EFFECTS OF BORIDE FORMER ELEMENTS ON RECOVERY AND RECRYSTALLIZATION OF REVERSE-TRANSFORMED AUSTENITE IN FE-19-PERCENT-NI ALLOY

Effects of various boride former elements (Ta, W, Nb, Ti, Mo) and/or B on recovery and recrystallization of reverse-transformed austenite we re examined in carbon-free Fe-19mass%Ni alloys. The combined addition of Nb and B was found to be the most effective in suppressing them; th e recrystallization temperature of the reverse-transformed austenite r ose up to 1200 K which is similar to 100 K higher than that of alloys with single addition of boride former element or B. TEM observation of extraction replicas indicated that the retardation of recovery and re crystallization was due to very fine precipitates of Nb-boride, It was made clear from the theoretical analysis on the solubility product of several MB(2)-type borides (M=Cr, Mn, Nb, Ti, Mo) that NbB2 was the m ost stable over a wide range of temperature (773-1173 K). This may be the reason why recovery and recrystallization do not occur up to the h ighest temperature in case of the combined addition of Nb and B.