Structure and chemistry of grain boundaries in Ni−16Cr−9Fe model materials

The grain boundary microchemistry of two model materials with composition Ni−16Cr−9Fe−0.03C (wt%) and Ni−16Cr−9Fe−0.03C−0.008B have been studied using optical microscopy, TEM and APFIM. The materials received heat treatment that resulted in semi-continuous precipitation at grain boundaries. The concentration of carbon in the matrix of both materials was reduced to about half the bulk value. Besides, boron totally abandoned the matrix of the material containing this element. TEM, EDX and APFIM investigations revealed the existence of Cr7C3 in the carbon containing material. Cr7C3 and Ni23B6 together with the unstable phases Cr23C6 and Cr7B3 were observed at the grain boundaries of the boron containing material. The composition of the observed intergranular precipitates, segregation of elements to the precipitate free segments of grain boundaries and chromium depletion are discussed.