Search

Aluminum alloys have a strong tendency to form alumina layers on their surfaces when exposed to atmospheric air, even at room temperature. This is a severe challenge for brazing aluminum alloys as the alumina layer acts as a diffusion barrier and hinders the interactions between the filler metal and the base material. In order to achieve a good metallurgical bond between the filler metal and the aluminum alloy, it is of crucial importance to remove the alumina layer as well as to simultaneously prevent further oxidation of the aluminum alloy. The current investigation focuses on the detailed micro-structural changes that occur during in-situ brazing of liquid filler metal, 95Sn-5Cu (wt.%) on an aluminum alloy, Al-7Si-0.3Mg. These in-situ studies were performed in a large chamber scanning electron microscope in order to monitor the interactions of the filler metal and the base material, particularly the role of Cu on alumina detachment. After the in-situ experiments, the local surface and cross-sectional regions were analyzed by scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy to understand the variation in chemistry across the wetted region, which includes the interfacial region between filler metal and the base material. As the alumina scale present on the aluminum alloy is very thin (2Cu intermetallic precipitates.

High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 °C was investigated in Ar–4%H2–4%H2O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr2O4 spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O2 oxide of 200–300 nm thickness was observed at the scale–alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.

Journal of the Electrochemical Society : JES 167(6), 061502 (2020). doi:10.1149/1945-7111/ab7d2e, Published by IOP Publishimg, Bristol