Nano-indentation hardness evaluation of 9Cr-ODS ferritic steel

An oxide dispersion strengthened (ODS) ferritic steel is the most prospective candidate cladding material for the sodium-cooled fast reactor (SFR) and other Generation IV reactors. Their target average burnup is 150 GWd/t and relevant neutron dose is as high as 250 dpa. For SFR fuel elements, Japan Atomic Energy Agency has been extensively developing 9Cr-ODS ferritic steel cladding, which exhibits the distinguished creep rupture strength at 973K. In order to elucidate its strengthening mechanism, hardness measurement using 1 mN load was conducted by means of nano-indentaion technique. The 9Cr-ODS ferritic steel exhibits the unique composite structure consisting of the ferrite grains designated as residual ferrite and surrounding tempered martensite grains. The nano-scopic hardness corresponding to individual grains, i.e. residual ferritic grains and tempered martensite grains, was directly measured at the condition of the normalizing and tempering. The inherent hardness of martensite grains reduces with increasing tempering temperature, whilst residual ferrite grains maintain almost constant hardness. TEM observation of the dispersed oxide particles suggested that a dispersion strengthening is slightly higher in residual ferritic grains, but dominant strengthening mechanism could be owing to extremely high density of dislocations, and their recovery retards in residual ferritic grains. The excellent high-temperature strength of 9Cr-ODS ferritic steel is attributed to optimum balancing between hard residual ferrite and soft tempered martensite grains. (authors)