Fabrication of Ni-Rich 58NiTi and 60NiTi from Elementally Blended Ni and Ti Powders by a Laser Powder Bed Fusion Technique: Their Printing, Homogenization and Densification

Compared to the equiatomic or near-equiatomic NiTinol alloys, Ni-rich NiTi alloys are suitable to be employed in structural applications as they exhibit higher hardness and are dimensionally stable. This research aimed to process two different grades of Ni-rich NiTi alloys, 58NiTi and 60NiTi, from Ni–Ti powder mixtures having about 58 wt.% and 60 wt.% Ni, respectively. This was performed by a laser powder bed fusion technique. At the first stage of this research, the printability of the used powder mixtures was investigated by applying different sets of printing parameters. Two appropriate sets were then selected to print the samples. Microstructural study of the printed parts revealed the existence of inhomogeneity in the microstructures. In addition, depending on the applied set of parameters, some amounts of cracks and pores were also present in the microstructure of these parts. Postprinting hot isostatic pressing procedures, performed at different temperatures, were developed to cause the reaction of phases, homogenize the parts, and possibly eliminate the existing flaws from the samples. Effects of these applied treatments on the microstructure, phase composition, density, dimensional integrity, and hardness of parts were sequentially studied. In essence, 58NiTi and 60NiTi parts having phase compositions complying with those of the equilibrium phase diagram were obtained in this research. However, the mentioned cracks and pores, formed in the microstructure of as-printed parts, could not be fully removed by postprocessing treatments.

Low-temperature degradation of high-strength Y-TZP (yttria-stabilized tetragonal zirconia polycrystal)

The objective of this study was to investigate the low temperature degradation characteristics of 2 types of high strength yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) in order to evaluate its suitability for implant body, implant superstructure or abutment. Disk-shaped conventional Y-TZP (0.25 mass% alumina) subjected to hot isostatic press treatment (HIP-Y-TZP) and Y-TZP/4Al₂O₃ with additional alumina (4.0 mass%) were mirror-polished. Accelerated aging tests with 134°C for 5 h at 0.2 MPa and 180°C for 5 h at 1.0 MPa were performed using an autoclave. Biaxial flexural strength and crystal phases were evaluated. Strength decreased as the proportion of monoclinic phase increased after accelerated aging treatment for both types of high-strength Y-TZPs. Despite the low alumina content, HIP-Y-TZP showed higher static strength and strength after accelerated aging treatment compared to Y-TZP/4Al₂O₃. However, both types of Y-TZP had adequate strength to be used as dental restorations even after accelerated aging treatment, therefore, its clinical suitability was considered high.

Effect of (Tb+Y)/Al ratio on Microstructure Evolution and Densification Process of (Tb0.6Y0.4)₃Al₅O12 Transparent Ceramics

(Tb_0.6Y_0.4)₃Al₅O₁₂ transparent ceramics were successfully fabricated by solid-state reactive sintering using Tb₄O₇, Y₂O₃, and α-Al₂O₃ powders as raw materials. The effect of (Tb+Y)/Al ratio on microstructure evolution and densification process was investigated in detailed. The results showed that the grain growth kinetics were significantly affected by (Tb+Y)/Al ratio. Al-rich and Tb-rich phases appeared in part of the samples of different ratios. Particularly, excess aluminum increased the diffusing process, leading to a higher densification rate, while samples with excess terbium ratios displayed a smaller grain size and lower relative density. The optical quality was highly related to the amount of the secondary phase produced by different (Tb+Y)/Al ratios. Finally, (Tb_0.6Y_0.4)₃Al₅O₁₂ transparent ceramics have been fabricated through pre-sintering in vacuum, followed by hot isostatic sintering (HIP), and the best transmittance of sample with a 4 mm thickness was approximately 78% at 1064 nm.