발간논문

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The objective of this study is to investigate the effect of microstructure and pre-strain on shape memory effect(SME) and transformation behavior in an Fe-32Mn-6.5Si alloy. The alloy was annealed at various temperatures from 600 to 1200℃ for 1 hour to vary microstructure after cold rolling. Shape memory effect and reverse transformation behavior were examined through bending test and TMA after deforming the alloy with various degrees of pre strain Austenite grain size increases with an increase in heat treatment temperature, except the specimen heat-treated at 600℃, which remains unrecrystallized. The specimen heat-treated at 600℃, retaining cold rolling effect, exhibits the best SME due to a high Md temperature. In the recrystallized specimens, the larger grain size, the better SME. As grain size increases, the reverse transformation range becomes shorter, indicating that reverse transformation takes place with less constraint. The better SME in a large grain size can be attributed to the easiness of the reverse transformation.

keyword : Shape memory effect, Pre strain, Reverse transformation behavior, Md temperature

The high temperature ductility behaviors of Al-Mg-Be alloy containg particles in grain have been studied using a tensile test and scanning electron microscopy. The results show that the reduction of area increased with increasing temperature up to 523K and the minimum ductility was found to be at 573K for 8.3 × 10-5s-1 strain rate. The rate of ductility loss of the Al-Mg-Be alloy deformed at 573K under strain rate, 8.3 × 10-4s-1 was higher than that of under a strain rate, 8.3 × 10-5s-1 However, the maximum ductility drop was occured at the same temperature, 573K for various strain rate. The reduction of area was not determined uniqely by Zenner-Hollomon parameter, Z=εexp(Q/RT). The cleavage fracture showing the ductility loss at higher temperature was associated with cavity growth as well as interlinkage of cavities. On the other hand, ductile fracture was observed on the fracture surface at lower temperature.

keyword : High-temperature ductility Loss, Strain rate, Reduction of area, Zenner-Hollomon parameter, Cavity growth, Cleavage fracure, Ductile fracture

The effects of additions of Al, Ti, Si and Sb on the mechanical properties, electrical resistivity and formation of discontinuous precipitate in Cu-9Ni-6Sn(wt%) alloy have been investigated. The physical and mechanical properties of the Cu-9Ni-6Sn alloy were strongly dependent on aging conditions. This alloy was much strengthened by aging at temperatures below 350℃ where the spinodal decomposition occurs, but the specimen does not be strengthened by aging at temperatures above 400℃ where the discontinuous precipitate forms. Al suppressed the formation of discontinuous precipitation and increased the electrical resistivity. The nucleation of discontinuous precipitate was significantly retarded by the small additions of Ti and Si and the discontinuous product has never formed up to 5h even at 450℃. The addition of Sb. on the other hand, was not affected markedly. In all of the alloys examined, the electrical resistivity was decreased by aging and the degree of reduction increased with increasing aging temperature and aging time.

keyword : Cu-9Ni-6Sn alloy, Spinodal decomposition, Metastable phase, Precipitation, Discontinuous precipitate, Tensile strength, Elongation. Electrical resistivity

In the present investigation, the low cycle fatigue(LCF) behavior of a high-strength steel heat-treated to different strength levels has been studied using the cylindrical smooth specimens under strain-controlled fully reversed pull-push condition: During the strain-controlled cycling the steel exhibits continuous softening behavior independent of the applied strain levels and heat-treatment. It has been found that the saturation of the plastic energy density per cycle(△W) proves to be a valid criterion to determine the stable hysteresis loops for establishing the cyclic stress strain(CSS) curve. The strain-life curve calculated on the basis of the CSS relationship is in good agreement with the experimental result, indicating a fine consistency between data from the CSS approach and from the strain-life approach. Then, the LCF properties are discussed with regard to their dependence on the heat treatments or strength levels of the steel investigated. Finally, the low-energy dislocation substructures during LCF have been identified using transmission electron microscopy.

keyword : Low cycle fatigue, High-strength steel, Cyclic softening, Plastic energy density per cycle, Low-energy Dislocation substructures, Slip bands

Microstructure and fracture behavior of the squeeze cast Mg-Al-Zn-Si alloys (AZ91-Si, AZ51-Si, AZ51 Si-Ca) have been investigated in the present study. To clarify the fracture behavior, in-situ fracture test and fractographic observation were conducted. The higher Al content promoted formation of the polygonal type Mg2Si, but resulting in reduction of the ductility due to increase in amount of Mg17Al12 and to presence of coarse Mg2Si. The lower Al content favored formation of the chinese script type Mg2Si which promoted the brittle fracture along the Mg2Si/matrix interface. AZ51-Si alloy had a higher ductility than AZ91-Si, because a number of slip lines were developed inside grains and cracks were blunted by matrix. The addition of Ca eliminated the chinese script type Mg2Si. Instead, fine Mg2Si particles were present in a polygonal shape, resulting in improvement of mechanical properties.

keyword : Polygonal type, Chinese script type, Mg2Si, Al content, Ca effect, Fracture behavior, Slip Line

Correlation of microstructure and fracture mode with the dynamic torsional behavior was investigated on tungsten heavy alloys whose size and shape were changed using the sintering conditions such as sintering temperature, time, and cycle. Dynamic torsional tests were conducted for seven specimens, four of which were fabricated by the repeated sintering process, using torsional Kolsk bar, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. With increasing the sintering temperature and time, the size of tungsten particles was increased, and their hardness became higher, thereby deteriorating fracture toughness. The dynamic torsional test results indicated that in the specimens whose tungsten particles were coarse and irregular-shaped, cleavage fracture occurred in the center of the gage section with little shear deformation, whereas shear deformation was concentrated into the center of the gage section in the conventionally processed specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of the impacted penetrator specimens and the in situ fracture test results. i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that there would be optimum tungsten particle size since the cleavage fracture mode would be beneficial for the self-sharpening.

keyword : Tungsten heavy alloy, Dynamic deformation behavior, Tungsten particle size and shape, In situ fracture test, Cleavage fracture

An investigation was conducted on the effects of microstructural parameters and dynamic deformation behavior on the cracking behavior occurring during cold forging of low-carbon steels. Detailed microstructural analyses of the cracked region showed that a number of adiabatic shear bands, along which cracks were propagated, were formed in the top interior part of the cold-forged pulley. Dynamic torsional tests were conducted using torsional Kolsky bar in order to investigate the dynamic deformation behavior during cold forging, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode, From the dynamic shear stress-strain curves, the steel containing considerable amount of nitrogen showed a smaller shear strain of 0.2 at maximum shear stress point, after which shear stress decreased rapidly just before fracture, whereas the other steel containing smaller amount of nitrogen showed relatively homogeneous shear deformation. This dynamic torsional behavior correlated well with the cracking and adiabatic shear banding behavior, together with yield point phenomenon occurring in the steel containing more nitrogen. Since the cracking occurring during cold forging was associated with the adiabatic shear banding and yield point phenomenon, the minimization of nitrogen and fast cooling rate during hot rolling would be suggested to prevent the cracking.

keyword : Low-carbon steel, Cold forging, Adiabatic shear band, Dynamaic torsional test, Yield point phenomenon

This study aims at evaluating fatigue properties of two high strength steels, i.e., a V-and Cr containing alloy steel and a 0.84%C steel, used for automotive diaphragm spring. Fatigue properties were investigated on microstructure and strength under the fixed hardness condition. The fatigue limits of the two steels were same when tensile strengths were nearly same (over 1400 MPa), and that they were linearly decreased in the alloy steel austenitized and tempered at higher temperatuies. These results were then interpreted based on microstructural parameters such as prior austenite grain size, amount of carbides and twinnings, and martensite lath width. The alloy steel having the higher tensile strength and the smalles amount of carbides showed better fatigue properties than the 0,84%C steel. Thus, the fatigue properties of the spring steels can be pre-estimated by tensile strength instead of hardness since tensile strength correlated well with the fatigue limit.

keyword : Diaphragm spring, Fatigue, Fatigue Limit, Carbide, Hardness

The cause of the edge scab defects in the hot-rolled ultra-low carbon steels was investigated through the examination of the shape and microstructure of the bar corner, the measurement of temperature distribution over the bar width and the physical modelling of the deformation behaviour at the bar corner by using a plasticine. It was found that the edge scab was originated from the abnormal micro-bulging at the bar corner, where ferrite was formed due to the local temperature drop. The amount of the defective strips was considerably decreased by reducing the number of descaling and by increasing the reheating temperature.

keyword : Edge scab, Hot-rolled ultra-Low carbon steel, Ferrite formation, Micro-bulging, Descaling

The texture of a drawn wire was measured using a method proposed by Van Houtte. In the method, wires are glued parallel to each other on the flat sample holder. The texture of the glued flat wire specimen is measured and analyzed to obtain the texture of wire. In order to estimate the accuracy of the method, l8μm thick aluminum foil was rolled to a 1.5mm dia. tube shape. The textures of the aluminum foil and tube-shape specimens were measured and compared. The similar orientation components were found in the both specimens, but the intensities of the orientation components decreased due to the lost of information. This method was applied to the texture measurement of the drawn aluminum wire. The measured pole figures showed that the (111) fiber texture developed in the surface layer and the {111} <112> circular texture developed in the sub-surface layer.

keyword : Circular texture, Alumimum wire, Shape effect correction