior of c.p. Ti (commercially pure Ti) is to make use of copper as an alloying element.

Ti alloyed with Cu is reported to provide adequate biocompatibility [5], reasonable corrosion resistance [4], [6] and comparatively lower melting temperature [4], [7], [8], which greatly favors casting procedures. Since the 1950s, a number of studies have been focused on the Ti–Cu alloys for industrial applications. Results from those investigations show that Ti–Cu alloys may present very high mechanical strength associated with good formability [4].

In the last 20 years, dental titanium casting technology has made considerable progress. Kikuchi et al. [4] have shown that centrifuged cast Ti–Cu alloys permits yield strengths as high as 650 MPa to be attained, a value which is considerable higher than that of c.p. Ti and of most of the dental casting Co–Cr alloys. Sun et al. [9] showed that the application of a solution heat treatment has strengthened Ti–2.5%Cu alloy due to the formation of Ti2Cu intermetallic particles after β phase decomposition into α-Ti and Ti2Cu.

One of the most inconvenient aspects in terms of biomaterials applications is the degradation caused by corrosion, which occurs due to the material interaction with body physiological fluids [10], [11], [12]. As a result, the corrosion resistance can be considered a fundamental characteristic of biomaterial components and it is associated with ions release of metallic species, which can be harmful for the organism. Degradation generally occurs on the surface of Ti-based components due to the chemical and electrochemical reactions between titania and ions in the surrounding media. In this context, electrochemical studies are very useful to establish correlations between the resulting Ti alloy microstructure and the kinetics of electrochemical corrosion and oxide film formation. The literature is scarce on reports concerning studies of Ti–Cu alloys assessing the influence of microstructure on the surface corrosion resistance. Takada and Okuno [13] studied the anodic polarization curves of a series of binary Ti–Cu alloys. Based on their experimental results, it was found that these alloys had the same anodic polarization curves as titanium. However, the precipitation of intermetallic Ti2Cu particles has provided a small increase in current density in the transpassive region beyond 1.4 V [13]. It is well known that the microstructure of metallic alloys has an important role on the mechanical properties and corrosion behavior of as-cast components [14], [15], [16], [17], [18], [19].

The aim of this study was to evaluate the general electrochemical corrosion resistance of Ti–5, 7.1 and 15 wt.% Cu alloy samples obtained by centrifuged casting and heat treatment. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were carried out by immersion in a naturally stagnant 0.15 M NaCl solution at 25 °C.

Section snippets

Experimental procedure

Commercially pure metals (Ti 99.86 wt.% and Cu 99.99 wt.%) were used to prepare the Ti–5%Cu, Ti–7.1%Cu and Ti–15%Cu alloys (weight percent) which are: hypoeutectoid, nearly eutectoid and hypereutectoid alloys, respectively, according to the Ti–Cu equilibrium phase diagram of Fig. 1 [4], [8]. These alloys were melted in an arc-melting furnace with a non-consumable tungsten electrode and water-cooled copper hearth under ultra-pure argon atmosphere. Initially, vacuum of 10−3 atm was created and then

Microstructures

SEM micrographs of centrifuged cast and heat treated Ti–Cu alloy samples are shown in Fig. 2, Fig. 3, respectively. As expected, rapid cooling of β grains limits the eutectoid coupled growth and gives rise to martensite formation. Fig. 2(a) depicts a typical micrograph of Ti–5%Cu alloy (hypoeutectoid) showing martensite structure only. However, under higher magnification (Fig. 2(d)), one may observe, besides martensite, evidences of α-Ti combined with Ti2Cu intermetallic, which were formed by

Conclusions

From the present experimental investigation with centrifuged cast and heat treated Ti–Cu alloy samples the follow conclusions can be drawn:

  • (i)

    The Ti2Cu intermetallic compound is always present in the microstructure, no matter the used processing condition. In addition, with increasing alloy Cu content, the volumetric fraction of Ti2Cu also increases.

  • (ii)

    Considering the X-ray diffraction results, one may conclude that is virtually impossible to distinguish α phase and martensite. Identification of both

Acknowledgements

The authors acknowledge financial support provided by FAPESP (The Scientific Research Foundation of the State of São Paulo, Brazil), FAEPEX-UNICAMP, CNPq (The Brazilian Research Council) and the Brazilian Synchrotron Light Laboratory (LNLS) for the use of their AFM mycroscope.

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