• 연구논문집
• /
• s.22
• /
• pp.127-139
• /
• 1992

The mechnial properties such as fracture strength, ductility and fatigue strength of Cu shape memory alloy are lower than those of Ti-Ni SMA, because of their high elastic anisotropy and large grain size. And in order to improve the mechanical property of Cu SMA, some techniques such as casting method by addition of refining element, powder metallurgy and rapid solidification process have been studied on the refinement of the grain size of Cu SMA. This study was carried out to refine the grain size of CuAlNi SMA by applying the melt spinning method. According to this study, the conclusions are as follows; - grain size of the melt-spun ribbon was about $1\mum$ - there was not change in grain size, although increasing of hot pressing temperature -grain size of the hot-extruded specimen was about $30-40\mum$, it is more refiner than that of castings

• Korean Journal of Materials Research
• /
• v.29 no.2
• /
• pp.97-105
• /
• 2019

In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the ${\gamma}$ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.

• Corrosion Science and Technology
• /
• v.17 no.2
• /
• pp.81-89
• /
• 2018

In this study, the effect of Nb alloying element on the corrosion fatigue properties of high strength steel is investigated by conducting fatigue experiments under corrosive condition and hydrogen induced condition, potentiodynamic polarization test, tensile test and surface analyses. Nb element is added to enhance the mechanical property of medium carbon steel. This element forms MX-type phases such as carbides and nitrides which are playing an important role in the grain refinement. The grain refinement is one of the effective way to improve mechanical property because both tensile strength and toughness can be improved at the same time. However, MX-type phase precipitates can be a susceptible site to localized corrosion in corrosive environment due to the potential difference between matrix and precipitate. The obtained results showed that Nb-added steel improved corrosion fatigue property by grain refinement. However, it is degraded for hydrogen-induced fatigue property due to Nb, Ti-inclusions acting as a stronger trap.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2003.10a
• /
• pp.132-135
• /
• 2003

Asymmetric rolling, in which the ratio of the rotation rates of the upper and lower rolls was 2, has been used to introduce an intense plastic shear strain for the purpose of grain refinement and developing shear deformation textures through the sheet thickness to improve the strength and plastic strain ratio of AA1050 aluminum alloy sheets. The alloy sheets were rolled at room temperature without lubrication. The textures and microstructures of the sheets were investigated by x-ray diffraction and electron back-scattered diffraction (EBSD) analyses with emphasis on effects of combinations of rot ling directions.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2004.10a
• /
• pp.668-671
• /
• 2004

The mechanics of the dendrite fragmentation is a very important aspect of grain refinement in rheocasting. In this work, the stress field of the dendrite stirred in the semisolid slurry was simulated by Metlab 6.0 software. The result shows that stress concentration at the root of the dendrite arms is great enough to cause plastic deformation though the agitation is moderate. Accordingly, dendrite fragmentation was suggested to be caused by fractured after fatigue erosion.

• Transactions of Materials Processing
• /
• v.16 no.5 s.95
• /
• pp.406-411
• /
• 2007

The microstructure and the hardness of interstitial free steel processed by equal channel angular pressing (ECAP) was investigated experimentally. ECAP processing of route A and route C was compared with regard to grain refinement by transmission electron micrographs. Micro hardness evolution was correlated with the gram structure produced by ECAP. Especially, the effects of the ECAP processing temperature and the number of processing passes were discussed in terms of grain refinement.

• Nuclear Engineering and Technology
• /
• v.37 no.4
• /
• pp.299-308
• /
• 2005

The behavior of fission gas in all major types of nuclear fuel has been reviewed with an emphasis on more recently discovered aspects. It is proposed that the behavior of fission gas can be classified in a number of characteristic types that occur at a high or low operating temperature, and/or at high or low fissile burnup. The crystal structure and microstructure of the various fuels are the determinant factors in the proposed classification scheme. Three types of behavior, characterized by anisotropic $\alpha$-U, high temperature metallic $\gamma$-U, and cubic ceramics, are well-known and have been extensively studied in the literature. Less widely known are two equally typical low temperature kinds: one associated with fission induced grain refinement and the other with fission induced amorphization. Grain refinement is seen in crystalline fuel irradiated to high burnup at low temperatures, whereas breakaway swelling is observed in amorphous fuel containing sufficient excess free-volume. Amorphous fuel, however, shows stable swelling if insufficient excess free-volume is available during irradiation.

• Korean Journal of Materials Research
• /
• v.21 no.7
• /
• pp.410-414
• /
• 2011

This study was carried out to evaluate the microstructures and mechanical properties of a friction stir welded Ni based alloy. Inconel 600 (single phase type) alloy was selected as an experimental material. For this material, friction stir welding (FSW) was performed at a constant tool rotation speed of 400 rpm and a welding speed of 150~200 mm/min by a FSW machine, and argon shielding gas was utilized to prevent surface oxidation of the weld material. At all conditions, sound friction stir welds without any weld defects were obtained. The electron back-scattered diffraction (EBSD) method was used to analyze the grain boundary character distributions (GBCDs) of the welds. As a result, dynamic recrystallization was observed at all conditions. In addition, grain refinement was achieved in the stir zone, gradually accelerating from 19 ${\mu}m$ in average grain size of the base material to 5.5 ${\mu}m$ (150 mm/min) and 4.1 ${\mu}m$ (200 mm/min) in the stir zone with increasing welding speed. Grain refinement also led to enhancement of the mechanical properties: the 200 mm/min friction stir welded zone showed 25% higher microhardness and 15% higher tensile strength relative to the base material.

• Journal of Korea Foundry Society
• /
• v.31 no.2
• /
• pp.79-82
• /
• 2011

Various grain refining alloying elements such as Sr, TiB, and Ca were added to AZ91D and their effects on the mechanical properties and mold filling ability were investigated. The average grain sizes of those alloys were significantly reduced by the small amounts of the alloying elements. Ca addition was the most remarkably effective in reducing the grain size, however it was found to deteriorate the mold filling ability and tensile properties. TiB addition was observed to be the most efficient for both grain refinement and mold filling.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.22 no.2
• /
• pp.132-139
• /
• 2002

The grain size of aluminum alloy was refined to the submicrometer level by using equal-channel angular pressing(ECAP). The effect of grain size refinement was evaluated by the tensile test, micro-hardness test, microstructure observations, ultrasonic test and acoustic emission test. The strength and the Vickers hardness were increased significantly according to grain size refinement after equal-channel angular pressed. The ultrasonic velocity was faster after equal-channel angular pressed, and the high frequency range appeared. The results of the ultrasonic velocity and the frequency range are expected to be basic data that can prove the grain size refinement