발간논문

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Steels containing 1.65% vanadium with 0.26% carbon have been isothermally transformed in the temperature range 600-750℃. The ferrite and the two types of vanadium carbides displayed in this experiment have been examined by thin foil electron microscopy and the crystallography between ferrite and carbides has been investigated with the aid of selected area diffraction and micro-diffraction. On the contrary to the previous reports, ferrite matrix and carbide precipitates possessed the Baker Nutting orientation relationship with no difference of carbide morphologies.

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The restoration processes that take place during and after hot compression of polycrystalline copper specimens, in the temperature range 673-1023 K and the strain rate range 7.4×10^(-4) - 4.55×10^(-2)s^(-1), have been investigated by means of an interupped technique and metallography. The specimens have two different average grain size, 11 ㎛ and 102㎛. All the specimens show the single or multiple peak stresses in the flow curves, which means that the dynamic recrystallization takes place. The multiple peaks in flow curves initiate at lower strain, lower temperature and higher strain rate in small grain sized specimens than in large ones. The static restoration after dynamic recrystallization during hot deformation proceeds on sequentially there distinct mechanisms, namely metadynamic recrystallization, recovery and recrystallization. Regardless of different grain sizes, the relative importance of contribution of the three mechanisms to the static softening is shown to be high1y sensitive to the fractional prestrain of the fast peak strain. There exists a critical fractional strain (-70%) below which no metadynamic recrystallization occurs.

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A study has been made of spheroidization of eutectiferous silicon in the aluminum-silicon cast alloy. AC4C, by annealing under various conditions The spheroidzation could not be achieved by annealing for 27 hours below 400℃, whereas it was achieved relatively easily at temperatures above 500℃. Coarsening of the spheroidized silicon particles was controlled by diffusion. The tensile strength of the specimens increased when annealed at 500 and 550℃ and decreased when annealed at 350 to 450℃, whereas the elongation of all the specimens increased with annealing time.

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A method of correlating with precision the thermodynamic properties and phase boundaries of the cadmium-lead system is described. For the liquid phase, the Krupkowski equation is used to correlate the enthalpy data of Kleppa, the excess free energy data of Zabdyr, and the eutectic point data of Hansen and Anderko. For the fcc (Pb) phase, a quasi-regular model is used. The calculated phase diagram of the cadmium-lead system using these formalisms agrees well with the experimental diagram.

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The effect of cooling rate on the morphology of the martensite and the mechanical properties of Fe-20% Ni martensitic alloy has been studied. The alloy was cooled from the austenite state at 1000℃ by the various practical cooling precesses iced brine, water, oil, air and furnace cooling Fe-20% Ni alloy is fully transformed to lath martensite even at very slow cooling rate such as 0.8℃/min. The size of block in the lath martensite structure increases with decreasing cooling rate. The mechanical properties such as hardness, tensile strength, elongation and reduction of area show nearly constant values, independent of cooling process However, although the impact energy shows a high and almost same vague at iced brine, water, oil and air cooling process, only the furnace cooling process shows a very low impact energy. Impact energy of the ail-cooled specimen, which was furnace-cooled again from a temperature just above M_s points is nearly same as or a little higher than that of the as-air cooled specimen. Hence, the reason that the furnace-cooled specimen shows a very low impact energy is considered to be due to packet/packet boundary segregation formed with assistance of movable dislocations during martensitic transformation between M_s and M_f temperature.

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The static and cyclic creep mechanism and behaviour of AISI 316 stainless steel have been investigated under the condition of constant and square wave stress cycling tension creep test in the temperature, stress and frequency range of 650-710℃ (0.55∼0.59 Tm), 21∼29 ㎏/㎜² (206 ∼ 285 MPa), and 3 cps, respectively. Also, the relationship between static and cyclic creep have been investigated The experimental result are follows ; The cyclic creep retardation is due to the interaction of solute atom and mobile dislocation Stress rised by dynamic strain aging was observed at high temperature tensile test. The stress exponent value for static and cyclic creep was observed to be about 5.8∼7.6 and 7.9∼7.3, respectively. The activation energy for static and cyclic creep deformation was 98.1∼92.0 and 102∼97.4 ㎉ mole, respectively. The metallographic examination of static and cyclic crept specimen have shown that the fracture type was mainly intergranular.

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The stress corrosion crack propagation of cold worked AISI 304 stainless steel in boiling MgCl₂solution was investigated. The crack propagation rate is not affected by the amount of cold work except the one cold worked 60%. It was found that the resistance to stress corrosion increases as the amount of stress induced martensite increases to certain amount, but beyond the amount the stress corrosion crack accelerates to shorter life. The higher degree of cold work shows the higher K_(1scc).

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Microstructure of Al-2.98wt%Cr splats which were rapidly solidified by twin piston apparatus after levitation melting was analysed. Transitian of microstructure from microcrystalline to cellular or dendritic occurred with the increase of interface temperature, and monotectic-like reaction i.e. high cone. Liquidid→Intermetallic Compound+low conc. Liquid seemed to occur at cell or dendrite boundary. With the increase of superheating of liquid drop, the thickness of splats reduced possibly due to the available time for thinning of the splats in liquid state, however little difference in lattice parameter was observed. Microstructure of transverse section was varied with the distance from center to edge part of the disk type splats. Precipitate clusters of the rapidly solidified Al-Cr alloy were verified by selected diffraction pattern to be Al_(11) Cr₂, which was supposed to be nucleated and grown in supercooled liquid.

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