발간논문

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Effects of dopant additions on interface stability in ZMR(Zone Melting Recrystallization) of silicon thin films have been investigated through computer simulation and experiments. According to computer simulation, it has been found that critical scanning speed (V^*) exists such that the critical wavelength λ at the solidifying interface increases with scanning speed below (V^*) due to radiative supercooling, while λ decreases with scanning speed above (V^*) due to constitutional supercooling. It turned out that (V^*) decreased with additions of B and P dopants on silicon thin films by ion implantation before ZMR and interface stability change with respect to scanning speed agreed well with expectations from computer simulation.

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A transmission electron microscopy study of the pulsed laser beam crystallized silicon thin film, deposited on SiO₂ at 520℃ by low pressure chemical vapor deposition, has been carried out. It has been shown that the amorphous silicon thin film irradiated by a pulsed laser beam was crystallized by an explosive crystallization process. The resulting polycrystalline matrix contained crystals which had either a diamond cubic or a hexagonal crystal structure. In the interior of the grains which have a hexagonal crystal structure, dislocations, extrinsic type stacking faults, and intrinsic type stacking faults were observed, but the defect density was apparently decreased compared with that in the grains crystallized by a soild phase crystallization. It is thought that the hexagonal crystal structure and many defects were formed by high temperature gradients at the liquid/solid interfaces and the growth breakdown.

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The purpose of this study is to investigate the effect of composition on the microstructure and mechanical properties of TiAl alloys. The composition ranges studied were Ti-xAl(x=43.7, 45.6, 47)-yMn(y=0.3). The mechanical properties have been characterized and correlated with microstructure, tetragonality, and deformation behavior. Two phase (α₂+γ) alloys have single γ grains and lamellar grains, which consist of twin-related L1。γ Phase and α₂phase. Addition of Mn promotes the formation of twin-related structure and the refinement of lameller structure and grain size. Increase in Ti/Al ratio promotes the refinement of grain size, but increases lamellar spacing. The modification of microstructure directly influences the compression properties and deformation mode of TiAl alloys. It has been found that Mn addition and increase of Ti/Al ratio enhance the plasticity of two phase (α₂+γ) alloys. The ductilization effect of Mn is partly due to its ability to promote the formation of deformation twins and dislocation generation at the twin intersections.

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The high temperature deformation behavior of 5083Al alloy has been examined through a series of tensile and load relaxtion tests, specifically in regard to the effect of grain size and test temperature. The experimental results were then analyzed using the recently proposed inelastic constitutive theory based on dislocation dynamics. The basic concept of this theory is that the flow behavior of fine grain crystalline materials at high temperature consists of grain boundary sliding accommodated mainly by grain matrix deformation caused by dislocations. Some of the important results obtained from this study is as follows. : 1) The high temperature deformation of fine grained 5083Al alloy is confirmed to consist of grain boundary sliding and accommodating grain matrix deformation, 2) The grain boundary sliding becomes a dominant deformation mechanism up to the strain rate of 10^(-2) and in a much lower temperature range of above 430℃ in this alloy, 3) The grain size effect appears to follow the Hall-Petch type relation replacing the flow stress by the internal strength variable(σ^*), 4) The activation energy for a lead dislocation to overcome grain boundaries is found to be Q¹=168.4 KJ/㏖ for this alloy.

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The microstructural evolution and alloying behavior with milling time during mechanical alloying (MA) of elemental powders with nominal composition Ni-17.5Al-8Cr-0.5Zr-0.4B(at%), and the effect of microstructure on the room temperature tensile properties of hot-extruded MA powders were investigated. The powders milled for 360min, which were considered to be in steady state, consisted of Al-supersaturated disordered solid solution with small amount of amorphous phase and α-Al₂O₃ particles produced by decomposition of process control agent during milling. Metallographic examinaton of hot-extruded powders showed very fine grains containing L1₂-type ordered Ni₃Al, disordered phase and finely dispersed oxides. The refined grain size and the oxide dispersoids constributed to the yield strength as high as 1593MPa and the ductility of 8.6%. The results in this study indicate that the mechanical alloying process is suitable for the production of very fine grained intermetallic compounds with very high strength.

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