Microstructural evolution of Alloy 709 during aging
Microstructural evolution of Alloy 709 during aging
复制标题
DOI：
10.1016/j.matchar.2019.06.018
复制
发表时间：
2019-08-01
影响因子：
4.7
通讯作者：
Bowen, Paul
中科院分区：
文献类型：
--
作者：
Ding, Rengen;Yan, Jin;Bowen, Paul
抗蠕变奥氏体不锈钢709合金(Fe-20Cr-25Ni(wt%)基钢)是下一代快中子反应堆的候选结构材料，其服役温度为500-550℃，但对709合金时效过程中的组织演变缺乏研究。因此，在本研究中，用电子显微镜研究了709合金在550、650和750℃静态时效后的微观组织。结果表明，709合金中的主要析出物是Nb(CN)，偶尔还会观察到棒状Z相(CrNbN)。在550℃时效至2000 h后，709合金的显微组织没有明显变化，这意味着709合金在550℃时效时相当稳定，650℃时效时在晶界产生球状M23C6相，在孪晶界和晶内产生片状M23C6碳化物，块状M23C6碳化物在Nb(CN)上形核。650℃时效500h后，位错上有细小弥散分布的Z相，其数量随时效时间和温度的升高而增加。650℃时效1000h后，晶界形成(Cr，Mo)(3)(Ni，Fe)(2)相，750℃时效后，β相在M23C6碳化物上形核，并由M23C6转变为Theta相，表明Theta相更稳定。550℃时效促进了铬和钼向晶界的偏聚，而650℃时效时，由于富铬的M23C6碳化物在晶界析出，出现了明显的贫铬现象。这种损耗在750℃时几乎消失。时效对709合金随后的机械行为的影响进行了简要讨论。
The creep-resistant austenitic stainless steel Alloy 709 (Fe-20Cr-25Ni (wt%) based steel) is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500-550 degrees C. However, the study of microstructural evolution of Alloy 709 during aging is lacking. In this study, thus, the microstructure of Alloy 709 has been investigated using electron microscopy in the as-received state and after static aging at 550, 650 and 750 degrees C. The results show that the prominent precipitate in the as-received Alloy 709 is Nb(CN), with the rod-like Z phase (CrNbN) observed very occasionally. After aging at 550 degrees C even up to 2000 h, no significant microstructure change was observed, which means that Alloy 709 is fairly stable at 550 degrees C. Aging at 650 degrees C produced globular M23C6 phase on grain boundaries, plate-like M23C6 carbides at twin boundaries and in the grain interior, and blocky M23C6 carbide nucleated on Nb(CN). Fine dispersoid Z phases were found on dislocations after aging at 650 degrees C for 500 h; their amount increases with aging time and temperature. (Cr,Mo)(3)(Ni,Fe)(2)SiN theta phase forms at grain boundaries after aging at 650 degrees C for 1000 h. After aging at 750 degrees C, theta phase nucleated on M23C6 carbide and a transformation of M23C6 to theta phase was found, which suggests that theta phase is the more stable. Aging at 550 degrees C promotes the segregation of Cr and Mo to grain boundaries whereas clear Cr depletion was observed at 650 degrees C due to the precipitation of Cr-rich M23C6 carbides at grain boundaries. Such depletion nearly disappears at 750 degrees C. The implications of aging for the subsequent mechanical behaviour of Alloy 709 are discussed briefly.