K480铸造镍基高温合金900℃高温时效过程中晶界粗化行为研究

INVESTIGATION ON COARSENING BEHAVIORS OF SERRATED GRAIN BOUNDARIES IN K480 NICKEL BASE SUPERALLOY DURING LONG TERM AGING AT 900℃

铸造K480镍基高温合金经固溶、时效处理后,在900℃长期时效3000 h.分别用OM和SEM对合金的晶界组织进行了观察,用EDS对合金中相的成分进行了测量.结果显示,K480合金的晶界为曲折的锯齿形晶界,该晶界由含碳化物晶界及含γ'晶界两部分构成.随着时效时间的延长,两部分晶界均发生粗化.其中含碳化物晶界中的MC碳化物分解形成M_6C相和η相,并随时间长大;而含γ'晶界中不连续γ'相逐渐相连并形成γ'相条带.含碳化物晶界的粗化比要高于含γ'晶界,且随时效时间延长,二者的差异变大.用Johnson-Mehl-Avrami-Kolmogorov(JMAK)方程量化了时效过程中碳化物晶界和-γ'晶界随时效时间的粗化行为,实验数据与计算结果吻合,表明二者的粗化行为随时间的演变规律是遵循着JMAK方程的变化形式.

Serrated grain boundaries in conventional cast nickel base superalloys can enhance high temperature mechanical properties of the alloy due to a large number of precipitates arranged in grain boundaries, which effectively inhibited the movement of dislocations. During service at elevated temperatures, the ripening, coalescence or degeneration process may occur in these precipitates, re- sulting in the coarsening of grain boundaries with time. This paper aims to investigate the coarsening kinetics and microstructure evolution of serrated grain boundaries in a conventional nickel base superalloy during long term aging. After solution and aging heat treatment, a long term aging treatment at 900 ℃ for 3000 h was carried out on a cast K480 nickel base superalloy. The microstructure of grain boundaries （GBs） was observed using OM and SEM respectively, and phase composition was measured using EDS. The results showed that the GBs of K480 alloy were the irregular serrated GBs composed of carbide GBs and η GBs respectively, both of which coarsened with aging time. During the course of aging, the MC carbides in the carbide GB were decomposed with the formation of new phases of M6C carbide and η phase; the discontinuous large-sized ηparticles in the 71 GB were coalesced with each other along the GB direction with aging time and ~ bands formed after aging treatment. The coarsening ratio of the carbide GB was higher than that of η GB at the beginning of aging treatment and the difference in these two values became larger with increase in aging time. A Johnson-Mehl Avrami-Kolmogorov （JMAK） type of function was employed to quantify the evolution of coarsening behaviors of the two kinds of GBs with aging time. The good agreement between calculated results and experimental data indicated that the coarsening behaviors of carbide GB and γ＇ GB in the serrated GB were evolved as a JMAK type of function of aging time in K480 nickel base superalloy long term aged at 900 ℃.