Fe-Ni基合金时效过程中γ'相析出的原子探针层析技术研究

Study on the Precipitation of γ' in a Fe-Ni Base Alloy During Ageing by APT

将一种Fe-Ni基沉淀强化奥氏体合金在980℃固溶后水淬,在620℃经过不同时间时效。利用硬度测试及原子探针层析技术(APT)研究时效过程中g'相的析出行为及其对材料硬度的影响。结果表明,当时效时间小于6 h时,合金硬度增加较快,由时效前的145 HV迅速增加至205 HV,随后硬度增加速率缓慢;时效120 h时,硬度为251 HV。APT结果表明,合金经固溶处理后,合金元素均匀分布在基体中。在时效最初阶段,Ti发生了较为明显的偏聚,形成含有Fe、Ni和Al等元素的富Ti纳米团簇。随着时效时间延长,富Ti纳米团簇中的Ni和Al原子的含量逐步增多,而Fe、Cr及Mo等原子的含量不断减少;当时效至120 h时,团簇中Ni与Ti+Al比值近似于3,即已完全形成g'相,表明g'相析出是形核-长大过程。合金硬度的变化与时效过程中g'相的数量密度和体积分数有关。

High strength Fe-Ni base austenitic alloys,such as A286 and JBK-75,are widely used in gas turbine jet engines and hydrogen service and so on because of their excellent corrosion resistance and low hydrogen embrittlement sensitivity.The ordered coherent g＇ [Ni3（Al,Ti）],precipitated during ageing,is thought to have the main contribution on the strength.Thus,it is very important to understand the characterization of the precipitation.However,few previous studies are focused on atomic scale evolution of the precipitated phase.Atom probe tomography（APT） is a unique microscopy technique that provides3 D analytical mapping of materials at near atomic resolution and a high detection sensitivity for all elements.The present research is focused on the microstructure evolution at ageing temperature at different time scales using APT.A Fe-Ni base austenite alloy were aged at 620 ℃ for different time after solution treated at 980 ℃ for 2 h.Hardness testing indicates that a sharp increase is observed when the ageing time is less than 6 h.The hardness is up to 205 HV from the initial 145 HV at the ageing time 6 h.After that the hardness increases slowly.The hardness is 251 HV at 120 h.APT results reveal that Ti-rich nanoclusters precipitate obviously at the initial stage of ageing,which contain Fe,Cr,Ni,Mo and Al elements.As the ageing time increases,more Ni and Al atoms are segregated in the Ti-rich nanoclusters while the Fe,Cr and Mo are ejected from the nanoclusters.When the ageing time is up to 120 h,the Ni/（Ti＋Al） ration is approximately close to 3.The precipitates can be identified as g＇ phase.The results reveal that the formation of g＇ involves nucleation and growth.Effect of the number density and the size of the g＇ precipitates on the hardening of the alloy has been estimated.