Additive manufacturing (AM) has shown the ability in processing titanium alloys. However, due to the unique thermal history in AM, the microstructure of AM-fabricated parts is metastable and non-equilibrium. This work was aiming to tailor the microstructure and to improve the mechanical properties of α+β Ti-6Al-4V alloy and metastable β Ti-5Al-5Mo-5V-1Cr-1Fe alloys by manipulating the post-process heat treatment. The results showed that Ti-6Al-4V exhibited a metastable α’ martensite microstructure in the as-fabricated condition, while a metastable β structure was formed in as-printed Ti-5Al-5Mo-5V-1Cr-1Fe. After post-process heat treatment, both lamellar and bimodal microstructures were obtained in Ti64 and Ti-5Al-5Mo-5V-1Cr-1Fe alloys. Especially, the Ti-6Al-4V alloy subjected to 950 °C annealing showed the lamellar structure with the highest fracture toughness of 90.8 ± 2.1 MPa.m^1/2. The one cyclically heat-treated has excellent combined strength, ductility and fracture toughness attributed to the bimodal structure. In addition, similar observations of lamellar and bimodal microstructure appeared in the post-process heat-treated Ti-5Al-5Mo-5V-1Cr-1Fe alloy. This study demonstrated that heat treatment is an effective way to tackle the non-equilibrium microstructure and improve the mechanical properties of selective laser melting (SLM)-fabricated titanium alloys.

We study the structural phase transition, elastic and thermodynamic properties of HfCr₂ from first principles calculations. The simulated results are very close to those of previous studies. The phase transition from C15 to C36 occurs at about 202.7 GPa, while the phase transition pressure from C36 to C14 is about 283.9 GPa. It can be seen from the phonon spectra and elastic constants that HfCr₂ is mechanically stable at C15 phase. It can be concluded that C15 phase HfCr₂ is ductile from 0 to 200 GPa, and the bigger the pressure, the greater the ductility from G/B value and Poisson's ratio. Then, through the analysis of density of states, the mechanism of metal properties is discussed. Finally, the relation of thermal expansion coefficient, Debye temperature and thermal capacity to pressure and temperature are discussed.

Tris(dialkylamino)aluminum (TDA) is a vital precursor for synthesis of aluminum nitride. Despite recent advances in TDA’s industrial production, its theoretical synthesis mechanism is still overlooked, remaining a hindrance for rational optimization of chemical reactions. In this study, we employed density functional theory (DFT) to investigate the synthesis mechanism of tris(dimethylamino)aluminum (Al(NMe₂)₃) from LiAlH₄, HNMe₂, and AlCl₃. We used Gaussian16 software at the B3LYP-D3BJ/6-311G(d,p) level to optimize the geometric structures of the reactants, transition states, and products, accounting for the solvent effect of tetrahydrofuran by the SMD implicit solvent model. Our results show that the reaction of LiAlH₄ with HNMe₂ to form LiAl(NMe₂)₄ occurs in four steps. Additionally, the reaction of LiAl(NMe₂)₄ with AlCl₃ proceeds in three endothermic steps. The calculated free energy barriers indicate that all reactions can occur spontaneously, consistent with experimental observations. Our work provides a promising avenue for efficiently evaluating and regulating the TDA reaction process.