발간논문

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The evolution of the through-thickness textures in a ferritic stainless steel sheet was investigated by means of texture measurements and microstructure observations. Pronounced through-thickness texture gradients were found in the hot band where the rotated cube orientation prevailed at the center layer and the Goss-orientation developed in the near-surface layers. The cold rolling texture was strongly dependent on initial preferred orientations of the hot band. The rotated cube orientation was quite stable during cold rolling. The weak initial texture of the hot band led to the formation of the typical bcc rolling texture. Upon annealing, the {557}$lt;583$gt; developed at the expense of the rotate cube orientation and the {111} fiber texture formed from the typical bcc rolling texture components. The evolution of texture gradients during rolling was discussed by means of the orientation stability based on the Taylor deformation theory. The recrystallization textures were explained in terms of the nucleation and the growth selection.

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In this study, training effects in Ni-Al-Fe-Mn shape memory alloys composed with β` and γ phase have been investigated. In particular, effects of the initial bending strain, volume fraction of γ phase and training cycles on shape recovery ability of the alloys were investigated. And the change of transformation temperature with training cycles were also investigated. The results of the training experiments showed that shape recovery ability increased rapidly in the early stage of training. The shape recovery ability of the alloy with low bending strain was poorer than that of the alloy with high bending strain in the first cycle of training, but it was reversed after the second cycle of training. A poor shape recovery ability in the first cycles of training was considered to be caused by the reduction of shape recovery energy of β` phase due to shape recovery of γ phase deformed plastically. The drastic increase of shape recovery ability in the second cycle of training was attributed to the increase of the recoverable detwinning strain of β` phase attained by the reduction of the increase rate in the dislocation density, owing to the strain hardening of γ phase occurred during the first cycle of training. The first-stage shape recovery decreased with increasing number of cycles of training after the second cycle of training, in contrast the second-stage shape recovery increased from the beginning but there was no rapid increase. This seemed to be due to the increase of the number of β` martensite variants transformed to β phase at higher temperature due to introduction of dislocation with training increase.

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Microstructure of a direct chill casted Al-10 wt.%Bi-2 wt.%Cu-1 wt.%Mg-0.5 wt.%Mn alloy φ180×2000 ㎜ in size was carefully investigated on the subsequent process after casting in order to search the optimum process condition suitable for mass production. Both the chemical modification by adding Sr into the melt and the rapid cooling rate by employing direct chill easting result in homogeneous cast structure consisting of primary Al and modified fine eutectic Si in spite of a large size of the ingot. The formation of ternary eutectics leads the segregation of CuMgAl₂ phase mainly in the center of the ingot. DSC reveals that ternary eutectics dissolves into the matrix around 520℃. The separation of network-structured fibrous eutectic Si to fine particle is found after one-hour heating above 460℃. Proper heat treatment causes equiaxed fine Si particle with the size of 10 ㎛ and complete dissolution of segregated CuMgAl₂ phase. Coarsening of separated Si particle is governed by diffusion controlled growth.

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The reason why the strength in the transverse direction is higher than that of the longitudinal direction has been identified through the investigation and quantitative treatment of the effect of the basal pole component (F) on the strength in Zr-2.5Nb pressure tube materials. Since the yield strength at room temperature has a linear relationship of σ_(YS) = σ_i + K_F F with the basal pole component in the tensile direction in the as-received and recrystallized material, it can be concluded that the origin of the higher yield strength in the transverse direction mainly is attributed to the higher basal pole component. The deformation mechanisms operated during tensile tests have been analyzed by the observation of the stress-strain curves and by the texture measurement before and after tensile test. The critical resolved shears stressed (CRSS) for $lt;a$gt; slip and (112＿1) and (101＿2) twinning system have been evaluated using the strength of the recrystallized materials and the relationship between the basal pole component and strength.

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Microstructure and mechanical properties of 10Cr-MoVNbN steels with increasing nitrogen contents were investigated. The nitrogen contents of the steels were 0.025 wt%(N02) and 0.045 wt%(N05), respectively. After tempering, the microstructure of the steels consisted of the tempered martensite matrix and various precipitates(M_(23)C_6, Nb(C, N), Cr₂N, V(C, N)). V-rich fine precipitates(5-20 ㎚) observed in the N05 steel hindered the movement of dislocation and consequently delayed formation of subgrain during tempering. It is clarified by transmission electron microscopy that Cr₂N nitride starts to precipitate at a lower tempering temperature and to decompose at a higher tempering temperature in the N05 steel than in the N02 steel. With increasing nitrogen content, hardness, tensile strength, high temperature creep strength and creep rupture elongation increased, but impact property and minimum creep rate decreased.

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The present study was focused on the correlation of the microstructure with the cryogenic fracture characteristics in the heat-affected zones (HAZs) of multi-pass welded cryogenic steel, i.e., QLT (quenching, lamellarizing and tempering)-processed 9% Ni steel used for LNG (liquefied natural gas) storage tanks in Korea due to the advanced cryogenic fracture toughness. The fracture behaviors of the various micro-zones within HAZ including LBZ (local brittle zone) were investigated using simulated HAZ specimens. From the Charpy tests using various synthetic CGHAZ specimens, the primary LBZ in this steel was found to be inter-critically reheated coarse grained HAZ (IC CGHAZ) like other structural steels. However, unlike other cases, IC CGHAZ in this cryogenic steel showed LBZ phenomena only at cryogenic temperature, not at room temperature. Micro-mechanical processes were identified by in situ observation within SEM (scanning electron microscopy) using miniaturized SENT (single edge notched tension) specimens. Based on the test results, the possible causes on cryogenic LBZ phenomena were analysed and discussed. These results suggest that the LBZ phenomena could be explained by total consideration of various metallurgical factors incluting carbon contents in M-A(martensite-austenite) constituents and their influences on fracture behaviors at cryogenic temperature.

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It is well known that the crack tip opening displacement (CTOD) test, widely used for heat-affected zone (HAZ) toughness evaluation, is very sensitive to complex distribution of microstructures in HAZ. So, as the second step of the researches on metallurgical analyses of fracture characteristics in HAZ of improved 9% Ni steel, this study aimed to investigate the microstructure distribution including local brittle zone (LBZ) and its influences on CTOD toughness. To study the correlation between microstructures and actual HAZ toughness in more systematic way, K-grooved HAZ specimens were prepared for the CTOD tests. To figure out the microstructure influence, the microstructure-distribution maps were constructed by semi-empirical method including the process of sectioning and observation of tested specimens. In the regions near fusion line, it was found that the crack initiations were ruled by weakest-link-typed failure, which resulted in relatively low toughness of the regions. In the regions of fusion line (F.L.) +2 ㎜ and +3 ㎜, i.e. relatively high toughness regions, the failure was ruled by rule-of-mixture type. It was also revealed that the minimum fraction of LBZs showing the weakest-link-typed failure was 17% in crack tip front of actual HAZ specimens. From the results obtained, the mechanism of toughness change based on metallurgical analyses was proposed and discussed.

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It is well known that Fe₃Al intermetallic compound shows an anomalous peak of the yield strength at about 500℃. That is, the yield strengths increase with increasing deformation temperature in the range of 300℃-500$quot;℃, and then decrease at higher temperatures. The dislocation structure was examined by transmission electron microscopy, and high temperature mechanical properties were examined by tensile and load relaxation tests. The flow stress curves obtained from lead relaxation bests were then analyzed in terms of internal variable deformation theory. It was found that the flow curves consisted of three micro-deformation mechanisms i.e inelastic deformation mode, plastic deformation mode and dislocation creep deformation mode, depending on both dislocation structure and deformation temperature. The flow curves could be well described by the constitutive equations of these three micro-deformation mechanisms based on the internal variable deformation theory.

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Carbon and titanium were added in two-phase(B2+L1₂) Ni-Al-Fe alloys for carbides precipitation, and the additions of 0.2 wt.%C and 3 at.%Ti have been found to be the most adequate for the formation of carbides. Carbon-doped alloys showed more refined microstructure than other carbon free alloys. The carbides, which were formed in the as-cast microstructure showing equiaxed dendrites, appeared to play a role to suppress the grain growth and the coarsening of second phases. The NAF29-10 alloy including no carbides showed the lamellar type microstructure, while the NAF29-10-C alloy including carbides showed much more refined mesh type microstructure. When the carbon free alloys were quenched into water, cracks occurred at the grain boundaries probably due to martensitic transformation. However, these cracks were not observed in the carbon doped alloy. Therefore, the carbon doped alloys showed a good ductility at room temperature. They also showed much higher yield strength than other alloys without carbides through the whole temperature ranges.

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Dynamic recrystallization behavior of two austenitic stainless steels, STS 310 and STS 304, with different stacking fault energy (SFE) was studied by performing high-temperature compression tests at 800∼1100℃ with strain rates of 10¹∼10^(-3)/sec. Both austenitic stainless steels with different SFE showed similar flow stress vs. strain behavior during the compression tests. However, their microstructural evolution nature with strain increase was found to be quite different. For STS 310 specimens having high SFE, dynamic recrystallization (DRX) takes place preferentially at/or near primary grain boundaries. The specimens experienced micro band (MB) and subgrain formation prior to the recrystallization. On the other hand, in the case of the STS 304 specimens with low SFE, less MBs formed and DRX occurred rather evenly through the whole area of primary austenite grains. These microstructural differences were discussed by relating dislocation movement with SFE magnitude. The mechanisms of DRX including the evolution of subgrain boundaries to high angle ones are also discussed.

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