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In order to understand the influence of the modification of deformation textures on the formation of annealing textures, the evolution of texture during the cross rolling and the subsequent annealing was investigated by employing X-ray texture measurments and microstructure observations. The β-fiber orientations were proved to be quite unstable during the cross rolling. The cross rolling led to the formation of {011}//ND fiber orientations. The cross rolled texture having uniform orientation densities along {011}//ND fiber resulted in the randomization of annealing textures. This indicated that the operation of both the oriented nucleation and the oriented growth was effectively suppressed in the cross-rolled specimen during recrystallization. Upon annealing at 500℃, the abnormal grain growth was responsible for the formation of strong {411}<011> component in the annealing texture.

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Effects of solution treatment(ST) and aging temperatures on the behavior of β decomposition and the mechanical properties of β-C alloy have been investigated mainly by means of TEM observations. In the initial stage of transformation at temperatures below 480℃, a modulated(or tweed) structure forms on a {110}_β and aligns <111>_β. And then a metastable β′ nucleates along the modulated structure. At the later stage of transformation, the equilibrium a phase nucleates at the β/β′interface and grows preferentially to <111>_β with Burgers relationship to β matrix. The β grain size and the fatigue limit are strongly dependent on ST temperature. The elongation, the yield and tensile strengths, however, are approximately constant independently of the ST temperature. Aging at 480℃ after ST at 800℃ gives the highest tensile strength but shows the lowest ductility in the present study. This result is considered to occurred by the large amounts of β′ and extremely fine needlelike α precipitates, which induce much elastic strains in the β matrix. Aging at 530℃ after ST at 800℃ gives improved tensile strength and ductility combination and represents the fatigue strength as high as 605㎫ with the high endurance ratio of 0.536. The mechanical properties of the specimens solution treated at 800℃ followed by aging treatment are better than those of the specimens solution treated at 900℃ followed by aging treatment. Therefore, it is indispensable for obtaining a better combination of strength, ductility and fatigue limit in the β-C alloy to select an adequate ST and aging temperatures.

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The thermal cycling fatigue behavior on the microstructure of hardfacing Fe-Cr-Ni-C stainless steels has been investigated using optical microscopy, scanning and transmission electron microscopy. x-ray diffractometry, differential scanning calorimeter, and high temperature tensile testing. The as-welded microstructures of Fe-Cr-Ni-C stainless steels exhibit a fully martensitic structure except for Fe-14%Cr-1Ni-0.11C alloy consisting of the small amount of δ-ferrite and the martensite. The thermal cycling fatigued microstructure varied as the distance of penetration increased from the top-surface to the inner region. A recrystallized ferrite structure of equiaxed grains followed by a lath-shaped ferrite structure and a tempered martensite structure were observed depending on C and Ni contents. The increase in the high thermal expansion coefficient due to low C and high Ni contents accelerated plastic deformation during thermal fatigue in hardfacing Fe-Cr-Ni-C stainless steel. In the case of alloys containing relatively high C and low Ni, the recrystallized ferrite region was not observed in thermally damaged region. This can be explained by low thermal expansion coefficient combined with the pinning effect of carbides.

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The glass forming ability and thermal stability has been investigated in Fe-P-C-B alloy system. The glassy allay containing 10-12at%P, 4-6at%C or 4-6at%B in the Fe_(80-x·y·z)(P_xC_yB_z)_(20) system was found to cause the extension of the supercooled liquid region, ΔT_x(=T_x-T_g) defined by the difference between crystallization temperature(T_x) and glass temperature(T_g) to 30K. The increase of ΔT_x is due to the increase of T_x exceeding the degree of the increase of T_g. The crystallization progress of the glassy alloys with ΔT_x about 30K takes place through a single stage of amorphous(Am)→(α-Fe + Fe₃P + Fe₃C + Fe₃B). On the other hand, the two stage process cause the disappearance of the glass transition phenomenon before crystallization. The reason for the high thermal stability of the supercooled liquid region and large glass-forming ability for the Fe-P-C-B alloy is interpreted to result from the combination of the following two contributions : (i) the suppression of crystalline nuclei due to the increase in the degree of dense random packed structure resulting from the significant difference in atomic sizes among P, C and B, (ii) the difficulty of the atomic rearrangements for the percipitation of the Fe-metalloid compounds.

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This research investigates the effects of heat treatment, displacement rate and orientation on fracture behavior of precipitation hardened Al-Zn-Mg-Cu alloy 7050 in air and aqueous NaCl solution utilizing fracture mechanics R curve concept. It was found that K_(th) value of Al 7050 increased with decreasing displacement rate from 10^(-3)㎜/sec to 10^(-5)㎜/sec, possibly due to the enhanced homogeneous deformation and reduced stress concentration at the microvoid nucleation sites. In 3.5% NaCl solution, on the other hand, the K_(th) value increased with the displacement rate down to 10^(-4)㎜/sec, and decreased with further decreasing displacement rate, demonstrating that the environmental effects become dominant at lower displacement rates. T73 tempered Al 7050 showed reduced resistance to fracture compared to the T6 counterpart as a result of coarse precipitates formation and large grain size. It was also found that Al 7050-T73 has superior stress corrosion cracking (SCC) resistance to Al 7075 in 3.5% NaCl solution. Constant displacement rate test method for K_(ISCC) and da/dt measurements requires extremely slow displacement rate for high SCC resistant alloys like Al 7050. (Received October 8, 1997)

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Tensile tests were conducted with varying strain rate (1×10^(-2)/sec∼2×10^(-4)/sec) at the test temperature range between room temperature and 750℃ to evaluate the effect of nitrogen on dynamic strain aging (DSA) behaviors in nitrogen added type 316L stainless steels. Yield and tensile strength increased without reduction of elongation with adding nitrogen. The temperature range and the critical strain for DSA increased with nitrogen addition. Types A and B serrations were observed at 600℃ and the strain for transition from type A to type B increased with nitrogen addition. Solute atoms for DSA were vacancies and chromium and the activation energy for DSA increased with adding nitrogen. The reason that nitrogen retarded DSA was explained by the fact that nitrogen reduced the chromium diffusion to dislocations due to the strong interaction between nitrogen and chromium.

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The microstructure and strengthening mechanism of Cu-Ag microcomposite was examined in this paper. The high strength of Cu-Ag composite was found to be associated with the fine filamentary microstructure. Dislocation density between silver filaments decreased with decreasing spacing of filaments and the density was not high enough to explain the high strength of Cu-Ag microcomposite. The decrease of ductility in Cu-Ag microcomposites heat treated at low temperatures can be explained by the reprecipitation of small shearable silver precipitates in same regions of copper rich a phase. The spheroidization temperature was predicted assuming that the rate of growth of a sinusoidal pertubation is governed by interface diffusion, which is in good agreement with the TEM observation. The predicted strength of heavily drawn Cu-24wt.%Ag using the modified rule of mixture was 985㎫, which is also in good agreement with the tensile strength of as-drawn wires of the present study.

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The strain induced α′-martensite formation and the strain hardening behavior of metastable austenitic stainless steel during cold drawing have been investigated. The strain induced α′-martensite nucleates mainly at the intersection of the mechanical twins rather than ε-martensite. It could be explained by the increase of stacking fault energy which arises from the heat generated during high speed drawing and, for AISI 304/Cu, the additional effect of Cu additions. The strain hardening behavior of austenitic stainless steel is strongly related to the microstructural evolution accompanied by strain induced α′-martensite. The work hardening rates of cold-drawn 304 increased with increasing interstitial element(C,N) contents which affect the strength of the strain induced α′-martensite.

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High cycle and low cycle fatigue properties of high strength hot-rolled steel sheets have been studied with respect to their tensile strength and strengthening mechanisms. Fully reversed low cycle fatigue test has been conducted at a rate of 6x10^(-3)s^(-1). Specimen for monotonic tensile tests and cyclic fatigue tests have been obtained from the topend of hot-rolled coils. Fatigue strength is increased with the increase in tensile strength up to 60㎏/㎟, then it is gradually leveled off above 70㎏/㎟. The C-Mn steel with low yield ratio shows lower fatigue strength ratio than Nb containing precipitation hardened steel with high yield ratio. The extent of cyclic softening in the C-Mn-Nb steels is higher than that of the C-Mn steels. This could be qualitatively interpreted by the low plastic strain amplitude formed during the cyclic deformation. The softening and the associated planar dislocations configuration could be qualitatively interpreted by assuming the existence of precipitation strengthening by interstitial atoms and the breakdown of the complexes during the cyclic deformation.

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The high temperature deformation behavior of Fe-2Ni-1Cr-Mo steels was investigated by hot compression tests performed in the temperature range 900℃ to 1200℃ with the strain rates of 10^0/sec, 10^(-1)/sec, 10^(-2)/sec and 10^(-3)/sec. Flow curves for 3.18Mo added steel were explained by hyperbolic sine relationship, ε˙ = A(sinh(0.0146σ_P))^(4.46)exp(-578KJ/RT) due to the dual effects of strain hardening and dynamic recovery during deformation. The activation energy for hot deformation increased with increasing the Mo contents. The softening at high temperatures was mainly induced by dynamic recovery and dynamic recrystallization. Microstructural change after hot deformation was interpreted by Zener-Hollomon parameters(Z). The relationship between microstructure and flow behavior was explained.

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