Breaking trade-off barriers of strength-ductility in Ti alloys and their composites has been a key challenge for their engineering applications. Herein, we developed a new strategy of introducing β-Ti ductile zones in heterostructured (Cu@CNTs/TC4)+TC18 composites, which were synthesized using two-stepped ball milling, spark plasma sintering and hot rolling processes. Such composites are composed of TC18-coarse grain zones (CGZ) and (TiC+α'')-reinforced TC4-fine grain zones (FGZ). They achieve an exceptional high strength and high ductility, which are superior to those of TC4 alloys and homogeneous Cu@CNTs/TC4 composites. Heterogeneous interfacial microstructures of TC4-FGZ/TC18-CGZ play an important role in strengthening and toughening effects. The grain-coordinated deformation between TC18-CG and TC4-FG is also the key reason for achieving an exceptional ductility.
Medium-entropy alloys (MEAs) typically exhibit outstanding mechanical properties, but their high Young's modulus results in restricted clinical applications. Mismatched Young's modulus between implant materials and human bones can lead to “stress shielding” effects, leading to implant failure. In contrast, β- Ti alloys demonstrate a lower Young's modulus compared to MEAs, albeit with lower strength . In the present study, based on the bimodal grain size distribution (BGSD) strategy, a series of high-performance TiZrNbTa/Ti composites are obtained by combining TiZrNbTa MEA powders with nano-scale grain sizes and commercially pure Ti (CP-Ti) powders with micro-scale grain sizes. Concurrently, Zr, Nb, and Ta that are β- Ti stabilizer elements diffuse into Ti, inducing an isomorphous transformation in Ti from the high Young's modulus α -Ti phase to the low Young's modulus β -Ti phase at room temperature, optimizing the mechanical biocompatibility. The TiZrNbTa/ β -Ti composite demonstrates a yield strength of 1490 ± 83 MPa, ductility of 20.7 % ± 2.9 %, and Young's modulus of 87.6 ± 1.6 GPa. Notably, the yield strength of the TiZrNbTa/ β -Ti composite surpasses that of sintered CP-Ti by 2.6-fold, and its ductility outperforms TiZrNbTa MEA by 2.3-fold. The Young's modulus of the TiZrNbTa/ β -Ti composite is reduced by 28 % and 36 % compared to sintered CP-Ti and TiZrNbTa MEA, respectively. Additionally, it demonstrates superior biocompatibility compared to CP-Ti plate, sintered CP-Ti, and TiZrNbTa MEA. With a good combination of mechanical properties and biocompatibility, the TiZrNbTa/ β -Ti composite exhibits significant potential for clinical applications as metallic biomaterials . Statement of significance This work combines TiZrNbTa MEA with nano-grains and commercially pure Ti with micro-grains to fabricate a TiZrNbTa/ β -Ti composite with bimodal grain-size, which achieves a yield strength of 1490 ± 83 MPa and a ductility of 20.7 % ± 2.9 %. Adhering to the ISO 10993-5 standard, the TiZrNbTa/ β -Ti composite qualifies as a non-cytotoxic material, achieving a Class 0 cytotoxicity rating and demonstrating outstanding biocompatibility akin to commercially pure Ti. Drawing on element diffusion, Zr, Nb, and Ta serve not only as solvent atoms to achieve solid-solution strengthening but also as stabilizers for the transformation of the β -Ti crystal structure. This work offers a novel avenue for designing advanced biomedical Ti alloys with elevated strength and plasticity alongside a reduced Young's modulus.
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