Microstructure of a boron containing high purity nickel-based alloy 640

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 degrees 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 degrees 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. (C) 2000 Elsevier Science S.A. Ail rights reserved.