It is known that the Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) segregates on the surface of temper-rolled HDG as Al-based oxides with increasing aging time in air at room temperature. In this study, the surfaces of Zn-0.2mass%Al HDG with and without temper rolling were investigated to clarify the segregation mechanism. Specimens with a Zn coating weight of 55-57 g/m 2 including 0.19-0.20 mass% of Al were used. The specimens were aged in air at 20°C or held in liquid nitrogen, and the surface and cross sections of the specimens were then observed and analyzed by XRF, SEM-EDX and EBSD. As a result, it was found that the velocity of Al-based oxide segregation on the surface of the temper-rolled HDG was much higher than that of the HDG without temper rolling. This was attributed to the difference in the area where formation of Al-based oxides was possible. It was also found that the Zn crystal grains in the coating layer were refined by recrystallization due to contact with the temper roll, resulting in an increased number of grain boundaries that can serve as Al diffusion paths. Some unrecrystallized grains also remained after temper rolling and could increase the number of formation sites for Al-based oxides, as they contain numerous dislocations that can serve as Al diffusion paths. These two different formation sites could lead to difference in the segregation rates observed in this study.

A new liquid-phase bonding method was developed to establish a strong bonding between Al bearing Mg alloys and steels by intentionally forming a thin and uniform reaction layer at the bonding interface. By adopting Ag as an interlayer between the Mg alloys and steel, Mg-Ag eutectic melt is produced at 773 K and nano-scale Fe-Al reaction layer is uniformly formed at the melt-steel interface during the isothermal solidification of the melt which is driven by the diffusion of Ag into the Mg alloy. At the completion of the solidification, exceptionally strong bonding, exceeding the yield strength of the base Mg alloys, is achieved.

MASAKI KOBA, TOSHIO ARAKI, SHOICHI NAMBU, JUNYA INOUE, and TOSHIHIKO KOSEKI Liquid-phase bonding between a Mg alloy (AZ31) and low-carbon steel was attempted at 773 K (500°C) using Ag as an interlayer that forms a eutectic melt with the Mg alloy at this temperature. On the AZ31 side, eutectic melting and subsequent isothermal solidification were observed, and it was confirmed that the solidification of the eutectic liquid was promoted by the diffusion of Ag into the AZ31 base metal. On the steel side, Al was transported from AZ31 during the eutectic melting and isothermal solidification. This transported Al was enriched at the steel surface and reacted with steel to form a uniform, thin Fe-Al intermetallic compound layer. After the isothermal solidification, strong bonding was achieved via the thin intermetallic compound layer between AZ31 and steel, and no Ag remained at the bonding interface. The strength of the joint was found to be higher than the yield strength of AZ31.

The fracture toughness of FeAl intermetallic compounds (IMCs), FeAl and Fe 2 Al 5 , that form as a thin layer on steel substrate was investigated. A model for evaluating the fracture toughness of a brittle thin layer on an elastoplastic substrate was applied, and the fracture toughness was evaluated from the thickness of the IMC layer and the crack interval in the IMC layer after uniaxial tensile testing. The phase and microstructure of the IMC layer were varied to investigate their effects on the fracture toughness of the IMC. The relationship between layer thickness and crack interval was in a good agreement with the theoretical model, and the fracture toughness was evaluated adequately using the model. It was clarified that FeAl has higher fracture toughness than Fe 2 Al 5 , and that fine-grained Fe 2 Al 5 has higher fracture toughness than coarse-grained Fe 2 Al 5 .

The Fe-Zn alloying reaction and selective oxidation behavior of 0.7 mass% Si-1.15 mass% Mn added hot-rolled steel annealed at 600-800°C were investigated by comparison with those of cold-rolled steel. The Fe-Zn reactivity of the hot-rolled steel improved from 600°C to 700°C but deteriorated from 700°C to 800°C. Above 700°C, the amount of Fe-Si-Mn oxide on the steel surface increased with increasing temperature, and this oxide deteriorated Fe-Zn reactivity. Below 700°C, a thin layer of Fe oxide on the steel surface deteriorated Fe-Zn reactivity. This oxide layer was reduced by Si and Mn that diffused from the steel substrate. Therefore, as the temperature increased from 600°C to 700°C, Fe-Zn reactivity improved due to the formation of reduced iron on the steel surface. In the case of the cold-rolled steel, the same selective oxidation behavior and reduction mechanism of the Fe oxide were also confirmed, and as a result, the Fe-Zn reactivity of the cold-rolled steel showed behavior similar to that of the hot-rolled steel. However, the Fe-Zn reactivity of the cold-rolled steel improved at a lower temperature than that of the hot-rolled steel. This can be explained by the faster diffusion rates of Si and Mn in the cold-rolled steel than in the hot-rolled steel. That is, reduction of the surface Fe oxide layer by diffused Si and Mn proceeded at a lower temperature, and as a result, the Fe-Zn reactivity of the cold-rolled steel also improved at a lower temperature.