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
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
0 1.-Micron School of Materials Science and Engineering, Boise State University , Boise, ID 83725-2090 , USA. 2.-Department of Chemical and Materials Engineering, California State Polytechnic University , Pomona, Pomona, CA 91768, USA. 3.-
1 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
2 Davis , Robles, Livingston, Johns, Ravi, Graugnard, and Hurley
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 (a) and beta (b) 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 b phase exhibited similar surface potentials, whereas that of the a 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 b phase.
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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,2–4 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 (a)
phase or a metastable bcc beta (b) phase, or a
combination of the two. While the a phase is
stronger, the b 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 a phase
and a vanadium-rich b 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.
Fig. 1. Co-localized backscattered electron (BSE) SEM (a) and SKPFM (b) images of Ti-6Al-4V alloyed with 0.01% B showing extensive grain
refinement. The smaller grain size leads to reduced contrast between the a and b phases in the SKPFM image as a result of spatial averaging.
Co-localized BSE SEM (c) and SKPFM (d) images of 0.43% B alloyed Ti-6Al-4V showing the formation of boron-rich needles, per corresponding
EDS maps of Al (e) and B (f). Red boxes in the SEM images indicate the approximate corresponding location of SKPFM, and the small black box
in d shows the representative area for the line scan analysis presented in Fig. 4.
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.7–9
Nevertheless, the contribution of TiB to the driving
force for microgalvanic corrosion relative to the a
and b 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
Ti6Al-4V.
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