Abstract:
This work presents a process optimization study for processing vapor grown carbon fibers and single walled carbon nanotubes into Si3N4 with the goal of developing advanced structural ceramic materials. Solid composite specimens were fabricated using a freeform fabrication technique called robocasting that uses high solids loading aqueous suspensions to fabricate near-net-shape-ceramic composite parts. Colloidal processing methods were used to manipulate the charging behavior between carbon nanofibers and silicon nitride particle surfaces in order to develop forty-five percent solids loading suspensions with a pseudoplastic theology that borders on dilatancy and is suitable for robocasting solid parts. Dispersion of nanofibers within each composite system was identified as was the starting dispersion of the nanofibers in the slurry.
Abstract:
The cyclic and static oxidation behavior in the processing of Ti-44Al-11Nb alloy has been studied at 900, 950, and 1000 °C in air for a period of up to 168 hours (or a week). A recreation of diffusion paths for the oxide to penetrate into the base metal during every cycle results in the lower oxidation rates during cyclic mode compared to static mode. An extended period of heating time allows for the coarsening of the structure for both forms of alloys: polycrystalline and directionally solidified crystals. Coarsening and phase transformation in the heating period involves large amounts of dislocation activities in the lamellar structure as well as at the interface of lamellar structure and islands of γ.
Abstract:
The composites of 6061 and 2014 aluminum alloys reinforced with alumina particles have been subjected to solutionizing treatment at 540 °C for various lengths of time up to 20 hours. The two composites show different behavior when the hardness is measured as a function of solutionizing time. The 6061 aluminum alloy containing 0.1, 0.15 and 0.2 volume fractions of alumina (VFAP) show a continuous increase in hardness values as a function of solutionizing time while 2014 alloy shows softening under almost identical experimental conditions. The grain growth law has been found to be observed in both composites and their monoliths. The aging behavior as influenced by the solutionizing seems to be affected by the solutionizing time in an identical fashion. Even though the monoliths indicate a decrease in time required to get the peak hardness (TPH) values the composites show an initial decrease and then increase in TPH values at 200 °C as a function of solutionizing time. The bonding between the particles and the matrix in the two composites has been found to improve as a result of longer solutionizing time as can be seen by the SEM fractographs for the samples deformed to fracture during room temperature tensile testing.
Abstract:
The grain growth law has been verified for a range of grain sizes produced in three composites containing 6061 aluminum alloy matrix and 0.10, 0.15 and 0.20 volume fractions of alumina particles (VFAP), with particle sizes of 10, 15 and 20 μm respectively, by solutionizing at 540°C for different times. The solutionizing time affects the (a) microstructures developed at the interface between the particles and the matrices and (b) age hardening characteristics, microstructures and microhardness values, at 200°C.
