A Review: Design from Beta Titanium Alloys to Medium-Entropy Alloys for Biomedical Applications

Abstract

β-Ti alloys have long been investigated and applied in the biomedical field due to their exceptional mechanical properties, ductility, and corrosion resistance. Metastable β-Ti alloys have garnered interest in the realm of biomaterials owing to their notably low elastic modulus. Nevertheless, the inherent correlation between a low elastic modulus and relatively reduced strength persists, even in the case of metastable β-Ti alloys. Enhancing the strength of alloys contributes to improving their fatigue resistance, thereby preventing an implant material from failure in clinical usage. Recently, a series of biomedical high-entropy and medium-entropy alloys, composed of biocompatible elements such as Ti, Zr, Nb, Ta, and Mo, have been developed. Leveraging the contributions of the four core effects of high-entropy alloys, both biomedical high-entropy and medium-entropy alloys exhibit excellent mechanical strength, corrosion resistance, and biocompatibility, albeit accompanied by an elevated elastic modulus. To satisfy the demands of biomedical implants, researchers have sought to synthesize the strengths of high-entropy alloys and metastable β-Ti alloys, culminating in the development of metastable high-entropy/medium-entropy alloys that manifest both high strength and a low elastic modulus. Consequently, the design principles for new-generation biomedical medium-entropy alloys and conventional metastable β-Ti alloys can be converged. This review focuses on the design from β-Ti alloys to the novel metastable medium-entropy alloys for biomedical applications.

Keywords: biomedical alloy; high-entropy alloys; medium-entropy alloy; metastable; β-Ti alloys.

Figure 1

Comparison of the modulus among published biomedical metastable Ti alloys, conventional biomedical alloys, and cortical bone [17,18,19,20,21,22,23,24,25,26,27].

Figure 2

Number of international journal publications on (a) high-entropy alloys and medium-entropy alloys from 2004 to 2022 and (b) biomedical high-entropy alloys/medium-entropy alloys from 2017 to 2022 (Source: www.webofscience.com).

Figure 3

The four core effects of high-entropy alloys.

Figure 4

Biocompatibility of SUS–316L, CP-Ti, equiatomic Ti–Nb–Ta–Zr–Mo, and non-equiatomic Ti_(2−x)–Zr_(2−x)–Nb_x–Ta_x–Mo_x (x = 0.6, 1.4) (x = 0.6, 1.4) Bio-HEAs. (a) Giemsa staining images of osteoblasts, (b) fluorescent images of osteoblast adhesion, and (c) quantitative analysis of fibrillar adhesion size regulation [32]. (Reprinted with permission under the terms of the Creative Commons CC-BY license from Elsevier: Scr. Mater.)

Figure 5

Types and proportions of elements used in biomedical high-entropy alloys/medium-entropy alloys (Bio-HEAs/MEAs) in international journals from 2017 to 2022 (Source: www.webofscience.com).

Figure 6

Microstructures of as-cast Ti-rich MEAs: Ti₅₀–Zr₂₅–Nb₁₅–Mo₁₀ (Ti50), Ti₅₈–Zr₂₃–Nb₁₂–Mo₇ (Ti58), and Ti₆₅–Zr₂₀–Nb₁₀–Mo₅ (Ti65). (a) Optical micrographs, (b) backscattering electron images (BEI), and (c) element mapping images obtained through electron microprobe analysis using wavelength dispersive spectrometers [73]. (Reprinted with permission from Elsevier: J. Alloys Compd. Copyright 2023, License: 5655690226569).

Figure 7

TEM images and SAED patterns of metastable Ti-rich MEA (Ti₆₅–Zr₁₈–Nb₁₆–Mo₁). (a) SAED of the [011] zone axes, (b) HR-TEM image in [011] direction (additional streaking is marked by blue arrows), and (c) SAED of the [111] zone axes, and (d) HR-TEM image in [111] direction (additional streaking is marked by blue arrows) [78]. (Reprinted with permission from Springer Nature: Met. Mater. Int. Copyright 2023, License: 501856029).

Figure 8

Microstructures and elemental analyses of as-cast Ti₆₅–Zr₂₀–Nb₁₄–Mo₁ and Ti₆₅–Zr₁₈–Nb₁₆–Mo₁. (a,d) backscattering electron images, (b,e) element mapping images obtained through electron microprobe analysis using wavelength dispersive spectrometers, and (c,f) line scan curves obtained through an electron microprobe analysis using wavelength dispersive spectrometers [78]. (Reprinted with permission from Springer Nature: Met. Mater. Int. Copyright 2023, License: 501856029).

Figure 9

Relationships between the moduli and thermodynamic parameters, as well as between the moduli and phase stability parameters, of biomedical high-entropy alloys/medium-entropy alloys [51,70,72,73,75,76,78,79,80,81,82,83,84,85,86,87]. The red line indicates data trend. (a) modulus vs. ΔS_mix/R, (b) modulus vs. ΔH_mix, (c) modulus vs. δ, (d) modulus vs. [Mo]_eq, (e) modulus vs. VEC, and (f) modulus vs. M_s.

Figure 10

Bo-Md diagram of biomedical high-entropy alloys/medium-entropy alloys (Bio-HEAs/MEAs) with a low elastic modulus (<80 GPa) [75,76,78,80,81,82,83,84,85,86,87].