Positron Lifetime Study of Defect Structures in B2 Ordered Co-Al Alloys

The positron lifetimes in the B2 intermetallic compounds Co 100−X Al X (X = 46.9 to 54.1) water-quenched, air-and furnace-cooled from high temperatures have been measured to reveal their defect structures. The positron mean lifetime depends entirely on the composition, irrespective of the cooling rate, and steadily increases from 161 to 180 ps as the Al concentration increases from 46.9 to 54.1 at%. A waterquenched Co 53.1 Al 46.9 was isochronally annealed for 900 s successively at 25 K intervals up to 1073 K, but its positron lifetime remained unchanged by the annealing. These results clearly show that vacancies are formed not only on the Co-sites but also on the Al-sites, and the total vacancy concentration is more than 10 −4 in both the Co-rich and the Al-rich CoAl. It was furthermore found that the population ratio of Al-vacancies to Co-vacancies gradually increases with the Co concentration, and the number of Al-vacancies is comparable with that of Co-vacancies in the vicinity of stoichiometric composition

Numerical Modeling of Distortion of Ti-6Al-4V Components Manufactured Using Laser Powder Bed Fusion

The laser powder bed fusion (L-PBF) process is a powder-based additive manufacturing process that can manufacture complex metallic components. However, when the metallic components are fabricated with the L-PBF process, they frequently encounter the residual stress and distortion that occurs due to the cyclic of rapid heating and cooling. The distortion detrimentally impacts the dimensional and geometrical accuracy of final built parts in the L-PBF process. The purpose of this research was to explore and predict the distortion of Ti-6Al-4V components manufactured using the L-PBF process by using numerical modeling in Simufact Additive 2020 FP1 software. Firstly, the numerical model validation was conducted with the twin-cantilever beam part. Later, studies were carried out to examine the effect of component sizes and support-structure designs on the distortion of tibial component produced by the L-PBF process. The results of this research revealed a good agreement between the numerical model and experiment data. In addition, the platform was extended to predict the distortion in the tibial component. Large distortion arose near the interface between the tibial tray and support structure due to the different stiffness between the solid bulk and support structure. The distortion of the tibial component increased with increasing component size according to the surface area of the tibial tray, and with increasing thickness of the tibial tray. Furthermore, the support-structure design plays an important role in distortion reduction in the L-PBF process. For example, the maximum distortion of the tibial component was minimized up to 44% when a block support-structure design with a height of 2.5 mm was used instead of the lattice-based support. The present study provides useful information to help the medical sector to manufacture effective medical components and reduce the chance of part failure from cracking in the L-PBF process