Simultaneous analysis of fracture surfaces by auger and energy dispersive x‐ray mapping

Abstract

AbstractThe grain boundary segregation of the metalloid impurities antimony and phosphorus to the prior austenite grain boundaries of an Fe‐3.5 Ni‐1Cr alloy during embrittlement at 430°C has been studied by simultaneous Auger and energy dispersive x‐ray analysis. The segregating species were found to be related to the tempering characteristics of the unembrittled microstructures. Initial intergranular embrittlement in a quenched and tempered martensite microstructure was associated with the prescence of phsophorus whereas the initial embrittlement in a bainitic microstructure was associated with the segregation of antimony. The difference in behaviour between the two microstructures has been explained in terms of site competition at the grain boundaries. In the case of the tempered martensite microstructure the carbon activity is reduced as a consequence of the formation of chromium rich precipitates at the grain boundary. Carbon activity is reduced as a consequence of the formation of chromium rich precipitates at the grain boundary. The reduction in carbon activity allowed phosphorus migration to the boundary producing intergranular embrittlement. In bainites the predominant precipitate M3C increased the carbon activity with a resultant decrease in phosphorus concentration the grain boundary and the absence of intergranular failure in the early stages of embrittlement. Prolonged embrittlement of the bainites produced a low energy intergranular failure. Increased nickel and antimony concentrations at the grain boundaries were associated with the formation of a fine grain boundary precipitate low in chromium. The increased carbon activity continued to prevent appreciable phosphorus segregation but was not sufficient to inhibit the cosegregation of nickel and antimony.