Microstructures and electronic characters of ß-Ga2O3 on different substrates: exploring the role of surface chemistry and structures.

Unveiling the microstructural and electronic properties of ß-Ga2O3 on different substrates is vital to realize the high quality and performance of ß-Ga2O3. Here, the microstructure disorder, defect characters and orbital structures of ß-Ga2O3 on the Al2O3, MgO, and SiC substrates with different terminations are studied. Although several growth mechanisms for ß-Ga2O3 are observed on the same substrate, ß-Ga2O3 prefers to deposit the octahedral Ga atom firstly on the Al2O3 and MgO substrates, and the latter can restrain the oxygen-vacancy formation and migration. The structural disorder, band offsets and gap states can be improved upon depositing ß-Ga2O3 on a substrate with metal terminations under the oxygen-poor conditions. Compared to the Al2O3 substrate, ß-Ga2O3 on the SiC substrate shows a smaller structure disorder and a higher defect formation energy, in particular under the oxygen-rich conditions, since ß-Ga2O3 prefers to deposit the tetrahedral Ga atom firstly on the SiC substrate to form a SiC-Ga2O3 interface with less dangling bonds. The type-II band alignment of the SiC-Ga2O3 interface can be changed into the type-I character with larger band offsets when ß-Ga2O3 is deposited under the oxygen-rich conditions, irrespective of the termination of the SiC substrate. These results provide a useful understanding of the effect of substrates on the quality and performance of ß-Ga2O3 and a scientific basis for the application of substrate-Ga2O3 interfaces.

Impact of Heat Treatment on the Mechanical Properties and Fracture Morphology of Ti555211 Alloy.

Heat treatment is important for optimizing the strength performance and improving the toughness of titanium alloys. In this study, we investigated the impact of three heat treatment methods (ß-annealing, double annealing, and solid-solution and aging treatment) on the mechanical properties and fracture morphology of Ti555211 titanium alloy. The results show that after ß-annealing treatment, the alloy retains a high strength, while showing almost no ductility, and no yield strength. The alloy after double annealing has a high elongation rate (~54%) and lower strength. After solid-solution and aging heat treatment, the alloy was able to retain both high strength and a certain degree of ductility. The optimal heat-treatment process is solid-solution treatment at 820 °C/2 h and aging at 560 °C/12 h, which results in a maximum tensile strength of 1404 MPa and an elongation rate of 11%.

Lamellar structure/processing relationships and compressive properties of porous Ti6Al4V alloys fabricated by freeze casting.

Lamellar pores have superior biocompatibility due to their similarity to the lamellar structure of natural bones. In the present work, porous Ti6Al4V alloys with lamellar pores were successfully fabricated by directionally freeze casting. The lamellar structure/processing relationships were systematically studied through analyzing the interaction between ice front and alloy powders. The structural feature of translamella bridges is observed in the lamellar structure. The volume shrinkage of porous Ti6Al4V alloys is in the range of 44-60%. This is much higher compared with that of the porous ceramics. The solid content in the slurry exerts a strong influence on the porosity, while the freezing ice front velocity affects the structural wavelength and pore width. With the increase in ice front velocity, the structural wavelength decreases by an exponential function. The lamella formation mechanism and porosity gradient along the freezing direction were discussed. Young's modulus and yield stress of porous Ti6Al4V alloys fall in the range of 2-12 GPa and 40-300 MPa, respectively. The dominant compressive deformation mode is lamella buckling and splitting. The fabricated porous Ti6Al4V alloys possess higher relative yield stress.

Microstructure Control of Welded Joints of Dissimilar Titanium Alloys by Isothermal Forging.

In this study, the welded joints of dissimilar titanium alloys Ti600/Ti-22Al-25Nb were strengthened by isothermal forging. Different deformation parameters, including temperature, deformation speed, and reduction, were chosen. By isothermal forging, the original coarse dendritic grains of the welded joints were broken up effectively to form a large number of equiaxed grains. Meanwhile, many second phases were precipitated in the grain. Additionally, the dynamic globularization kinetics of second phases within the welded joints were quantitatively characterized and investigated. The results showed that the dynamic globularization kinetics and globularization rate were sensitive to the deformation conditions, and were promoted by a reduced strain rate and an elevated deformation temperature.