N18合金经轧制变形及780℃/5 h热处理,在500℃/10.3 MPa过热蒸汽中发生了疖状腐蚀.采用TEM和SEM研究了合金及腐蚀生成氧化膜的显微组织.结果表明,发生疖状腐蚀的原因是试样在(α+β)两相区的热处理过程中,合金元素铌、铁、铬等大量迁移至α-Zr晶界处,形成Zr(NbFeCr)2和β-Zr相,造成α-Zr晶粒内部合金元素严重贫化.对于这类合金,应注意其热加工过程,尽量避免进入(α+β)相区并保温时间过长,以防止疖状腐蚀的发生.
Nodular corrosion was found in N18 zirconium alloy processed by rolling deformation and 780 ℃/5 h heat treatment in the superheated steam of 500 ℃/10.3 MPa.The microstructure of N18 zirconium alloy and the oxidation films were observed by SEM and TEM.The results show that the heat treatment carried out in the (α+β)intercritical temperature range promoted the movement of alloying elements such as Nb,Fe and Cr to the boundaries of α-Zr grains and the formation of Zr(NbFeCr)2 and β-Zr phases,thus led to the severe depletion of alloying elements in α-Zr grains and resulted in nodular corrosion in the heat treated N18 zirconium alloy.It is suggested that the heat treatment of N18 alloy should avoid to be performed in the (α+β) intercritical temperature range for long time so as to prevent nodular corrosion.
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