Phase Separation in Ti-6Al-4V Alloys with Boron Additions for Biomedical Applications: Scanning Kelvin Probe Force Microscopy Investigation of Microgalvanic Couples and Corrosion Initiation
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To investigate the effect of boron additions on the corrosion behavior of Ti-6Al-4V for potential use in biomedical implants and devices, cast samples of Ti-6Al-4V were alloyed with 0.01% to 1.09% boron by weight and subjected to hot isostatic pressing. Subsequent analysis via scanning Kelvin probe force microscopy and scanning electron microscopy/energy-dispersive spectroscopy revealed the presence of both alpha (α) and beta (β) phase titanium, enriched in aluminum and vanadium, respectively. At all concentrations, boron additions affected the grain structure and were dispersed throughout both phases, but above the solubility limit, needle-like TiB structures also formed. The TiB needles and β phase exhibited similar surface potentials, whereas that of the α phase was found to be significantly lower. Nevertheless, when subjected to high applied electrochemical potentials in saline solutions, corrosion initiation was observed exclusively within the more noble β phase.
Titanium and its alloys are used in numerous applications, ranging from aerospace and high-end bicycles to dental and biomedical implants. In regards to biomedical applications, these alloys have been used as replacements for hip and knee joints, as well as for components in artificial hearts and pace makers.1 Commercially pure titanium (CP Ti; UNS R50400) is used in dental applications such as crowns and bridges, as well as for components such as screws,2 whereas UNS R56400 (Ti-6Al-4V; Ti64) is used as a structural biomaterial for orthopedic prostheses.1 The success of these alloys in the medical field is a result of their excellent corrosion resistance, biocompatibility,2, 3, 4 high strength, and lower Young’s modulus in comparison with other implant alloys such as stainless steels and cobalt-chromium alloys.3
At room temperature, titanium alloys can exist in either of two phases, a stable hcp alpha (α) phase or a metastable bcc beta (β) phase, or a combination of the two. While the α phase is stronger, the β phase is more ductile, and accordingly, many titanium alloys are designed to contain a mixture of both phases to optimize alloy properties for the intended application. The Ti64 alloy is composed of an aluminum-rich α phase and a vanadium-rich β phase. Even though this alloy has achieved remarkable success as an implant material, it would be desirable to increase its useful life span in light of the projected increase in human life expectancy.5 A potential alternative based on a Ti64 matrix is described in this article.
Boron additions to Ti-6Al-4V have been shown to increase yield and tensile strengths of the alloy with the likely mechanism being grain size reduction of the alloy. Dramatic decreases in grain size have been reported even for small amounts of boron additions, i.e., in the 0.01 wt.% to 0.1 wt.% range.6 When boron additions exceed the solubility limit, titanium monoboride (TiB) precipitates are formed. Previous work determined that with the addition of low levels of boron, particularly up to 0.02 wt.% B, there is an increase in the corrosion resistance.7, 8, 9 Nevertheless, the contribution of TiB to the driving force for microgalvanic corrosion relative to the α and β phases has not yet been determined. We have used a combination of nanoscale imaging techniques to elucidate composition, structure, and resultant galvanic potential differences between the microstructural phases present in Ti-6Al-4V alloy samples alloyed with varying weight percentages of boron. Here, we report the results of these studies in an effort to understand the effect of TiB ceramic precipitates on localized corrosion initiation in Ti-6Al-4V.
Materials and Methods
Alloys and Sample Preparation
Test coupons were machined out of cast and hot isostatically pressed (HIPped) cylinders of Ti-6Al-4V alloyed with 0.01% to 1.09% boron by weight.7 The coupons were metallographically prepared by sequential grinding down to 800 grit, sonication in deionized (DI) water followed by polishing with 6-µm, 3-µm, and 1-µm diamond slurries (Allied) using lapping oil as a lubricant. After again sonicating in DI water, a 0.05-µm aqueous alumina slurry was used for final polishing, followed by sonication in DI water and acetone. Each of the sonication steps were carried out for 3 min. After preparation, the coupons were subjected to electrochemical testing in an artificial physiological electrolyte (modified Hanks Balanced Salt Solution) following the ASTM F2129-15 test method for determining the corrosion susceptibility of small implant devices. In addition, alloy coupons were subjected to high peak potentials in saline solutions to examine differences in their corrosion behavior (described elsewhere).7, 8, 9
Scanning Kelvin Probe Force Microscopy (SKPFM)
SKPFM imaging was conducted using a Bruker Dimension Icon AFM operating in frequency modulation (FM) PeakForce KPFM mode.10, 11, 12 After polishing, each test coupon was cleaned with UHP N2 prior to imaging to remove any loose surface particulates. A PeakForce amplitude of ~75 nm and a lift height of ~85 nm were used to acquire the topography and surface potential images, respectively. The same PFQNE-AL probe (Bruker) was used to image all samples. Image processing was performed in NanoScope Analysis V1.80, and the resultant surface potential histograms were exported for quantitative analyses.
Scanning Electron Microscopy (SEM)
SEM imaging was performed in a Hitachi S-4300N equipped with a tungsten filament electron source. In addition to standard secondary electron (SE) imaging, backscattered electron (BSE) imaging was also conducted at 10 keV. Energy-dispersive x-ray spectroscopy (EDS) maps were obtained at 15 keV. Z-contrast in BSE imaging mode permitted ready identification of TiB precipitates, whereas EDS was used to construct elemental maps to differentiate between the Al-rich α phase and the V-rich β phase. All SEM imaging was carried out after SKPFM because of the potential for carbon deposition by the electron beam, which has been shown to affect the measured surface potentials.10,13
Results and Discussion
Effect of Boron on Microstructure
Phase Composition and Volta Potentials
Figure 2 presents representative SKPFM images of Ti-6Al-4V samples containing varying amounts of added boron. At boron concentrations greater than the saturation limit of ~0.02 wt.%, blocky or needle-like TiB phase precipitates were observed and confirmed through EDS analysis. Although the TiB precipitates are present in the higher B concentration samples of Fig. 2, they are not readily distinguishable in most SKPFM images because their relative Volta potential and aspect are similar to that of the β phase. Instead, it is necessary to correlate features seen in the SKPFM images with co-localized BSE SEM images and EDS maps, as shown in Fig. 1e and f, to verify phase identity. Co-localized BSE SEM and SKPFM images with corresponding EDS maps for all nine sample compositions studied (<0.001% to 1.09% B) are shown in Figs. S1 to S9 in the supplementary materials.
The V-rich β phase was found to be noble relative to the α phase and possessed a Volta potential ~200–400 mV higher than the α phase depending on the sample. The direction of galvanic coupling is in agreement with that of unalloyed (i.e., boron-free) Ti-6Al-4V. Nevertheless, the results reported here indicate a significantly higher potential difference between the β and the α phase than previously reported by others, perhaps because of higher spatial resolution in the current SKPFM scans resulting in less averaging of the measured potentials near phase boundaries.16 This averaging effect occurs regardless of the method used to determine the potential difference between phases (e.g., fitting of histograms versus cross-sectioning of selected features; see Figs. S10 to S24 and Table S1) but can be somewhat counteracted in the case of cross-sectional analysis, which uses the potential measured at the interior of the phase in preference to that at the phase boundary (and hence is to be preferred in terms of giving a more accurate reflection of the true interphase potential difference). The observed difference in potential between β and α phases suggests that when corrosion occurs under natural conditions, the lower relative potential of the α phase will make it the expected anode site, thereby experiencing preferential dissolution supported by cathodic activity within the more noble β phase.
Because B has a very low solubility limit in titanium, it is expected that the phase composition of α and β are independent of B content and, hence, that the Volta potentials are minimally affected. Thus, although B additions and the amount of cold working can significantly impact the mechanical behavior of Ti-6Al-4V alloys, neither is significantly detrimental to the resulting Volta potential difference between the α and β phases or overall corrosion resistance of the alloy.16
The relative Volta potential of the TiB phase is higher than that of the α phase, indicating it could act as a preferential cathode during corrosion in addition to (or in preference to) the β phase. Specifically, the potential difference implies there is a thermodynamic driving force to favor preferential dissolution of α phase supported by coupled reduction reactions on adjacent TiB phases, unless it is kinetically limited, for example, by the presence of a passive layer. Nevertheless, the TiB phase was not found to be an effective cathode in previous studies because the precipitated TiB phase did not decrease or seem to influence corrosion performance under simulated physiological conditions.8 Hence, the additions of B and the presence of the TiB phase appear to be neutral factors regarding corrosion resistance of Ti-6Al-4V + B alloys.
The Volta potential difference between α and β phases indicates a significant driving force for local galvanic corrosion, but uniform passivation of the alloy surface significantly prevents microgalvanic couple-driven corrosion initiation, including under physiologically relevant conditions.
Above the B solubility limit, the precipitated TiB phase is noble to both the α and β phases, but it does not appear to influence corrosion initiation, propagation, or passivation in electrochemical tests carried out in an artificial physiological electrolyte.
Under forced corrosion conditions at very high applied potentials, any observed corrosion damage appears limited to preferential dissolution of the β phase, regardless of the presence of B or precipitated TiB phase. The TiB phase did not appear to influence the location for corrosion, and the β phase was selectively but uniformly attacked.
Boron additions do not detrimentally impact overall corrosion resistance or inhibit the robust inherent passivity of Ti-6Al-4V, while offering a significant beneficial effect in regard to the mechanical properties for biomedical implant and device applications. In fact, based on the limited data presented here for forced corrosion, increased boron content may improve corrosion resistance and warrants further study.
Assistance from Dr. Nick Bulloss of the Boise State Center for Materials Characterization (BSCMC) with the SEM/EDS work is gratefully acknowledged.
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