Electronic Origin of α″ to β Phase Transformation in Ti-Nb-Based Thin Films upon Hf Microalloying

Abstract

We present results on thin Ti-Nb-based films containing Hf at various concentrations grown by magnetron sputtering. The films exhibit α" patterns at Hf concentrations up to 11 at.%, while at 16 at.% Hf, the β-phase emerges as a stable structure. These findings were consolidated by ab initio calculations, according to which the α"-β transformation is manifested in the calculation of the electronic band energies for Hf contents between 11 and 18 at.%. It turns out that the β-phase transition originates from the Hf 5d contributions at the Fermi level and the Hf 6s hybridizations at low energies in the electronic density of states. Bonding-anti-bonding first neighbor features existing in the shifted plane destabilize the α"-phase, especially at high Hf concentrations, while the covalent-like features in the first neighborhood stabilize the corresponding plane of the β-phase. Thin films measurements and bulk total energy calculations agree that the lattice constants of both α" and β phases increase upon Hf substitution. These results are important for the understanding of β-Ti-based alloys formation mechanisms and can be used for the design of suitable biocompatible materials.of Hf can slightly affect the Ti-Hf's dynamic Young's moduli, while they increase the tensile strength of the system [23].Ti-Nb-Hf ternary alloys have a strong potential to combine the biocompatibility of these three elements. Especially in the thin film form, these Ti-Nb-Hf alloys could be of valuable use as coatings. However, to our knowledge, reports focusing on the stabilization of the β-phase of Ti-Nb-Hf are rather scarce. Aiming to reveal the preferred phases and their electronic origin, we present a combined theoretical and experimental study on Ti-based systems with low Nb substitutions (around 17 at.%-where the α" phase emerges in the Ti-Nb alloys) and various Hf content (between 5.55 and 25.00 at.%). Thin Ti-based films were grown by magnetron sputtering (MS) and characterized by XRD measurements and energy dispersive X-ray spectroscopy (EDS). Linearly augmented plane wave density functional calculations DFT-LAPW TiNb 18.75 Hf x (0 ≤ x ≤ 25 at.%) were used to reveal the preferred crystalline phases and their electronic origin. These results are adequate for understanding the β-Ti based alloy formation mechanisms and can be used for the design of suitable biocompatible materials.

Electronic and Structural Properties of Antibacterial Ag–Ti-Based Surfaces: An Ab Initio Theoretical Study

“…The available first neighbour environments (FN) offer Ag the opportunity to have Ti, Nb or TiNb in the first neighbourhood (named Ti_FN, Nb_FN and TiNb_FN). Ag prefers Ti_FN showing a total energy difference of −4.80 eV against −4.29 eV of the mix TiNb_FN and 3.00 eV of Nb_FN systems in line with previous calculations of Ga, In, Sn and Hf in β-TiNb [6,21,47]. Concerning the mechanical properties, the bulk modulus of Ti 68.…”

“…Aiming to reveal the energetically favoured Ag sites inside it, we consider Ti 68 Nb 30 Ag 2 (at%) 3 × 3 × 3 supercells (i.e., close to 30 (at%) Nb composition) where the β solid solution phase is stable [6]. One Ag atom substitutes a Ti atom in different first neighbour environments, resulting in the composition Ti 68 Nb 30 Ag 2 (at%) in line with previous calculations of Ga, In, Sn and Hf in β-TiNb [6,21,47]. The available first neighbour environments (FN) offer Ag the opportunity to have Ti, Nb or TiNb in the first neighbourhood (named Ti_FN, Nb_FN and TiNb_FN).…”

“…Ag in bulk β-TiNb: In Section 3.1, for the Ti 68 Nb 30 Ag 2 (atomic units at%) alloy a 3 × 3 × 3 supercell (9.831 Å × 9.831 Å × 9.831 Å for the calculated 3.277 Å lattice constant) was considered and a 5 × 5 × 5 k-point sampling was employed by applying periodic boundary condition in the three dimensions (3D) to mimic the bulk system while by reordering the Ti and Nb atoms, a different first neighborhood was achieved for Ag atoms in line with previous calculations [6,21,47]. Section 3.2.…”

“…The prototypical β-TiNb system we assume is Ti 70 Nb 30 [6,21,47]. Aiming to reveal the energetically favoured Ag sites inside it, we consider Ti 68 Nb 30 Ag 2 (at%) 3 × 3 × 3 supercells (i.e., close to 30 (at%) Nb composition) where the β solid solution phase is stable [6].…”

“…In addition, the lowest valence electron state at −6.2 eV has clearly a Ag 5s character and an antibonding character with respect to the Ti neighbouring atoms, as presented in the (001) WF projection in the inset of Figure 1a. The presence of bonding and antibonding states at energies far from the Fermi level introduces system instability [6,21,46,47,49].…”