Effect of particle morphology and microstructure on strength, work-hardening and ductility behaviour of ODS-(7-13)Cr steels

The effect of particle morphology and grain refinement to the nanometer scale on strength, work-hardening and tensile ductility of reduced activation ODS-(7-13)Cr steels has been modelled with a dependence on deformation temperature (T=RT-700 deg. C) and a superimposed irradiation hardening. The Orowan model predictions describe as the upper limit the observed particle strengthening of various ODS-(7-13)Cr-(≤0.5 wt% yttria) steels. An optimum particle size dp* congruent with 7-22 nm (fv=0.004-0.05) and strength, together with a lower limiting ultra-fine grain size dK,c≥90 nm result in maximum uniform ductility increase by grain refinement and dispersion hardening (DIGD). Optimum size dp* increases with increasing particle volume fraction fv and deformation temperature and decreases with irradiation hardening and grain refinement. The region of DIGD is limited to achieve a critical strength σL corresponding to a critical particle volume fraction fv,c and grain size dK,c, above which uniform strain becomes limited by the strong drop of fracture strain. Grain refinement and irradiation hardening decrease σL, fv,c and increase dK,c. In accordance with experimental results of ODS-Eurofer, nominal uniform strain increases with increasing fv by about εu,n=Be+Aelnfv, most strongly around 300 deg. C, but weakly at the 600 deg. C minimum. The strong ductility increase above 600 deg. C results from a reduction of dislocation annihilation and structural recovery of strength. At T<300 deg. C, grain refinement increases uniform ductility up to dK,c for lower fv toward a saturation value which increases with increasing ratio of shear modulus to Hall-Petch constant. The enhanced uniform ductility at T≥300 deg. C is otherwise strongly decreased by grain refinement, more pronounced at lower fv and for strengths above σL