Effect of surface nanocrystallization on the corrosion behavior of Ti–6Al–4V titanium alloy

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Highlights

  • A nanocrystalline surface layer was formed on a Ti–6Al–4V alloy by means of SMAT.

  • Electrochemical experiments identified beneficial impacts of SMAT on corrosion behavior of Ti6Al4V alloy.

  • XPS analysis has highlighted the presence of various oxides in particular TiO2.

Abstract

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline surface layer was formed on a Ti–6Al–4V alloy. The corrosion behavior of Ti–6Al–4V in a Ringer's solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Overall results of all studies identified beneficial impacts of SMAT on corrosion behavior of Ti–6Al–4V alloy.

The surface oxide film formed on Ti–6Al–4V and its stability in biological environments play a decisive role for the biocompatibility of implants. In this study, passive oxide films formed on Ti–6Al–4V surfaces and their natural growth in a Ringer's solution have been investigated by microhardness and X-ray photoelectron spectroscopy (XPS).

Introduction

Today, titanium alloys are one of the most important metallic materials used in orthopedics and dental surgery [1]. Despite their high corrosion resistance in the physiological environments and their high specific strength, the revision surgeries of damage at the metal surface and consequent periprosthetic metallosis indicate that failures of the implant occur in vivo.

In addition, the degradation of metallic biomaterials in vivo mainly due to combined wear and corrosion processes results in the formation of particulate and ionic metallic debris, which are recognized as major factors limiting the lifespan of joint replacements [2].

Generally the passive films formed on titanium and its alloys consist mainly of amorphous titanium dioxide [3], [4]. The physico-chemical and electrochemical properties of the oxide film as well as its long-term stability in biological environments play a major role in the bio-compatibility of titanium implants [5].

Most material failures occur on the surfaces including stress fracture, wear and corrosion. Accordingly, controlling the surface properties can effectively improve the overall behavior of the material.

Thus, many efforts in the last two decades have focused to solve this problem in order to confer work hardening to the surface region of the workpiece and to improve the corrosion resistance. Recently, a new family of severe plastic deformation (SPD) processes has attracted considerable scientific interest in order to generate nanocrystalline surface layer. These later advances enhance several mechanical properties while keeping a reasonable corrosion resistance [6], [7]. These processes include for example ultrasonic shot peening (USSP) [8], equal channel angular pressing (ECAP) [9], surface nanocrystallization and hardening (SNH) [10], or surface mechanical attrition treatment (SMAT) [11].

Among the above-mentioned techniques, SMAT is a relatively new process derived from conventional shot peening (SP). Both of these processes are based on repeated impacts on the sample surface in order to induce compressive residual stresses as well as work hardening to the surface region of the workpiece [12], [13].

However, compared with SP, SMAT induces a multidirectional plastic deformation and deeper compressive residual stress along with the creation of a superficial nanocrystalline layer [14]. This later is mainly due to the difference of the shot size, impact velocities and especially the multidirectional plastic deformation. In fact, in the SP method, a stream of spherical shots with sizes in the range 0.25–1 mm is blasted against the workpiece at impact velocities in the interval 20–150 m s 1, under a controlled atmosphere [15]. While, the SMAT method entails impacting repeatedly the workpiece multidirectionally, but using balls with sizes typically in the range 2–10 mm and impact velocities in the interval 5–15 m s 1 [16].

The repeated impacts, in the SP method, confer residual compressive stresses and work hardening to the surface region of the workpiece [12], [13], but unfortunately do not always induce surface nanocrystallization [12], [13], [15]. On the other side, SMAT involves higher kinetic energies than SP, and is therefore expected to result in thicker nanocrystalline and work-hardened surface layers as well as deeper surface regions with larger residual compressive stresses [14].

SMAT has been developed to induce in the workpiece routinely the desired nanocrystalline (nc) surface layer [17]. Using SMAT, nanocrystalline layers in the surface of various materials, such as stainless steel [19], [21], pure Cu [20], pure Fe [8], Al alloy [18], and Mg alloy [22] have been successfully produced.

Nevertheless, in the literature, few investigations were carried out to predict the effect of SMAT on the corrosion behavior of biomaterials.

Therefore, the aim of this investigation is to characterize and to discuss the microstructure evolution and the corrosion behavior of Ti–6Al–4V subjected to surface mechanical attrition treatment. The samples were subjected to different conditions of SMAT in order to form a nanocrystalline layer on the surface. Electrochemical tests were carried out in Ringer's solution to investigate the corrosion behavior of this material. In addition, Vickers hardness tests as well as X-ray photoelectron spectroscopy (XPS) analyses were performed on titanium alloy prior to and after SMAT treatment.

Section snippets

Material

The material tested in this study is a titanium alloy Ti–6Al–4V, its chemical composition is given in Table 1.

The specimens, 20 mm × 10 mm × 3 mm in dimensions were cut from rectangular plate. They were ground and successively washed ultrasonically with acetone and distilled water, and dried before SMAT. The material was delivered without prior treatment and without aging.

Surface mechanical attrition treatment (SMAT)

This process is based on the vibration and movement of spherical shot by means of an ultrasonic power generator [23]. The pellets

Microstructural observations

The initial microstructure of Ti–6Al–4V sample can be seen in Fig. 2. We can distinguish:

  • The primary α phase appears as nodular, the average diameter of the nodules is of the order of 10 μm.

  • The secondary α phase has a lamellar structure.

  • The residual β phase between the α lamellae.

To confirm the presence of a nanostructure, TEM observations of the topmost surface were realized. An examination of the surface treated by SMAT reveals a microstructure characterized by nanometric grains uniformly

Surface hardness

Microhardness tests have been performed prior to and after SMAT treatment to gain information regarding the effect of SMAT on the mechanical properties of the surface.

Fig. 5 shows the surface hardness as a function of loading after SMAT (conditions 1 and 2). By comparing the obtained results, we can see that samples treated by SMAT have a maximum hardness in comparison with the untreated sample.

Indeed, the treated sample (SMAT2) has a maximum hardness of 450 Hv0.2, while for the untreated

Open circuit potential

The evolution of the open circuit potential (OCP) of Ti–6Al–4V obtained in Ringer's solution (pH = 7.2, at 37 °C) as a function of time is presented in Fig. 6.

Initially, OCP was approximately − 0.55 V/SCE for the untreated Ti–6Al–4V. During the first hours of immersion, we notice a sudden displacement of OCP towards positive potentials. This initial rise could be related to the formation of an oxide film on the alloy surface. This layer may improve the stability of the material during the immersion.

Chemical analysis of surfaces by XPS

The XPS characterization was performed on three samples of titanium alloy (Untreated, SMAT1 and SMAT2). Analyses were performed on samples after immersion in Ringer's solution at 37 °C for one week. These samples were removed from the corrosive medium, rinsed with distilled water and ethanol and then dried.

The composition of titanium surfaces before and after SMAT may be determined on the basis of survey spectra (Fig. 11, Fig. 12, Fig. 13).

Typical XPS survey spectra evidenced the presence of Ti,

Conclusion

Initially, nanocrystalline surface layer was successfully synthesized on Ti–6Al–4V alloy by SMAT. The average grain size in surface layer was 50 nm.

Then, an electrochemical study has highlighted the effect of SMAT on the corrosion behavior of Ti–6Al–4V alloy, where the stability of the passive film was observed and an increase of free potential of samples treated with SMAT.

Surface treatments, such as SMAT, bring about an increase of passive and corrosion values as a consequence of the formation

Acknowledgments

The authors wish to thank the Regional Council of Champagne-Ardenne (France) for financial support (NANOSURF project).

References (35)

  • M. Geetha et al.

    Prog. Mater. Sci.

    (2009)
  • H. Gleiter

    Prog. Mater. Sci.

    (1989)
  • Y. Lin

    Acta Mater.

    (2006)
  • X. Wu et al.

    Acta Mater.

    (2002)
  • Y. Iwahashi et al.

    Acta Mater.

    (1998)
  • K. Dai et al.

    Acta Mater.

    (2004)
  • K. Lu et al.

    Mater. Sci. Eng. A

    (2004)
  • L. Wagner

    Mater. Sci. Eng., A

    (1999)
  • K. Dai et al.

    Mater. Sci. Eng., A

    (2007)
  • J.W. Tian et al.

    Mater. Sci. Eng., A

    (2008)
  • N.R. Tao et al.

    Acta Mater.

    (2002)
  • H.W. Zhang et al.

    Acta Mater.

    (2003)
  • Y.S. Zhang et al.

    Wear

    (2006)
  • M. Chemkhi et al.

    Surf. Coat. Technol.

    (2013)
  • Y.H. Wei et al.

    J. Alloys Compd.

    (2008)
  • J. Tian et al.

    Mater. Sci. Eng., A

    (Oct. 2008)
  • D. Song et al.

    Corros. Sci.

    (2011)
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