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 6 modulus of about 30 GPa, indicating that it mimics the mechanical property of human bone. Note also that the CCAS alloys can be elastically strained to more than 2.5%, and this material survived from a fatigue test up to 107 cycles at a stress of 380 MPa corresponding to an elastic strain of 1.65%. Figure 2b shows the tensile stress-strain curves at higher strains, where the superelastic behavior is revealed. Both the 58Co and 59Co samples show good superelasticity, and for 59Co, a recoverable strain up to 17% was obtained. The CCAS alloys exhibit large elastic anisotropy, which may be one of the reasons for the low Young’smodulus along FEATURE the <001> direction. For 59Co, elastic constants were determined as c11 = 203.2 GPa, c12 = 184.4 GPa, and c44 = 122.2 GPa. Figure 3 shows the crystallographic orientation dependence of the Young’s modulus calculated using elasticity equations[8]. Young’s modulus along <001> was found to be only 27.7 GPa, whereas along <111> it was as large as 302.1 GPa. Wear and corrosion resistance of the CCAS alloys were also investigated[4]. Ball-on-disc wear tests were conducted in air and confirmed good wear resistance comparable to conventional fcc CoCr alloys, and much better than that of stainless steels and Ti-base alloys. Results of anodic polarization tests show that the CCAS alloys exhibit similar trends to conventional fcc Co-Cr alloys, indicating high corrosion resistance. Figure 4 summarizes the characteristics of the CCAS alloys and conventional metallic biomaterials. In conventional biomaterials, except for NiTi, there is a trade-off (“banana curve”) between a low Young’s modulus and high wear resistance, as shown in Fig. 4a. The CCAS alloys go far beyond the banana curve with a low Young’s modulus similar to human bone and a high wear resistance comparable to conventional fcc CoCr alloys. Figure 4b shows the superelastic recoverable strain against corrosion resistance. The recoverable strain of CCAS was about 17%, twice that of commercial NiTi and three times that of MgSc and Ti-Zr-Nb-Sn alloys[9,10]. The authors’ CCAS alloys with high corrosion resistance offer a new option for shape memory alloys suitable for biomedical applications. SUMMARY Newly developed Co-Cr-Al-Si (CCAS) shape memory alloys feature a low Young’s modulus, similar to the flexibility of human bone. The authors report this is the first time that a low Young’s modulus, high corrosion and wear resistance, Fig. 2 — (a) Stress-strain curves of the <001> single-crystalline CCAS alloys compared with conventional metallic biomaterials. The Young’s moduli of CCAS alloys are the closest to human bone; and (b) superelastic behavior of the CCAS alloys[4]. (a) (b) Fig. 3 — Crystallographic orientation dependence of Young’s modulus calculated using the elastic constants of 59Co-33Cr-3.4Al-4.6Si. 87 8
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