발간논문

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Finite element calculations are performed for crystalline solids subjected to plane strain tensile loading. Using Asaro`s double slip model, shear band developments in single crystals are analyzed. The effect of various rate sensitivities and latent hardening parameters on microscopic plastic behavior was clarified. Moreover the deformation behavior of polycystals which have grain boundaries was compared to that of single crystals.

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The creep behavior of PM1000 alloy, ODS Ni-base superalloy with 0.6wt%Y₂O₃ was investigated in order to understand the role of dispersoids during high temperature creep deformation as a modeling alloy. Nimonic75 alloy which has no dispersoids but has almost the same chemical composition as PM1000 was selected as a reference alloy. Creep tests were performed at 800℃ 900℃ and 1000℃. In contrast to Nimonic75, the PM1000 alloy showed the threshold behavior at certain stress level under which no detectable creep strain proceeds, TEM observations indicated that there was an attractive interaction between particles and dislocations. A new model to predict the threshold stress in high temperature creep was proposed in this study. The predicted threshold stresses by this model agreed well with the experimental results.

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The influence of heat treatment condition on the texture and the formability in Al-7wt%Mg alloy sheet has been studied. The formability is strongly affected by the texture. Therefore, it is important to control the recrystallization textures in order to improve the formability. For two different heat treatment condition in Al-7wt%Mg alloy sheet, the textures were analyzed by using the three dimensional orientation distribution function(ODF) method, the plastic anisotropy strain ratio(R) and forming limit diagram(FLD). Lankford parameter(R) was predicted by the continum mechanics of textured polycrystals (CMTP) method, which were compared with experimental R values. The relationship between the textures and R values, texture and formability was investigated in this study.

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A study on the microstructure and mechanical properties of two stainless steels, STS 304 and STS 430J1L, was performed in order to investigate their sticking mechanism during hot rolling. A simulation test was carried out using a high-temperature wear tester capable of controlling rolling speed, load and temperature. The results of the mechanical tests, together with high-temperature hardness tests, explained why sticking of the STS 430J1L took place more severely upon hot rolling, although the two steels had similar room-temperature hardness. The microstructural analyses revealed that the sticking process proceeded with three stages, viz, nucleation, growth, and saturation, for the both stainless steels. The coarse Cr carbides in the roll were also found to play no particular role on sticking.

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The high temperature deformation behavior of Al-7wt%Mg alloy was investigated with the strain rates of 10^(-1)/sec, 10^(-2)/sec and 10^(-3)/sec and the test temperatures of 350℃, 400℃ and 450℃ by hot compression test. Flow curves were well explained by hyperbolic sine relationship, ε=A(sinh(0.012σ_p))^(2.954) exp(-139.5KJ/RT). The softening of Al-7wt%Mg alloy during hot deformation was mainly induced by dynamic recovery and dynamic recrystallization. Microstructural change of this alloy was interpreated by Zener-Hollomon parameters(Z). Microstructure induced by the recovery was composed of elongated grains above Z parameter of 10^9. Below this value, the recrystallization was occurred and its structure consisted of small equaxial grains of about 30㎛.

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Fe-Cr-Ni alloy(AISI 303 stainless steel) was prepared by air induction melting(AIM). The machinability and corrosion resistance of Fe-Cr-Ni alloy were investigated. SEM/EPMA/IA were used to analyze the amount, size and morphology of nonmetallic inclusions. Electroslag remelting(ESR) method was used to refine the air induction melted Fe-Cr-Ni alloy. ESR ingots exhibited clean surface finish, and showed a defect free cast structure with directionally grown grains. The amount and size o1 inclusions such as sulphide were reduced and the morphology of inclusions was changed to the round shape during ESR. The machinability and corrosion resistance of ESR-treated Fe-Cr-Ni alloy ingots, in the as cast condition, were much improved on those of AIM ones. The improvement was attributed to the change of inclusion amount, size, morphology as well as uniforrn distribution of inclusions due to ESR process.

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A detailed examination has been carried out for clarifying the effects of rare-earth metals (La, Ce) addition on precipitation behaviors in Al-Li-Cu-Zr alloy. Present work was performed by means of the measurements of hardness, tensile strength and electrical resistivity, and the observation of transmission electron micrograph during ageing. The La and Ce elements added to 2090 alloy not only play leading role in retardation of peak age condition but in increasing strength and elongation of 2090 alloy. The δ′ and T₁, phases in the 2090 alloy containing La and Ce elements homogeneously distributed in the matrix and were finer than those of 2090 alloy at all ageing conditions. The δ′ phases, particularly, exist spherical shape in the specimen aged at 190℃ for 45hrs. Therefore, considering strength and elongation, the La, Ce may be estimated positive alloy addition element because of the improvement of strength, elongation in La, Ce addition alloy.

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Directional solidification have been studied on the GS-36Ni russian single crystal superalloy at various temperature gradients and solidification rates. A liquid metal cooling technique was employed to take advantage of its high temperature gradient capability in directional solidification which is essential for single crystal growth. The dendrite spacings(λ) with temperature gradients (G) and solidifiation rates(V) were satisfied by well known equation λ ∝ G^(-½) V^(-¼). The finer dendrite spacings in the single crystal superalloy were produced at the higher temperature gradients, which results in decreasing sharply the homogenization time. The solid/liquid interface instability was also investigated by quenching the solid/liquid interface during directional solidification.

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The steady state of growth of rod during spray forming was calculated by 3 dimensional transient model and 2 dimensional steady state model. When the substrate rotation velocity is small, the rod shape shows asymmetry. But the average vertical growth velocity of rod per cycle of substrate rotation converges to substrate withdrawal velocity. When the average vertical growth velocity of rod per cycle of substrate rotation is the same as the substrate withdrawal velocity, it should be defined as steady state of growth. After growth velocity of rod vertex converges to steady state, the whole profile of rod surface converges to that of steady state. The rod shape of steady state calculated by 3-d transient model and by 2-d steady state model at high substrate rotation velocity shows approximately same results.

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The effect of initial eccentric distance, substrate withdrawal velocity, spray angle and substrate shape on steady state behavior was studied, using 3 dimensional model of spray forming of rod by computer simulation. The mechanism to reach steady state at various conditions was analyzed through the growth velocity at vertex of rod, G_v. Above a critical eccentric distance, G_v, increased from the beginning of forming and reached steady state. Below a critical eccentric distance, G_v, decreased and reached steady state. Below a certain critical withdrawal velocity, the growth velocity of vertex reached steady state. .Above the critical value of withdrawal velocity, G_v, increased to critical value and decreased to zero growth velocity of vertex. The critical withdrawal velocity was the maximum velocity where spray axis coincide with vertex.

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