발간논문

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The objective of this study is to investigate the effect of deformation degree on damping capacity, transformation behavior and mechanical property in Fe-16%Mn-2%Cr and Fe-22%Mn alloys. The Fe-16%Mn2%Cr and Fe-22%Mn alloys undergo γ→ε→α` and γ→ε martensitic transformation by deformation, respectively. In the two alloys, the increase in degree of cold rolling cause an increase of reverse transformation temperatures of ε→γ because the dislocations introduced by cold rolling hinder the movement of γ→ε boundaries. The decrease in mobility of γ→ε boundary is responsible for a rise in driving force, raising the reverse transformation temperature. At low strain amplitudes of 1×10^(-4)∼4×10^(-4), damping capacity increases with increasing rolling degree up to 5-10%. At a high strain amplitude, the 0% rolled alloy exhibits good damping property. The results indicate that area of γ→εboundaries at a low strain amplitude is more dominant than their mobility. With a higher strain amplitude, however, mobility becomes a major factor for damping capacity.

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Microstructures of steel are the major variable which determines mechanical properties such as strength, toughness and fatigue limit. Ultrasonic wave propagating in polycrystalline metals is mainly attenuated by grain boundary scattering. Effect of ultrasonic attenuation on microstructure changes in plain carbon steel consisting of phases such as ferrite+pearlite, ferrite+bainite, bainite and martensite was evaluated. Ferrite+pearlite steel yielded greater attenuation than bainitic steel, and bainitic steel showed higher attenuation than martensitic steel. From analysis of relationship between attenuation and microstructure comprising grain and subgrain boundaries, it was confirmed that the main scatterer of martensite structure could be packet boundary. This result shows that microstructures of steel could be identified nondestructively by the measurement of ultrasonic attenuation.

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Dispersion strengthened Al-Ti alloys with very fine microstructures were fabricated by double mechanical alloying (DMA) process, and the elevated temperature mechanical properties of these alloys were investigated. The alloys prepared by DMA process represented better distribution and smaller size of precipitates than those by single mechanical alloying(SMA) process. The analysis of XRD and TEM observation indicated that the size of Al₃Ti phase was smaller than 30 ㎚. The lattice parameter of Al was decreased from 4.0485 to 4.0401Å during MA, which indicates the dissolving of Ti into Al matrix. In this study, the maximum solid solubility of Ti during MA was estimated to be 8.82 wt%. Inspite of effective dispersion of intermetallic compound by DMA process, the alloys did not show improved mechanical properties as expected, due to too high volume percentage of intermetallic dispersoids.

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It has been reported that SA508 Class 3 RPV steel exhibits dynamic strain aging phenomenon(DSA) at the operating temperature, 288℃. Therefore, DSA effect on the low cycle fatigue behaviors must be necessarily considered for precise life prediction. In this work, the influence of temperature and strain rate on tensile and low cycle fatigue properties of two kinds of steels which was produced by the same steel making method has been investigated. Test temperatures and strain amplitudes at strain rate of 4×10^(-3)/s and 4×10^(-4)/s were varied from room temperature to 400℃ and 0.6 to 1.8%, respectively. Steel with smaller grain and fine carbide size and homogeneous distribution exhibited longer fatigue life and higher mechanical properties. DSA region was considered as the temperature range which appeared inverse strain rate dependence of tensile stress and stress amplitude. Marked secondary strain hardening took place on cyclic stress response curve at DSA temperature, whereas cyclic softening remarkably occurred at different temperatures. As increase of temperature and decrease of strain rate, fatigue life decreased. But DSA resulted in the increase of fatigue life. It was believed that occurrence of secondary crack and surface multicrack improved fatigue crack growth resistance.

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The effects of tramp elements on the microstructures and mechanical properties of high strength low carbon TRIP aided cold-rolled steel sheets were investigated in the present study. Four kinds of 0.15C-1.5Mn-1.5Si-(Cu, Cr, and Ni) cold-rolled steel sheets were prepared, and microstructural observation and tensile tests were conducted after intercritical annealing and isothermal treatment. The main emphasis has been placed on understanding the effects of tramp elements (Cu, Cr, and Ni) on microstructures, retained austenite volume fractions, and mechanical properties of the cold-rolled steel sheets. The addition of Cu or Cu+Ni resulted in a great increase of the retained austenite volume fraction as well as a discontinuous yielding behavior, and the elongation and the strength-elongation balance were higher than that of the Cr containing cold-rolled steel sheets. The addition of Cr or Cr+Ni showed a dual phase stress-strain curve behavior having high tensile strength up to 1,000 ㎫ and low elongation about 20%.

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A mechanism of longitudinal surface crack formation in the continuous casting mold for the peritectic medium carbon steel was investigated by measurement of the fracture strength, the heat flux and the shrinkage of solidifying shell. The fracture strength of medium carbon steel shell is higher than that of other carbon steels and it exhibits an anomalous decrease of the heat flux on the mold surface due to the surface roughness of the solidifying shell caused by the δ→γ phase transformation just after solidification. Because of the larger shrinkage and higher strength, the peritectic medium carbon steel shell forms larger air gap between the shell surface and the mold wall, resulting decrease of the heat flux to the mold and hence higher recalescent shell temperature. The recalescence of the shell will induce the crack in the sub-surface at low ductile interdendritic area or grain boundary of the solidifying shell if combined with the stress due to the bending and frictional force. The sub-surface crack will propagate to the shell surface via grain boundraries at a later stage beacause of thermal and mechanical tensile stresses.

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Ni/Ni-aluminidel/Ti/Ti-aluminide laminate material, considered as a functionally gradient material, was manufactured by hot-press and Ti, Ni and Al foils. Thick intermetallic layers of NiAl and TiAl₃ were formed by self-propagating, high temperature synthesis (SHS) reaction, and thin layers of Ni₃Al and TiAl were formed by diffusion in the solid state at the interface between metal and aluminide layers. Bending strength in metal-intermetallic composite depended on loading direction in bending test. As a result of fracture toughness measurements crack arrester had greater fracture resistance than crack divider due to interruption of crack growth in metal layers. Pure metal Ni and Ti layers showed ductile rupture. Also Ni₃Al and NiAl in Ni-aluminide layers revealed cleavage fracture and intergranular fracture, respectively, while Ti-aluminide layers showed typical cleavage fracture.

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NiAl intermetallic compound was fabricated by mechanical alloying. The microstructure of the intermetallic compound was investigated as a function of mechanical alloying time and heat treatment condition by the combined analysis of XRD, neutron diffraction, SEM, and TEM. Mechanical alloying of Ni-50 at.%Al powders for 120 hours resulted in nanocrystalline grains with diameter of 2-5 ㎚. X-ray diffraction and neutron diffraction spectra indicated that only NiAl intermetallic compound was formed during mechanical alloying. NiAl superlattice peak (100) was still retained after mechanical alloying for 120 hours. Heat treatment of mechanically alloyed powders at 600℃ for 90 hours encouraged grain growth up to 20-30 ㎚, while long range order parameter remained almost constant. The lattice parameter was reduced from 2.903Å for as-milled NiAl compound to 2.884Å for the case of heat treatment at 600℃ for 90 hours. The activation energy for the reduction of lattice parameter was also evaluated as 0.5-0.7 kJ/㏖ as a function of heat treatment time by the Arrhenius equation. As the heat treatment temperature increased, the activation energy for the reduction of lattice parameter gradually decreased. It is concluded that the ordered nanocrystalline NiAl intermetallic compound was obtained by the ordering transition from topological disordered state during mechanical alloying of Ni-50 at.%Al up to 120 hours.

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Surface composite layers reinforced with TiC particles were fabricated by high-energy accelerated electron beam. TiC powders were mixed with MgO/CaO flux powders with varying the flux mixing ratios of 5, 10, 20, and 40 wt.% to process four kinds of TiC/flux mixtures. The TiC/flux mixtures were deposited evenly on a plain carbon steel substrate, on which electron beam was irradiated to fabricate TiC reinforced surface composite layers. The microstructural analysis was conducted using X-ray diffraction and Mo¨ssbauer spectroscopy as well as optical and scanning electron microscopy. The microstructure of the surface composites was composed of surface composite layer, interfacial region, coarse-grained heat affected zone (HAZ), and finegrained HAZ. A few TiC agglomerates and residual micropores were found in the composite layer of the specimen processed without flux because of density difference, but their number was significantly decreased in the specimens processed with a considerable amount of flux. As a result of irradiation, TiC particles were homogeneously dispersed throughout the composite layer of 2.5 ㎜ in thickness, whose hardness was greatly improved. The proper flux mixing ratio was 10˚-20% to obtain excellent surface composite layers containing homogeneously dispersed TiC particles.

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Process parameters of PACVD process using titanium tetraisopropoxide(Ti(C₃H_7O)₄) organic compound precursor as metal donor instead of TiCl₄ were investigated to deposit the Ti-base hard layer on tool steel surface in order to decrease the deposition temperature and improve the step coverage of substrate with complicate shape. Ti(NCO) composite layer was deposited on tool steel by MO-PACVD process at approximately 500℃ using gas mixture of titanium tetraisopropoxide, hydrogen and nitrogen. Process parameters such as precursor volume %, substrate temperature and H₂/N₂ gas ratio were optimized. Deposited layers were characterized in terms of hardness, surface morphology, corrosion resistance and composition. Layers of good quality were obtained when the content of Ti(C₃H_7O)₄ vapor in the gas mixture was lower than 5%. The layer obtained with H₂/N₂ gas ratio of 1:1 showed the highest hardness of 1645H_(v0.05). The layers had lower hardness when using H₂/N₂ gas ratio of 7:3 and 3:7. The layer obtained at 500-550℃ with H₂/N₂ gas ratio of 1:1 had good corrosion resistance.

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