Inhibiting effect of palladium additions on embrittlement of Fe–Ni low expansion alloys by copper containing brazing alloys
AbstractThe Pd containing brazing alloys were invented as filler materials that (i) had good spreading and gap filling properties, (ii) did not badly erode the high temperature metals for which their use was intended, (iii) were ductile, and (iv) did not contain volatiles, which made their use in radio valves attractive particularly for glass–metal seals, where the Ag–Cu eutectic was a favoured alternative, but could lead to cracking of parent metal during the brazing cycle unless preplated with Ni. The work reported here traced the mechanism of attack of Fe–Ni alloy by Ag–Cu eutectic with three modifications containing nominally 5, 10, and 25 wt-%Pd. A static beam loading technique was used in conjunction with scanning electron and optical microscopy, electron probe microanalysis, X-ray (XPS), and Auger (AES) spectroscopy. The alloys containing 5%Pd or less were hot short, but were ductile when cold bend tested; those containing 10 and 25%Pd were ductile at all temperatures tested. Ductility was associat...