Application of EPMA and analytical TEM to brazed metal-supported catalytic converters

摘要

The compositional and structural properties of brazed and braze-influenced areas can significantly affect the product lifetime of catalytic converters. Electron Probe Microanalysis (EPMA) with wavelength dispersive detection and Transmission Electron Microscopy (TEM) equipped with scanning capability and EDS were applied. The TEM samples were prepared in the Focused-Ion-Beam (FIB)-Workstation with a so-called In-situ-Liftout technique. Two samples were investigated: one with a high lifetime in a test bench, the other with a very low lifetime due to oxidation and corrosion. In both cases NiAl-phase precipitates with a Ni/Al-ratio of 1.2, respectively 1.4 were found in the braze-influenced areas, but the size, structure and the composition of the surrounding matrix are different. The high lifetime sample consists of a structure similar to eutectic appearance with small needle-like phases surrounded by small globular phases within a homogeneous matrix, whereas the low lifetime sample contains large polygonal phases. It has been found by TEM that interconnected structures of large polygonal phases of NiAl exist in brazed and braze- influenced areas with inclusions of matrix elements. As reported elsewhere [4], such phases can oxidise very heavily in exhaust gas whereas small globular and small needle-like phases of NiAl do not oxidize under the same conditions. One of the reasons for the occurrence of undesired large polygonal phases is limitations in the diffusion of nickel into the iron-based alloy, probably due to observed carbides of chromium in the steel foil material. The formation of Cr-carbides reduces the Cr- content of the surrounding matrix, which results in a reduction of the oxidation resistance.