A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 2 3 5 TOUGHYET FLEXIBLE SUPERELASTICALLOYS MEET BIOMEDICAL NEEDS Novel bcc CoCr-base alloys exhibit a Young’s modulus similar to human bone combined with superelastic strain twice that of NiTi alloys. Xiao Xu, Takumi Odaira, Sheng Xu, Kenji Hirata, Toshihiro Omori, Kosuke Ueki, Kyosuke Ueda, Takayuki Narushima, Makoto Nagasako, and Ryosuke Kainuma, Tohoku University, Japan Stefanus Harjo and Takuro Kawasaki, Japan Atomic Energy Agency Lucie Bodnárová, Petr Sedlák, and Hanuš Seiner, Czech Academy of Sciences Metallic biomaterials are widely used to replace or support failing hard tissues due to excellent mechanical properties and high wear resistance, with demand increasing as the global population continues to age[1]. It is widely accepted that successful metallic bio- materials should have good biocompatibility, high corrosion resistance, and strong wear resistance. In addition, a low Young’s modulus similar to human bone is now recognized as another important factor, in order to avoid bone atrophy due to the stress shielding effect[2]. While the Young’s modulus of stainless steels and conventional fcc CoCr alloys is as high as 190-240 GPa, for β-type Ti-base alloys it is generally in the range of 50-80 GPa. Young’s modulus values are as low as 35 GPa for Ti-Nb-Ta-Zr, close to that of human bone at approximately 10-30 GPa[3]. However, Ti-base alloys come with the compromise of low wear resistance. In fact, alloys that feature a low Young’s modulus along with high wear resistance have been difficult to realize. Featuring unique superelastic behavior, shape memory alloys, specifically NiTi alloys, are widely used as self- expanding vascular stents and orthopedic bone staples. Due to concerns about nickel allergies, Ni-free β-type Ti-base shape memory alloys were developed. However, their re- coverable strain is less than 5%, approximately half that of NiTi alloys. This article explores the recently developed bcc CoCr- base alloy Co-Cr-Al-Si (CCAS) as a potential solution to these issues, i.e., the difficulty in combining a low Young’s modulus with high wear resistance, and the challenge of realizing large superelastic strains[4]. CCAS alloys are considered flexible because they can exhibit a low Young’s modulus— around 10-30 GPa—similar to human bone. They are also tough, exhibiting high wear resistance and good corrosion resistance. Further, they exhibit superelasticity with significant recoverable strain up to 17%, showing promise for use as shape memory alloys. CCAS ALLOY DEVELOPMENT In 2013, the authors began to develop Co-Cr-Ga-Si shape memory alloys, which exhibit unique reentrant martensitic transformation behavior and cooling-induced shape memory effect[5]. In 2015, the team discovered a relatively inexpensive Co-Cr-Al-Si systemwith a chemical composition close to stoichiometry Co50Cr25(Al12.5Si12.5) (in atomic ratio) of the Heusler phase[6]. Ductility was low and the concentration of Al, an undesirable element for biomedical applications, was large. The researchers then reduced the concentration of both Al and Si to 7-8 at.%, and finally reached the following compositions: Co51Cr34Al7Si8 (atomic ratio, 58Co-34Cr-3.6Al- 4.4Al in mass ratio, 58Co) and Co52Cr33Al6.5Si8.5 (atomic ratio, 59Co-33Cr-3.4Al-4.6Si in mass ratio, 59Co). These CCAS alloys contain lower Al concentration than Ti-6Al-4V (mass ratio) and can be hot-rolled at 1473 K. Moreover, single crystals with lengths up to several centimeters can be easily fabricated using a cyclic heat treatment technique[7]. Figure 1 shows CCAS single crystalline sheets. Figure 2 shows the tensile stress-strain curves of the CCAS alloys and conventional metallic biomaterials. Figure 2a focuses on the elastic region. While the conventional metallic biomaterials show high Young’s moduli, the <001>-oriented CCAS single crystals have a low Young’s FEATURE Fig. 1 — Single crystals of CCAS alloys prepared by cyclic heat treatment[11]. 6 7
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