The effect of Sn addition on phase stability and phase evolution during aging heat treatment in Ti–Mo alloys employed as biomaterials
Introduction
In recent decades, the Ti–6Al–4V alloy, which was originally developed for the aerospace industry, has been used extensively in the manufacture of orthopedic implant devices, primarily due to its high corrosion resistance, high mechanical strength and satisfactory biocompatibility [1], [2], [3]. However, its elements may be the source of severe long-term health problems. A number of investigations have found that the release of aluminum and vanadium ions may result in inflammatory reactions with body tissues and neurological disorders, such as Alzheimer's disease [4], [5].
Although the Ti–6Al–4V alloy has a low elastic modulus in comparison with Fe and Co-based alloys, its stiffness is still much greater than that of bone. A mismatch between the stiffness of implants and bone results in an insufficient load transfer from the orthopedic implant to the adjacent bone, a phenomenon known as stress shielding [6]. According to Wolff's Law, remodeling of a bone's internal structure and external shape may occur in response to mechanical stress; therefore, a lack of mechanical stress causes bone mass to be lost around the implant, leading to failure on the bone/implant interface [6]. As a result, to overcome the mechanical behavior and toxicity shortcomings of the Ti–6Al–4V alloy, metastable β Ti alloys have been introduced for biomedical applications with the most promising alloys being those composed of Ti and Mo.
Metastable β Ti–Mo alloys have high biocompatibility, low elastic moduli and enhanced mechanical behavior. Mo is a powerful β stabilizing element and the addition of only 10 wt.% Mo to Ti can completely stabilize the β phase at room temperature after water quenching [7], [8]. In addition, cytotoxicity studies have suggested that Mo ions are less toxic than Al and V ions [9], [10], [11].
Their solute content, thermo-mechanical processing conditions and phase transformation pathway imply that metastable β Ti–Mo alloys can display a variety of microstructures and phase arrangements, which directly affect their mechanical behavior. It is well known that rapid cooling applied to near-β Ti alloys at temperatures above the β transus may result in the appearance of the orthorhombic martensite phase (α″), the precipitation of the metastable athermal ω phase and β phase retention. If a posterior aging heat treatment is performed, α phase precipitation may occur, which enhances the alloy's mechanical behavior. Decomposition of these metastable phases and α phase precipitation comprises an intricate transformation pathway that includes the reverse transformation of α″ phase into β phase and the appearance of the isothermal ω phase, which can significantly affect the heterogeneous nucleation of the α phase. The isothermal ω phase is assumed to be a transition phase in the transformation of the β phase into α phase and may be formed during aging heat treatments at intermediate temperatures. The presence of the athermal ω or isothermal ω phases usually causes embrittlement of Ti-based alloys; therefore, its presence must be avoided. According to Maeshima et al. [12], Sn addition to Ti alloys can suppress the ω phase formation. These authors have evaluated the shape memory properties of Ti-5 or 6 mol% Mo with additions of 1 to 5 mol% Sn. Due to Sn addition, isothermal and athermal ω phase formation was suppressed and Ti–5Mo–1, 2 and 3Sn alloys presented a typical shape memory behavior, with no brittle fracture after 5 loading–heating cycles. After solution heat treatment and water quenching, the only phase detected by X-ray diffraction in Ti–5Mo-1 to 5Sn alloys was the β phase. This indicates that Ms (martensite start) temperature decreases with Sn addition, the same effect observed in Ti–Nb–Sn alloys [13].
The relationship among alloy composition, heat treatment conditions, microstructure and mechanical behavior in Ti–Mo–Sn alloys is of paramount importance because it is the basis for the development of new β Ti alloys. Therefore, the present study discusses the effect of Sn addition on the microstructure of Ti–8Mo–xSn (x = 0, 1.5 and 6.0 wt.%) and it is mainly focused on the phase transformation temperatures, mechanisms of α phase precipitation and also, microstructure–property relationships after aging heat treatments performed under different heating rates.
Section snippets
Experimental details
Samples with nominal compositions of Ti–8Mo, Ti–8Mo–1.5Sn and Ti–8Mo–6Sn (wt.%) were arc melted under an inert argon atmosphere using non-consumable tungsten electrodes. The ingots were melted at least five times to achieve high chemical homogeneity. The samples' chemical composition was determined by X-ray fluorescence spectroscopy (Rigaku RIX 3100). The amount of interstitial elements O and N was obtained using a Leco TC400 analyzer. These samples were then heat treated in quartz tubes under
Results and discussion
Table 1 presents the chemical composition of the Ti–8Mo–xSn (x = 0, 1.5 and 6.0 wt.%) experimental alloys. The interstitial (oxygen and nitrogen) contents conform to the ASTM-E-1409-13 standard [15]. The oxygen content is in the range of 0.15 wt.% to 0.17 wt.%.
It is expected that the addition of Sn to Ti–Mo alloys causes suppression of the ω phase [16]. In addition, although Sn is considered a neutral Ti alloying element, it can alter the phase transformation temperatures of Ti–Mo–Sn system and,
Conclusions
It follows from this study that the addition of Sn into a Ti–Mo system decreases the elastic modulus of the samples by suppressing ω phase precipitation. The results suggest that the electronic approach to phase prediction (Bo × Md) cannot be considered completely accurate regarding the elastic modulus. The diagram suggests that the addition of Sn increases the elastic modulus, but this modulus decreases due to suppression of the metastable phases.
The microstructural analyses suggest that Sn
Acknowledgments
The authors gratefully acknowledge the LME/LNNano/CNPEM for access to the SEM/TEM facilities; the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) Grant # 2014/06099-1 and CNPq (National Council for Scientific and Technological Development) Grant # 484379/2012-7 for their financial support.
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