A titanium alloy containing large amounts of β-stabilizing elements such as Nb, Ta and Mo, so that it has the BCC β phase even at room temperature. Its low Young's modulus reduces stress shielding in medical use.
Japanese name: β型チタン合金
β titanium alloy appears in 2 practice questions across 1 topic, including Biomaterials.
Field: Biomaterials
Pure titanium has the HCP α phase below 882 °C and the BCC β phase above it. Mo, V, Nb and Ta dissolve widely in the β phase and lower the transformation temperature, while Fe, Cr and others act as eutectoid-type β stabilizers. When enough of these elements is added, the β phase is retained to room temperature on quenching or air cooling, and such alloys are called β titanium alloys. Most are metastable at room temperature, and aging precipitates fine α phase and other phases. Al, O and N, on the other hand, stabilize the α phase, and Ti-6Al-4V is an α+β alloy consisting of the two phases. Which phase dominates changes the properties greatly.
The Young's modulus of β alloys is about 55–80 GPa for biomedical compositions, lower than the roughly 110 GPa of Ti-6Al-4V. The closer the composition of the BCC β phase is to the limit at which it can exist stably, the lower its resistance to deformation that shears atomic planes, and the lower its Young's modulus. Tuning the composition so that the β phase is only just retained therefore gives a low Young's modulus. Ti-Nb-Ta-Zr and Ti-Mo alloys are examples. They still do not reach bone (about 10–30 GPa), but compared with stainless steel (about 200 GPa) or Co-Cr alloys they are much closer to bone.
A β alloy that has only been solution treated and quenched has a low Young's modulus but not very high strength. Aging to precipitate fine α phase raises the strength greatly, but because the α phase has a higher Young's modulus, the overall modulus also rises. Aging at lower temperatures can also form a fine phase called the ω phase, which raises the Young's modulus and causes embrittlement. For biomedical use, therefore, cold working and aging conditions are chosen to adjust the microstructure so that the required strength and fatigue strength are obtained while keeping the Young's modulus low. Combining low modulus with strength is at the heart of alloy design.
Because β alloys are BCC, they are easy to cold work and can be heat treated to high strength, so they have also been used for aircraft springs and fasteners and for sheet that is formed cold. For biomedical use, another advantage is that compositions can avoid V and Al and be made of elements regarded as having low cytotoxicity, such as Nb, Ta and Zr. On the other hand, Nb and Ta are heavy and expensive elements, which raise density and cost. Like titanium alloys in general, they also have poor wear resistance and are unsuitable for sliding parts. Ni-free superelastic and shape memory alloys are also being studied in β-type Ti-Nb and related systems.
Answer: A. Retaining the BCC β phase down to room temperature makes the Young's modulus lower than that of Ti-6Al-4V and reduces stress shielding
Titanium takes the HCP α phase at low temperature and the BCC β phase at high temperature, and adding large amounts of β-stabilizing elements such as Nb, Ta and Mo retains the β phase even at room temperature. β alloys have a Young's modulus of about 55–80 GPa, lower than that of the α+β alloy Ti-6Al-4V (about 110 GPa) and closer to that of bone (about 10–30 GPa), so they reduce stress shielding. Nb, Ta and Zr are also considered to have low cytotoxicity, with the added advantage of avoiding V and Al. It is the α phase that is HCP; calling the β phase HCP confuses the crystal structures. Raising the stiffness of an implant works in the direction of strengthening stress shielding. β alloys also form a passive film on their surface and keep their corrosion resistance, and losing the film would only increase ion release, not lead to bonding with bone.
The classification is by which phase mainly makes up the alloy at room temperature. α alloys consist of the HCP α phase and excel in high-temperature strength, creep strength and weldability. The α+β alloy Ti-6Al-4V balances strength and workability well and is the most widely used. β alloys are BCC, easy to cold work, can be heat treated to high strength and can have a low Young's modulus.
Because concerns have been raised about the cytotoxicity of V and about the effects of Al on the nervous system in Ti-6Al-4V. Problems have not been confirmed in normal use, but for implants left in the body for a long time, alloys using elements regarded as having small effects if they dissolve, such as Nb, Ta and Zr, have been developed.
Not as a dense alloy: even the lowest-modulus compositions are about 40–50 GPa and cannot reach the roughly 10–30 GPa of cortical bone. To get closer, this is combined with making a porous body whose pores lower the Young's modulus. Increasing porosity also lowers strength, however, so the two are balanced.
Switching to a β titanium alloy with a Young's modulus of about 55 GPa raises the bone's share to 20/75 ≈ 27%.
A rod of cortical bone (Young's modulus 20 GPa) and a rod o…Young's modulus Stress shielding Passivity Biocompatibility Biodegradable polymer Bone cement Hydroxyapatite Osseointegration Superelasticity