Won-Chan Seo

Dielectric and ionic conduction properties in LiYF4 single crystals

Abstract A LiYF 4 single crystal was grown by the Czochralski technique in an Ar-gas atmosphere. The complex dielectric constants and complex impedances are measured in the range of 100 Hz–10 MHz and up to 200°C for a c -plate of a LiYF 4 single crystal. The LiYF 4 reveals a strong low frequency dielectric dispersion below 300 kHz, from room temperature to 200°C. It is believed that the strong dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of Li + -ions between neighboring sites, with an activation energy of Δ=0.49 eV. The conduction mechanism seems to be the non-interacting Debye type and the relaxation times satisfy the Arrhenius relation.

Microstructure and Impact Toughness of Weld Metal in Multipass Welded Super Duplex Stainless Steel

The effects of reheating during welding on the microstructure and impact toughness of weld metal in 25% Cr super duplex stainless steels were investigated. Using different heat inputs, weld metals with different reheated regions were obtained. This showed that, depending on the reheating temperature, the microstructure in the reheated region was quite different from that of the as-deposited microstructure. When reheated into the temperature range, fine intragranular austenite was formed in the as-deposited columnar structure. However, when reheated above the solvus temperature range, most of the columnar structure disappeared and fine equiaxed austenite and ferrite were formed. Because of the larger amount of fine austenite in the reheated region, a higher impact toughness was obtained in the weld metal with a higher amount of reheated region.

Effects of Diffusible Hydrogen Content and Hardness on Cold Cracking in High Strength Weld Metal

*Division of Advanced Materials Science and Engineering, PKNU, Busan, KoreaKEY WORDS: Weld metal 용접금속, Cold cracking 저온균열, Diffusible hydrogen content 확산성수소량, Hardness 경도ABSTRACT: The effects of the diffusible hydrogen content and hardness on the cold cracking in high strength weld metal were investigated . The diffusible hydrogen contents were influenced by welding paramet ers such as the voltage and contact tip-to-work distance (CTWD) . The diffusible hydrogen content increased with an increase in voltage. However , it was decreased with an increase in CTWD. CTWD also influenced the weld metal hardness,especially when the wire used had a higher stren gth than the base metal. This showed that weld metal hardness h ad a more powerful effect on weld metal cold cracking than the diffusible hydrogen content in this experiment.교신저자 방국수: 부산광역시 남구 용당동 산 100, 051-629-6379, ksbang@pknu.ac.kr