Design and development of Ti–Zr–Hf–Nb–ta–Mo high-entropy Alloys for metallic biomaterials

Applying empirical alloy parameters (including Mo equivalent), the predicted ground state diagram, and thermodynamic calculations, noble nonequiatomic Ti–Zr–Hf–Nb– Ta–Mo high-entropy alloys for metallic biomaterials (BioHEAs) were designed and Jo ur na l P re -p ro of Journal Pre-proof

formation tendency in multicomponent alloys have already been suggested and applied to alloy design in HEAs [4,5]. Furthermore, thermodynamic and ab initio calculations was employed in attempt to design alloys in HEAs [4,5]. We suggested an alloy design using the ground state diagrams predicted by Materials Projects [20,21] for designing multicomponent alloys, including metallic glasses [22] and HEAs [23][24][25]. We also demonstrated that thermodynamic calculations were effective in predicting the constituent phases in BioHEAs [8 -10,14]. In the present study, alloy design concepts based on the combination of empirical alloy parameters, including newly suggested parameters, predicted ground state diagrams, and thermodynamic calculations were investigated. Furthermore, the first development of new Ti-Zr-Hf-Nb-Ta-Mo BioHEAs that exhibits biocompatibility comparable to that of CP-Ti, higher mechanical strength than CP-Ti, and an appreciable room-temperature tensile ductility, was reported.
nonequiatomic Ti-Zr-Hf-Nb-Ta-Mo BioHEAs was designed and developed. The Ti-Zr-Hf-Nb-Ta-Mo alloy system is a combination of Ti-Zr-Nb-Ta-Mo BioHEAs and Hf. Hf is a metallic element with good biocompatibility, as confirmed by animal implantation tests [29]. Moreover, Hf has been used as an additive element in Ti-based biomaterials [30,31]. To enhance the room-temperature ductility in BioHEAs, three strategies were considered: (1) attaining the mixing entropy of S mix ≥ 1.5 R for satisfying the entropy-based definition of HEAs [4,5], where S mix is evaluated by the mixing enthalpy of the ideal/regular solid solution and R is the gas constant, (2) decreasing the melting temperature for the suppression of casting defects such as cold where ̅ is the compositional average of the atomic radius of the constituent elements ( ̅ = ∑ • i constituent elements in multicomponent alloys can be evaluated using; however, these cannot be discussed using H mix , (H mix )and. The empirical alloy parameters in Table 1 indicate the high solid solution formation tendency in equiatomic TiZrHfNbTaMo (TZHNTM-Eq) and nonequiatomic Ti-Zr-Hf-Nb-Ta-Mo (TZHNTM-X, X = 1, 2, 3). and the VEC for new alloy design to predict the phase stability of BCC/HCP phases in medium-entropy alloys (MEAs) for biomedical applications (BioMEAs) and BioHEAs.
In Table 1, the values of the VEC and Moeq in TZHNTM-X (X = Eq, 1, 2, 3) are given, along with the other empirical alloy parameters H mix , (H mix ), , and . Figure 1 shows    In Fig. 2a, the green dot indicates the alloys whose Ti concentration was at and above 35 at.% as the Ti-rich alloys. In Fig. 2b, hollow blue circles (○) and red closed circles (•) indicate Ti-Zr-Hf-Nb-Ta-Mo alloy investigated in the present study, and the indices Eq, 1, 2, and 3 denote the TZHNTM-X (X = Eq, 1, 2, 3), respectively.

J o u r n a l P r e -p r o o f
We also discussed the relationship between the Moeq and empirical alloy parameters for predicting solid solution formation tendency (H mix , (H mix ) , ). Mo alloys with S mix ≥ 1.5R, the parameters H mix (Fig. 3a), (H mix ) (Fig. 3b), and  ( Fig. 3c)     T S in TZHNTM-Eq (Fig. 5a) is significantly larger than that in TZHNTM-X (X = 1, 2, 3) (Figs. 5b-5d). The single BCC phase is not a thermal equilibrium state at the temperature below the BCC phase's decomposition temperature (T D ). Moreover, the T D J o u r n a l P r e -p r o o f in TZHNTM-Eq (Fig. 5a) was significantly higher than that in nonequiatomic TZHNTM-X (X = 1, 2, 3) (Figs. 5b-5d). The HCP phase was thermally stable at a considerably lower temperature than the T D in TZHNTM-X (X = Eq, 1, 2, 3), regardless of the alloy composition. The thermodynamic calculation revealed a high tendency for the formation of a BCC phase during solidification in TZHNTM-X (X = Eq, 1, 2, 3), where the BCC phase formation was also predicted using the Moeq in Fig. 1. Therefore, the TZHNTM-X (X = Eq, 1, 2, 3) alloys were selected and fabricated for further validation based on the above-described alloy design and prediction.
J o u r n a l P r e -p r o o f  Figure 6 shows the XRD patterns of arc-melted ingots of the TZHNTM-X (X = Eq, 1, 2, 3) alloys. In the XRD patterns (Fig. 6a), the calculated XRD intensities of   (Fig. 7b). An equiaxial dendrite structure composed of white contrast dendrite and gray contrast interdendrite regions was observed in the SEM-BSE image of the TZHNTM-X (X = Eq, 1, 2, 3) alloys (Fig. 7a). In the EDS element mapping images of Ti and Zr elements (Fig. 7b), the enrichment of Ti and Zr in the J o u r n a l P r e -p r o o f interdendrite region and the enrichment of Ta in the dendrite region were observed in the equiatomic TZHNTM-Eq (Fig. 7b1, left). A significant difference in the Hf, Nb, and Mo elements between dendrite and interdendrite regions was not observed in EDS element mapping images. The dual BCC phases in the XRD patterns corresponded to the dendrite BCC and interdendrite BCC phases in TZHNTM-Eq. No significant difference in the composition of the dendrite and interdendrite regions was observed in the EDS element mapping images in the nonequiatomic TZHNTM-3 (Fig. 7b2, right).

Results and discussion
The difference in the segregation behavior between TZHNTM-Eq and TZHNTM-3 Mo-related compounds was not detected from XRD analysis (Fig. 6), SEM observation ( Fig. 7a), and EDS mapping (Fig. 7b) in TZHNTM-X (X = Eq, 1, 2, 3).  BioHEAs [51]. The thermomechanical process was considered to be effective in eliminating solidification segregation. Further investigation of the mechanism of room-temperature ductility and further evaluation of the mechanical properties of TZHNTM-3 using specimens prepared by AM and/or thermo-mechanical processes will be reported in future work.

Figure 9
Biocompatibility of the arc-melted ingots in the Ti-Zr-Hf-Nb- Ta The analysis of the mechanical properties by the tensile test (Fig. 8) and biocompatibility (Fig. 9)

Conclusions
In conclusion, nonequiatomic Ti-Zr-Hf-Nb-Ta-Mo BioHEAs with superior biocompatibility and mechanical properties were successfully designed and developed.
The results are summarized as follows: (1) BCC phases without intermetallic compounds were detected by the XRD patterns of the arc-melted ingots of the equiatomic Ti 16 (2) In addition to the VEC parameter, it was found that the Moeq parameter was useful for the alloy design and prediction of BioMEAs and BioHEAs.
(3) Alloys design and prediction by the combination of empirical alloy parameters (including Moeq), the predicted ground state diagram constructed by the Materials Project, and thermodynamic calculations using the SGTE database, was effective to develop nonequiatomic Ti-Zr-Hf-Nb-Ta-Mo BioHEAs.