Papers by Keyword: Pilling-Bedworth Ratio

Abstract: Oxide scale behaviors by surface segregation of Mg, Ca and Be in Al and their effects on oxidation resistance at melt temperature were investigated. With the addition of Ca and Be in Al-7.5mass%Mg alloy, the samples showed a suppressed weight gain. However, in the initial oxidation, Ca added samples exhibited improved oxidation resistance. As a result of oxide layer observation by microscopy, Ca added Al-7.5mass%Mg alloy exhibited the region overlapped by constituent elements, indicating multi-element oxide is formed on the surface. In the oxidation of Al-Mg-Be system, BeO is formed as primary oxide and mixed layer with MgO, while Ca addition in Al-Mg system causes no change in the primary and secondary oxides, but formation of CaMg₂Al₁₆O₂₇. BeO and BeAl₂O₄ may contribute to balanced layer by combination between constituent oxides in the Al-Mg-Be system. In the case of Ca addition, CaMg₂Al₁₆O₂₇ acts as a filler of the cracks in MgO layer.

Abstract: The candidate gases for the replacement of SF6 in magnesium melt protection were reviewed from the viewpoint of environmental effects, melt protection properties, and chemical properties. The surface films formed under fluorine bearing gases were analyzed by using XPS. The XPS results revealed that the surface films consist of MgO and MgF2. The fluorine content of the surface film increases with increasing gas concentration and exposure time. The Pilling- Bedworth ratio (PBR) of MgO was 0.73, and the porous magnesium oxide could not cover the whole melt surface. PBR of MgF2 was 1.29, and the surface film which contains MgF2 could cover a lager area of the melt surface. This higher PBR of composite surface film prevented further oxidation and evaporation of magnesium.

Abstract: The oxidation behavior of carbides on Inconel 718 surface was investigated. Oxidation tests were performed at the temperature range of 300 to 650°C. Two types of carbide, NbC and TiC were detected, and the amount of NbC was significantly higher than that of TiC. The oxidation of NbC started at the carbide surface and the matrix interface, and propagated into the interior of the carbide and eruption subsequently occurred with increasing oxidation time. In case of TiC, the oxidation occurred at the outer surface of the carbide and the matrix interface, but did not propagated into the interior of the carbide. Oxides formed on the outer surface of the TiC contained a relatively large amount of elemental Nb. The cross-section of oxidized specimen was investigated. NbC was oxidized more easily due to the significantly lower Gibbs free energy of oxidation than that of TiC. Eruption occurred mainly at the NbC because the Pilling-Bedworth ratio of Nb is higher than the critical value of 2.3 where eruption typically occurs.