Jung-Woo Cho

Interface reactions and reaction synthesis of a high temperature composite system

Reactive diffusion during isothermal annealing was examined in a Ti−Al−Si temary system. When a TiAl/TiSi2 reaction couple was annealed at 1373 K for 200 h, the product phase sequence was observed as TiAl/TiAl2/Ti2Al5/TiAl3/Ti5Si4/TiSi/TiSi2, in which the integrated diffusion coefficient of Ti2Al5 showed the lowest value. A Ti-rich Ti5Si3 phase appeared when an additional Ti flux was provided between the TiAl/TiSi2 reaction. The growth kinetics for both the TiAl/TiSi2 (direction reaction) and TiAl/Ti/TiSi2 reactions (Ti biased reaction) were identified, and a comparison of the morphology of the product phases was provided. The chemical potential variation and mass balance exhibited guidance in predicting the diffusion pathway during isothermal annealing. It appears that a biasing reaction (i.e., controlling component flux) is an effective tool for the phase selection and morphology changes during isothermal annealing.

Practical application of diffusion pathway analysis for SiC-metal reactions

Diffusion pathway analyses and operation kinetics for SiC-/metal interface reaction have been investigated To select the materials for an interlayer between an SiC and Ni reaction couple, a constructed chemical potential diagram has been utilized. When Cr was selected as the interlayer material for the SiC/Ni diffusion couple, the reaction pathway and product phase morphology have been modified by controlling the component flux. The strategy of modifying the diffusion pathway provides insight for achieving a stable interface design and for controlling the interface morphology within a kinetic bound.

Dynamic compressive properties obtained from a split Hopkinson pressure bar test of Boryeong shale

Dynamic compressive properties of a Boryeong shale were evaluated by using a split Hopkinson pressure bar, and were compared with those of a Hwangdeung granite which is a typical hard rock. The results indicated that the dynamic compressive loading reduced the resistance to fracture. The dynamic compressive strength was lower in the shale than in the granite, and was raised with increasing strain rate by microcracking effect as well as strain rate strengthening effect. Since the number of microcracked fragments increased with increasing strain rate in the shale having laminated weakness planes, the shale showed the better fragmentation performance than the granite at high strain rates. The effect of transversely isotropic plane on compressive strength decreased with increasing strain rate, which was desirable for increasing the fragmentation performance. Thus, the shale can be more reliably applied to industrial areas requiring good fragmentation performance as the striking speed of drilling or hydraulic fracturing machines increased. The present dynamic compressive test effectively evaluated the fragmentation performance as well as compressive strength and strain energy density by controlling the air pressure, and provided an important idea on which rock was more readily fragmented under dynamically processing conditions such as high-speed drilling and blasting.