Impacts of trace carbon on the microstructure of as-sintered biomedical Ti–15Mo alloy and reassessment of the maximum carbon limit
Introduction
Beta-titanium alloys that can offer a low modulus of elasticity, high strength, good fatigue strength, excellent corrosion resistance and biocompatibility continue to attract significant attention in the medical and surgical device industry. The Ti–15Mo alloy (in wt.% unless stated otherwise) is one such important alloy, where a minimum 10 wt.% Mo is necessary to ensure near full β phase retention at room temperature [1]. It was developed as a generic metastable β-Ti alloy but it can be manufactured as a two-phase (β + α) alloy as well in order to achieve an optimum combination of strength and ductility when aged after cooling from a forging or solution heat-treatment temperature [2], [3]. To address the safety concerns over vanadium and aluminium for medical applications, Ti–15Mo, together with four other β-Ti alloys, was proposed and evaluated in the late 1980s for surgical implant applications in the USA [4], [5]. The assessments, including two-year implantation tests in animals, revealed a satisfactory level of localized biological response [5]. Further research has been carried out since on Ti–15Mo and other binary Ti–Mo alloys for medical applications [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. These studies confirmed the excellent suitability of Ti–15Mo, including its biocompatibility for medical applications from different perspectives. Sufficient data permitted the creation of an ASTM materials standard for wrought Ti–15Mo alloy for surgical implant applications [16], originally approved in 2001 [17], designated as F2066 (UNS R58150).
Various product forms of Ti–15Mo, including bar, rod, sheet and wire made to specifications by Allegheny Technologies Incorporated (ATI), are now commercially available for orthopaedic, trauma, spinal, dental, orthodontic and cardiovascular applications [2]. Also, Ti–15Mo can be manufactured into an ultra-high-strength (yield strength >1300 MPa) β-Ti material for orthopaedic implant applications that are more concerned with higher strength than lower modulus due to a number of notable benefits [16]. On the other hand, there are many orthopaedic implant applications that will benefit from a lower modulus. Recent developments have indicated that porous Ti–15Mo fabricated by powder metallurgy (PM) routes [18], [19] can offer the same modulus of elasticity as that of human bones. In addition, powder metallurgy is capable of manipulating the pore sizes to best accommodate tissue growth. The combination of these two features is expected to encourage more innovative designs and fabrications of Ti–15Mo by PM routes for implant applications.
Apart from the mechanical property requirements, corrosion of metallic implants in human bodies is a pertinent clinical issue caused by the electrochemical attack of human body fluids and can lead to potential mechanical failures [20]. As with other metals, the corrosion resistance of a β-Ti alloy implant depends critically on its chemical homogeneity and microstructural uniformity. The influence of impurity is an important concern in this regard. Recent work has revealed that impurities at the ppm level can markedly affect the microstructure of as-sintered unalloyed Ti by forming additional grain boundary (GB) phases [21]. The ASTM standard F 2066-08 defined the chemical requirements for wrought Ti–15Mo for surgical implant applications as follows (maximum limit in wt.%): 0.05N, 0.015H, 0.20O, 0.10C, 0.10Fe, 14.00–16.00Mo and balance for Ti [17]. Of these impurities, Fe is a β-Ti stabilizer and a fast diffuser in β-Ti. In addition, Fe and Ti do not form Ti–Fe intermetallics in Ti alloys containing even up to 6.0 wt.% Fe [22]. Hence, Fe is not a concern in terms of microstructural uniformity. As for O, N and H, the specified limit for each impurity is much lower than their respective solubility limits in β-Ti. Therefore they are not expected to be problematic. However, the specified limit for carbon seems to be just on the borderline. Fig. 1 shows the Ti–C binary phase diagram up to 1.2 wt.% C. To avoid TiC formation in β-Ti, the carbon content should be limited to ∼0.08 wt.% by Fig. 1. This may be regarded as the basis of ASTM standard F 2066–08 for the maximum carbon limit of 0.1 wt.% specified for Ti–15Mo.
The formation of brittle titanium carbides in a biomedical titanium alloy, particularly when it forms along the GB with a complex three-dimensional (3-D) morphology, has the risk of decreasing the fatigue properties, ductility and corrosion resistance of the alloy while increasing the modulus of elasticity. This could lead to unexpected consequences and should be avoided metallurgically. This paper presents a detailed microstructural study of an as-sintered biomedical Ti–15Mo alloy with a focus on the impacts of trace carbon, including use of 3-D tomography to determine the 3-D morphology of the identified GB carbon-enriched phase. Based on the experimental findings, the maximum carbon limit for Ti–15Mo has been reassessed. For comparison, similar studies have been performed on as-sintered biomedical unalloyed Ti, Ti–6Al–4V and Ti–16Nb.
Section snippets
Materials and sample preparation
Titanium hydride (TiH2) powder (99.5% purity; 95–106 μm) and elemental Mo powder (99.5% purity; <63 μm) were used to fabricate Ti–15Mo alloy. The selection of the TiH2 powder instead of hydride–dehydride (HDH) Ti metal powder was due to the resulting higher sintered density and lower oxygen content [23]. The powder mixture was blended in a Turbula mixer for 45 min and then compacted into cylinders of 10 mm both in diameter and height under a uniaxial pressure of 600 MPa. The samples were sintered by
Carbon-enriched GB phases in as-sintered Ti–15Mo
The as-sintered microstructure of Ti–15Mo is shown in Fig. 2. The alloy was easily sinterable and achieved a nearly pore-free microstructure after 120 min at 1350 °C (see the SEM secondary electron, SEM-SE, image in Fig. 2a), indicative of the promise of powder metallurgy as a versatile low-cost near-net-shape fabrication approach to the alloy. The SEM-SE image in Fig. 2b also suggests the existence of primary α-Ti phases (αp-Ti), which are susceptible to corrosion and were etched off from the
Discussion
The current ASTM standard specification F2066-08 sets the maximum carbon limit of 0.1 wt.% for Ti–15Mo for surgical implant applications [17]. The as-sintered Ti–15Mo, which contains only 0.032 ± 0.006 wt.% C, shows the clear presence of GB Ti2C. Because of its much higher hardness and Young’s modulus than the β-Ti matrix (see Fig. 7), the formation of the brittle GB Ti2C structure (see Fig. 5) can be extremely detrimental to the fatigue properties of the alloy. In addition, because the interface
Summary
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GB titanium carbide (Ti2C) forms in as-sintered Ti–15Mo alloy that contains as low as 0.032 ± 0.006 wt.% C. The morphology of the carbide has been revealed by 3-D tomography, similar to primary α-Ti. It has an fcc crystal structure with a lattice parameter of a = 8. ± 0.1 Å when indexing all the electron diffraction spots observed into integer Miller indices. The overall carbon content of the GB Ti2C structure varies in the range of ∼29.9–38.4 at.% measured by TEM and SEM EDX. The Ti2C phase shows a
Acknowledgements
The authors would like to acknowledge the support of the Queensland Centre for Advanced Materials Processing and Manufacturing. M. Yan further acknowledges the support of a Queensland Smart Future Fellowship (Early Career). The authors wish to specially thank one of the reviewers for his/her constructive comments and suggestions on the TEM-EDX analysis of carbon and the identification of the grain boundary Ti2C phase presented in the paper.
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2021, Materials and DesignCitation Excerpt :Relatively long grain boundaries (GBs) of MIM Ti-Nb-Zr are usually decorated with GB α-precipitates and GB TiC-inclusions, as can be seen in Fig. 3; their formation is related to GB-regions enriched with impurities [34]. In fact, a so-called single TiC-particle with widely recognized elongated-shape [35] in PM β-class Ti-alloys is often assembled from a plurality of TiC-particles with irregular shape, indicated in Fig. 3b. On the other hand, the GB α-precipitates as a whole frequently present a typical “fishbone”-like morphology, and evidence for this is shown in Fig. 3c. Furthermore, these side-plates of a certain GB α-phase, in essence, belong to distinct crystallographic α-variants in the different spatial directions inside adjacent β-grains, and therefore are in most cases more or less misoriented against each other [24]. In most cases, the backbone of the GB α-phase has an identical orientation with one of the side-plates of a GB α-phase but within the same GB α-phase this can occasionally alternate so that some sections are crystallographically oriented to the side plates on one and other sections parallel to the side plates on the other side [36].