•Phase stabilities of β, α phases and compounds in Ti alloys were explicitly compared.•Alloying element dependence of the elastic properties were accurately determined.•The electronic structures of Ti alloys were systematically analyzed.•Temperature effect related to mixing entropy is significant to stabilize the phases.Phase stability and elastic properties of Ti1-x-Xx alloys (X = substitutional Mo, Nb, Al, Sn, Zr, Fe, Co, and interstitial O) in body-centered cubic (bcc) (β phase) and hexagonal close-packed (hcp) (α phase) crystal structures were studied using first-principles calculations. The formation energy was used to determine the dependence of the phase stability on the atomic concentration x (0 ≤ x ≤ 0.5 for substitutional elements and 0 ≤ x < 0.02 for O) of the alloying elements and atomic configurations (ordered, disordered, and other compound structures). The disordered configurations of the atoms in the alloys were considered within the framework of the special quasi-random structures (SQS) method. The composition dependence of elastic constants, isotropic elastic moduli and density of states (DOS) were also evaluated.The predicted formation energies and elastic properties agrees well with previous experimental and theoretical results. Addition of Mo was found to stabilize the β phase significantly while destabilizing the α phase. Nb was found to be a weaker β-stabilizer. In Ti-Al and Ti-Sn systems, the compounds form preferentially at a higher concentration. Zr shows little effect on the phase stability, thus Zr is a neutral element. For Ti-O systems, the octahedral site is the most stable site for interstitial oxygen in both bcc and hcp structures. The comparison of formation energies showed that O acts as an α stabilizer. Considering the configurational entropy contribution, we concluded that temperature effect can be significant to stabilize the phases. The elastic constants calculation revealed that adding Mo, Nb, Fe, or Co increases the mechanical stability of bcc Ti, whereas bcc Ti-Al, Ti-Sn, and Ti-Zr systems are mechanically unstable for all the calculated concentrations. For their hcp counterparts, Ti-Mo and Ti-Nb have lower mechanical stability than the Ti-Al, Ti-O, and Ti-Sn systems. It was found in the electronic structures calculation that increasing addition of Nb, Mo, Fe, and Co weakens the covalent-like bonding of hcp system and strengthens the metallic bonding of bcc system. This variation of the TDOSs of these Ti alloys explains why the bcc phase becomes more stable than the hcp phase at high concentration. Due to the similarity of electronic structure, Zr does not change the DOS of the bcc and hcp phases significantly: The Fermi level in DOS profile of Ti-Zr was found to be identical to that of pure Ti system. Ti-Al and Ti-Sn alloys keep the covalent-like bonding, which explains why hcp Ti-Al and Ti-Sn are stable.

Prompt phase transformations make grains in the heat-affected zone (HAZ) smaller during welding of 9% chromium (9Cr) heat-resistant steels leading to premature failure under creep conditions, which is well known as a type IV fracture. Because the type IV fracture shortens the creep lifetime of the steels, suppressing the fracture is an urgent task in the energy industry. The present study shows that boron addition and nitrogen reduction inhibit grain refinement after welding because of a change in the morphology of the precipitate at prior austenite grain boundaries. In conventional 9Cr steel (ASME Gr. 92 steel), a high amount of MX was unable to pin interface migration of the phase transformation and generated fine grains in the HAZ. In the new B-added steels, B-stabilized M23C6 became the dominant precipitate and showed a larger pinning effect of the phase transformation than MX, which resulted in coarse grains in the HAZ. This suggests that designing stabilized M23C6 forms a superior welded microstructure and results in a longer creep lifetime of 9Cr steels.