Microstructure of a boron containing high purity nickel-based alloy 690

A high purity model alloy with major composition Ni–30Cr–10Fe–0.024C–0.006B (wt.%), corresponding to the commercial Alloy 690, was investigated using secondary ion mass spectroscopy, atom probe analysis and transmission electron microscopy in combination with energy dispersive X-ray analysis and electron energy loss spectroscopy. Solution annealing at 1200°C for 20 min put most carbon and boron into solution, but some intergranular borides and carbides, probably formed during quenching, were observed. Heat treatment at 700°C for 1 and 100 h resulted in substantial intergranular precipitation of Cr23C6 and Cr2B. The influence of boron on the precipitation process in Alloy 690 is discussed and compared with precipitation in Alloy 600.