Effect of dose on irradiation-induced loop density and Burgers vector in ion-irradiated ferritic/martensitic steel HT9

ABSTRACT

Samples of F/M steel HT9 were irradiated to 20 dpa at 420°C, 440°C and 470°C in a transmission electron microscope with 1 MeV Kr ions so that the microstructure evolution could be followed in situ and characterised as a function of dose. Dynamic observations of irradiation-induced defect formation and evolution were made at the different temperatures. Irradiation-induced loops were characterised in terms of their Burgers vector, size and density as a function of dose and similar observations and trends were found at the three temperatures: (i) both a/2 <111> and a <100> loops are observed; (ii) in the early stage of irradiation, the density of irradiation-induced loops increases with dose (0–4 dpa) and then decreases at higher doses (above 4 dpa), (iii) the dislocation line density shows an inverse trend to the loop density with increasing dose: in the early stages of irradiation, the pre-existing dislocation lines are lost by climb to the surfaces while at higher doses (above 4 dpa), the build-up of new dislocation networks is observed along with the loss of the radiation-induced dislocation loops to dislocation networks; (iv) at higher doses, the decrease of number of loops affects more the a/2 <111> loop population; the possible loss mechanisms of the a/2 <111> loops are discussed. Also, the ratio of a <100> to a/2 <111> loops is found to be similar to cases of bulk irradiation of the same alloy using 5 MeV Fe²⁺ ions to similar doses of 20 dpa at similar temperatures.

KEYWORDS:

• In situ TEM
• ion irradiation
• radiation damage
• ferritic/martensitic steel
• HT9

Acknowledgements

This work was a part of the NEUP-IRP project named as ‘High Fidelity Ion Beam Simulation of High Dose Neutron Irradiation.’ The access to the IVEM facility was through the Rapid Turnaround Experiment awarded by the US Department of Energy Nuclear Science User Facilities. The authors thank Pete Baldo, Ed Ryan, Jing Hu, Meimei Li and Mark Kirk at the IVEM facility for their assistance. Sample preparation and STEM characterisations were performed at the Analytical Instrumentation Facility (AIF), North Carolina State University.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by U.S. Department of Energy [grant number DE-NE0000639]. The AIF is a member of the North Carolina Research Triangle Nanotechnology Network which is supported by the State of North Carolina and the National Science Foundation [award number ECCS-1542015].