Search

There has been increasing effort to achieve high strength and ductility for Ti-based alloys. However, there are limited reports investigating the effect of multiple solute elements on the diffusion mechanisms at the atomic level. In this study, the diffusion mechanism of interstitial solute elements was investigated in α-Ti using the first-principles calculations. Solute elements B, C, N, O, and F were confirmed to be the most stable at octahedral sites with their calculated formation energies consistent with the reported phase diagrams of Ti and solute elements. The migration energies of C, N, and O are high, approximately 2 eV, while those of B and F were approximately 1 eV. A high correlation was observed between the migration energies and remarkable difference in the charge densities of the solute atoms between stable and transition states. These results indicate that migration energies and diffusion could be determined by considering not only the atomic radius of solute atoms but also their charge transfer with Ti atoms.

The diffusion mechanism of interstitial solute elements in α-Ti was investigated using first-principles calculations. Solute elements, B, C, N, O and F, were confirmed to be the most stable at octahedral sites with their calculated formation energies. The migration energies of C, N and O are high, approximately 2 eV, while those of B and F were approximately 1 eV. A high correlation was observed between the migration energies and difference in the charge densities of the solute atoms between stable and transition states. These results indicate that migration energies and resultant diffusion could not be determined only by the atomic radii of solute atoms, and charge transfer must also be taken into consideration. The charge transfer between matrix and solute atoms affects diffusion mechanism of solute atoms in Ti.

Commercial pure Ti alloys are favorable for biomedical applications; however, they need to be strengthened by the addition of alloying elements. Herein, Ti-Fe alloys modified with Zr were explored to produce alloys that exhibit high strength and ductility and are biocompatible. Ti-Fe extruded materials with Zr solutes were prepared by combining solid sintering and hot extrusion. X-ray diffraction results revealed that the lattice constant of α-Ti and β-Ti increased proportionally with the Zr content, indicating the Zr solid solution behavior. In situ and ex situ observations of the microstructure formation revealed the element Zr, a weak β-stabilizer, and a decreased phase transformation temperature. However, during the cooling process after hot extrusion, the grain growth stagnated at the same temperature, irrespective of the Zr content, resulting in both α-Ti grain refinement and increased β-phase area fraction. The tensile test results demonstrated that 1010 MPa 0.2% yield strength of the Ti-4Fe-13Zr extruded material increased to 156%, compared with that of the base Ti-4Fe material. Furthermore, Labusch model and the Hall-Petch equation were used to quantitatively evaluate the strengthening mechanism derived from the Zr solid solution and the changes in the grain size.

Titanium is well-known for its high strength, good corrosion resistance, and biocompatibility. For further improvement of its mechanical properties, it is alloyed with other elements, such as zirconium. In this study, pre-mixed pure Ti powder and ZrH2 particles were employed to prepare extruded Ti–Zr alloys with a low hydrogen content via dehydrogenation after sintering. The Zr solutes were uniformly dissolved in the α-Ti matrix by applying the optimal conditions for homogenization and water-quenching heat treatment. The synthesized Ti–Zr alloys consisted of the equiaxed fine α-Ti grains via dynamic recrystallization during extrusion. When the ZrH2 particle content was less than 10 wt%, the extruded Ti–Zr alloys showed a large tensile elongation of more than 25% at room temperature with a yield stress of approximately 850 MPa. In the case of Ti–10% ZrH2 powder, the mean α-Ti grain size was 2.7 μm, which was smaller than that of extruded pure Ti (3.5 μm). By applying the Hall–Petch equation for grain refinement and Labusch model for Zr solid solution to quantitatively estimate the increase in the yield stress, it was clarified that both factors had an equal effect for Ti–5 and –10 wt% ZrH2 powder even with the dominance of solid-solution strengthening by Zr solutes for alloys with small Zr content.

Single β phase titanium alloys are used for fabricating biomedical devices, and it is desirable to improve the strength of these alloys. In this study, the tensile strength of Ti-35 at.% Ta alloys with a single β phase was improved using oxygen solid solution and the strengthening mechanism was also investigated. The lattice constants of these alloys calculated from X-ray diffraction data increased with an increase in the oxygen content. This result suggests that oxygen atoms formed solutes in the alloys, which was supported by results obtained from first-principles calculations. Microstructural analyses of the alloys indicate that Ti, Ta, and O were homogeneously distributed. Tensile tests were conducted for the alloys, and the 0.2%YS increased with the increase in the O content of the alloy. The increments of 0.2%YS which was determined experimentally correspond well with the theoretically determined trend, and this observation is a result of the effect of the change in grain size and O solid solution. The contribution of the grain refinement was negligibly small compared with that of the O solid solution, and the latter is the major reason for the observed increase in the 0.2%YS of the Ti-35 at.% Ta alloys.

Ion-conducting solid electrolytes are widely used for a variety of purposes. Therefore, there is a high demand for the design of highly ion-conductive materials. Theoretical simulations have become effective tools for investigating the performance of ion-conductive materials because of advancements of computers and computational codes, respectively. However, it can be significantly expensive to conduct an extensive search using theoretical computations. Further, dynamic conductivity and static stability must be simultaneously satisfied for practical applications. Therefore, in this study, we propose a computational framework that simultaneously optimizes dynamic conductivity and static stability; this is achieved by combining theoretical calculations and the Pareto hyper-volume criterion-based Bayesian multi-objective optimization. In our framework, we iteratively select the candidate material that maximizes the expected increase in the Pareto hyper-volume criterion; this is a standard optimality criterion of multi-objective optimization. We show that ion-conductive materials with high dynamic conductivity and static stability can be efficiently identified by our framework via two case studies on diffusion of oxygen and lithium.

Ti-based materials with high oxygen solute contents were fabricated from mixtures of Ti powder and TiO2 particles by selective laser melting (SLM). Uniformly dissolved oxygen (O) from the TiO2 particles caused c lattice expansion in α-Ti crystals, which effectively increased the strengths of as-built SLM Ti–O materials. The as-built SLM Ti–O material using 1.5 wt% TiO2 showed a yield stress of 962 MPa and 15.3% elongation. The yield stress increases calculated by the Hall–Petch equation and Labusch model were equivalent to those observed experimentally, and O solid-solution strengthening was dominant in increasing yield stress.