Compositional screening of Zr-Nb-Mo alloys with CALPHAD-type model for promising bio-medical implants

The increasing demands for biological hard tissue replacements (HTR) press on the development of biomedical implant materials with good biological and mechanical compatibility. Three criteria of screening promising alloy compositions for bio-medical application were proposed: (1). High thermodynamic stability of biological compatible phase to avoid β_bcc→α_hcp and β_bcc→αʹ_hexagonal transformations. (2). Positive tetragonal shear elastic constant C′ and shear modulus to avoid the acoustic lattice softening induced structural transformation. (3). Appropriate Young's modulus of β_type alloy in or close to 10-30 GPa to avoid the “stress-shielding effect”. To validate the proposed criteria, the initiatives of computational screening for candidate compositions of Zr-Nb-Mo alloys were implemented by CALPHAD based modeling. With the thermodynamic calculation, not only the plot of T0 contour was employed to indicate the boundaries of non-classical decomposition of β_bcc phase and martensitic transformation, but also the composition range in which the alloys exhibit relative low critical driving force to trigger nucleation and growth of α_hcp was determined. The Young's modulus at body temperature and the contour of the vanishing tetragonal shear elastic constant were predicted within the entire composition range. By superimposing the composition ranges which individually satisfy the criteria, the “candidate composition” were determined as Zr-(18−24)Nb-(0−3)Mo (atom%). They are in good agreement with the composition in the experimentally fabricated biomedical alloys, thus further exemplify the reliability of the criteria. The proposed criteria together with the CALPHAD based computational methodology as a whole open up a new way to develop the potential bio-medical materials in the framework of Materials Genomes.