발간논문

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To investigate the effect of the microstructure prior to high temperature annealing on the secondary recrystallization in grain oriented electrical steels, the specimens, containing 3.1%Si, 0.034%C, 0.017%Al, 0.008%N, were annealed at 400, 600 and 850℃ for 5 min, in 10% N2+90%H2 atmosphere after the second stage cold rolling and then together with as-rolled specimens, heat-treated again for secondary recrystallization. It has been found that having a fully recrystallized matrix prior to high temperature annealing is not a precondition for complete development of the secondary recrystallization. As the microstructure prior to the final annealing changed from a fully recrystallized to an as-rolled state, the sizes of the secondary grains became larger and their formation temperature higher due to the differences in the microstructures and textures during the heating stage of the final annealing. The specimens with as-rolled and recovered matrices showed better magnetic induction values than the fully recrystallized, of which implication was more ideal orientation of the secondary grains.

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The microstructural development of the adiabatic shear band formed by ballistic impact studied in a Weldalite 049 alloy. The microstructure of the adiabatic shear band was examined by optical and transmission electron microscopy, revealing the existence of fine, equiaxed grains within the adiabatic shear band. This microstructure was considered to be formed in an extremely short time by the combined effects of the highly localized shear deformation and a high temperature rise occurred with the shear band. Dynamic recrystallization is suggested as a possible mechanism to explain the microstructural development of the adiabatic shear band. Although thermal effects were apparent, no evidence was found to support the melting of the alloy.

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An investigation was made to establish the pre-treatment conditions for hot forging of aγ-TiAl alloy. The investigation was based on the fractional ratios of the phases formed under varying rates of heating during reaction. The chemical composition of the experimental alloy was Ti-46.6Al-1.4Mn-2Mo(at%) and elemental powders were used as raw materials to synthesize the alloy. Representative phases in reaction-sintered elemental powders consisted of TiAl, Ti₃Al and TiAl₃, and a fast heating rate increased the amount of Ti₃Al but decreased that of TiAl₃. From the EPMA and EDS analysis it was concluded that the retention of TiAl₃, a source of porosity, was governed by the extent of solid-solid reaction below the melting temperature of Al. With the effect of the heating rate on the TiAl₃confirmed, we then experimented high frequency induction heating as a means to substantially increase the heating rate. A heating rate of 300℃/min thus obtained essentially removed the TiAl₃ phase and reduced porosity in the final material. As a result, a sound γ-TiAl with a fully lamellar microstructure of 50 ㎛ in grain size was produced in heat-treated condition.

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Effect of grain size and inclusions on tensile strength and the Charpy V-notch impact absorption energy of HY180 steels has been investigated by means of optical microscopy, energy dispersive X-ray spectrometry, scanning electron microscopy, tensile tests and Charpy V-notch impact tests. Grain growth during austenizing treatments in Ti-modified alloy was smaller than that in Ti-unmodified alloy. and consequently Ti addition was very effective to grain refinements of HY180 steels. Inclusions containing Ti and S were formed in Ti-modified alloy, while MnS in Ti-unmodified alloy. Tensile strength decreased with raising austenizing temperature, which was attributed to the grain growth. The Charpy V-notch impact absoprtion energy of Ti-unmodified alloy increased with rasing austenizing temperature, while it kept almost constant in Ti-modified alloy, although grain size of the alloy increased.

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The low cycle fatigue (LCF) life prediction model for directionally solidified(DS) Mar-M247LC superalloy has been stuided as the variables of plastic strain and stress range. The plastic strain range is the general criterion for LCF life prediction model like Coffin-Manson equation. On the other hand the stress range is also important to the criterion for LCF life evaluation. The complex effect of plastic strain and stress range on LCF life could be considered as the calculation of hysteresis loop area in fatigue testing. The LCF life as a function of plastic strain and stress range has been measured at the variables of strain rate and DS direction.

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Shape recovery stress of Ti-50.2Ni and Ti-(50-X)Ni-XCu(at%)(X=5-30) shape memory alloys has been measured by means of thermal cycling tests under constant load. In each alloy, the shape recovery stress increased with increasing applied stress up to the critical stress for slip deformation, above which it decreased with increasing applied stress. This was due to decrease in recoverable elongation by the slip deformation. The critical stress for slip deformation depended on Cu-content. With respect to the maximum shape recovery stress, for the alloys whose Cu-content was less than 5at% and more than 15at%, it decreased with increasing Cu-content. For the alloys whose Cu-content was between 5at% and 15at%, however, it increased with increasing Cu-content. Cu-content dependence of the maximum recovery force did not correspond to that of recoverable elongation. Thermal cycling test under costant load method was effective to measure a heating temperature dependence of the shape recovery stress.

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The crystal rotation during deformation was calculated using the Taylor-Bishop-Hill full constraint model and the Renourd-Wintenberger second work minimization method. The changes in orientations of fcc metals with the strain ratio, x of shear strain to plane strain were calculated. The {4 4 11}<11 11 8> orientation component is calculated to be stable at x=0(plane strain), whereas the {1 0 0}<0 1 1> and {1 1 1}<1 1 2> components are stable at x= √2. The stable orientation map at x=1.5 was obtained in the whole orientation space, which shows that {1 1 1}<1 1 2> and {1 0 0}<0 1 1> are stable for initial orientations of γ-fiber and {100} fiber in the Euler space, respectively. The rotation behavior of crystal could be easily understood from the crystal rotation vector at the ψ₁=π/2 section of the Euler space including ε-fiber.

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The purpose of this study is to design and fabricate a new zirconium alloy with higher mechanical properties and to characterize it. The new experimental zirconium alloy was Zr-1.0Nb-0.5Sn-0.5Cr-xMo with addition of 0.2, 0.5, 0.7wt.% Mo, respectively. In order to characterize the experimental alloys, annealing heat treatment was done from room temperature(20℃) to 800℃ for 1hr. The tensile strength at room temperature and Vickers micro-hardness have been measured, and the microstructure was observed to identify the distribution and the shape of the precipitates with annealing temperature. Results showed that micro-hardness, ultimate tensile and yield strengths increased with increasing annealing temperature from 300℃ up to 450℃, while ductility remained practically unaffected by annealing. Transmission electron microscopic examination revealed decomposition of α prime phase and formation of fine cylindrical precipitates during annealing at 450℃ for 1hr., and SADP analysis revealed that the precipitates were mainly ZrMo₂, while annealing at 550℃, caused formation of cell boundary and coarsening of the precipitates. Molybdenum, therefore, was found to be a fairly effective element to inhibit the movement of the dislocations during recovery and thus retard the formation of the recrystallized grains.

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A study has been made to investigate microstructural influence on the fatigue crack propagation(FCP) of an Al-Li 8090 alloy in relation to intrinsic and extrinsic nature at room (300K) and cryogenic temperatures (77K). By applying different heat treatment paths, two types of microstructures were produced : one is the δ`(Al₃Li) phase dominant microstructure exhibiting planar slip characteristics and the other is the S`-δ` microstructure showing homogeneous slip behavior. Crack closure loads measured by the crack-opening displacement method have been used to obtain intrinsic fatigue resistance. At cryogenic temperature of 77K. crack growth rate curves of both microstructures shifted toward higher ΔK region as compared to those at room temperature. The morphology of fatigue fracture surface was significantly changed with decreasing temperature in two microstructures. In a microstructural aspect, such variation of fracture surface was attributed to the transition of slip behavior with decreasing temperature. In the δ` microstructure, planar deformation mode inducing faceted cracks at room temperature changed into rather homogeneous deformation mode at cryogenic temperature. Whereas in the S`+δ` microstructure, intensity of planar slip mode increased at cryogenic temperature resulting in higher degree of surface roughness comparable to that of δ` microstructure.

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Crystallographic texture of as-cast AISI 430 strips both in roll-width direction and through thickness direction was studied. In the center region of strips, typical solidification texture with {100}components was observed and weak component of γ fiber, {111}∥rolling plane, was observed. In the edge region of strips, strong component of γ fiber texture was observed, whereas, typical solidification texture was weakly developed in the whole strip. Crystallographic rotation near the surface layer was observed, which showed the asymmetry of pole figure and development of rolling texture. Near the edge of the strip, orientation density of γ fiber and white band near the central layer increases because of heavy deformation in proportion to height of solidification end point. In case of heavily deformed strips, texture change through the thickness direction was observed. The increasing intensity of γ fiber from surface layer to central layer is due to the increase of shear deformation near central layer.

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