Study Of Irradiation Hardening In Oxide Dispersion Strengthened Steels

Oxide dispersion strengthened（ODS）steel is one of the promising candidate materials for structural components in the Generation-Ⅳfission reactor and the future nuclear fusion reactor structural components for its excellent high temperature resistance and radiation resistance.Exposed to a high dose of neutron radiation,the mechanical properties of ODS steel degrade due to radiation-induced hardening.Studies suggested that irradiation-produced dislocation loops are the main causes of irradiation hardening.The fomation of dislocation loops is closely related to the irradiation conditions and the initial microsture of ODS steelsA set of ODS steels including two kinds of 14Cr-steels and three kinds of9Cr-steels were used for the study on the microstructure,hardness,irradiation hardening and post-irradiation annealing behaviors.This study can be dived into three sections:1)The relationship between the hardness and microstructure of ODS steelsFor the five kinds of steels,their microsture including grain size distribution,oxide nano-particles distribution and dislocation distribution were characterized by using EBSD,SANS and TEM,respectively.Their Vickers hardness and nano-hardness were measured.We found that the hardness of ODS steel with higher Cr content,smaller grain size,higher dislocation density and finer oxide particle distribution was higher.Based on the microstructral parameters of the five steels,their theoretic yield stresses were calculated according to the corresponding strengthening mechanism.The calculated yield stresses of the three 9Cr-steels agree well with their experimental values derived from Vickers hardness.There is a good linear relationship between the Vickers hardness and the nano-hardness of the five kinds of steels,which can be expressed as H_V=0.78H_IT.2)Irradiation-induced hardening in ODS steelsThree-step irradiations with Fe ions(0.5 MeV Fe⁺/3.7×10¹⁵ cm^-2+2 MeV Fe⁺/6.1×10¹⁵cm^-2+5 MeV Fe²⁺/1.15×10¹⁶ cm^-2)were conducted at 300 and 500℃,respectively,to produce roughly uniform damage layer.The irradiation-induced hardness change was measured by using nano-indentation.Significant irradiation hardening was detectived in all the samples irradiated at300 and 500℃.Weak irradiation hardening was observed in the near surface layer of the samples irradiated at 500℃.One possble reason is that the sample surface can act as a two-dimensional sink capturing point defects caused by irradiation.Another possible reason is that a fraction of the irradiation-induced defects introduced during steps 1 and 2 of the irradiation will heal up during steps 2 and 3,when the whole sample is again exposed to the irradiation temperature.14Cr steels irradiated at 300℃ showed additional hardening compared with 9Cr steels,which might be caused by the irradiation-enhanced formation ofα’phases.Compared with the three kinds of 9Cr steels,the two kinds of 14Cr steels exhibited less hardness increase for irradiations at both 300 and 500℃.Finer grain distribution and higher dialocation density can effectively improve the irradiation resistance of ODS steel,although the corresponding sink strengths are minor compared with the calculated sink strength of particle-martrix interfaces.Having exactly the same paticle distribtution as9Cr-ODS-A,9Cr-ODS-B has smaller grain size and higher dislocation density after additional heat treatment.As a result,9Cr-ODS-B showed less hardening after irradiations at 300 and 500℃.It was found that the irradiation-induced hardness increase as a fuction as total sink strength could be expressed by a equation:H_IT=170×(S_total)^-1.5.3)Post-irradiation annealing behavior of ODS steelsThree kinds of ODS steels were selected to perform single-step irradiation with Fe ions(5 MeV Fe²⁺/1.15×10¹⁶ cm^-2)at 300 and 500℃.Post-irradiation isochronal annealing was carried out from the irradiation temperature up to 600℃ with an interval of 50℃.Nanoindentation was used to measure the hardness change of irradiated layer after each step of annealing.The irradiation-induced hardening was completely eliminated for all alloys via post-irradiation annealing at sufficiently high temperature.9Cr-ODS-A and9Cr-ODS-B irradiated at 300℃ showed similar annealing behavior.The hardness of irradiated layer kept unchanged after annealing for 300 to 400℃,and gradually dropped down to unirradiated level during annealing from 400 to 550℃.This indicated that irradiation-induced nanofeatures in 9Cr-ODS-A and 9Cr-ODS-B might have the same category but different size and density distributions.The cross section morphology of 9Cr-ODS-A irradiated at 300℃ showed an obvious damage band at the location of dpa peak.A large number of irradiation dislocation loops were observed in the damage band.These dislocation loops were suggested to be the main cause of irradiation hardening.Considering TEM observations and published results in references,we assume that these irradiation-induced dislocation loops shrink and/or disappear by capturing themal vacancies during annealing,which leads to hardness reduction.Based on this assumption,a model was established to calculate the hardness variation caused by annealing.The calculated hardness as a fuction of annealing temperature for 9Cr-ODS-A irradiated at 300℃ agreed well with the experimenatal results.14Cr-ODS showed a different annealing behavior as compared with 9Cr steels,which may be related to the evolution ofα’phases.