D. Preininger, Forschungszentrum Karlsruhe GmbH Technik und Umwelt (Germany)
The superimposed effects of oxide dispersion strengthening and grain refinement to the nano-crystalline (nc) level (dk>3 nm) on tensile ductility as uniform and fracture strain and Charpy-impact behaviour of ODS-Ta-W alloys have been examined by micro-mechanical deformation and stress-induced ductile/dynamic fracture models. The impact model takes into account stress intensity and local plasticity and fracture within the plastic zone of crack. Especially, the grain size versus ductility and impact properties as the ductile-to-brittle transition temperature DBTT and upper shelf energy USE have been examined as function of dispersion parameters like the mean size dp and volume fraction fv together with a superimposed strengthening by £40at.% W alloying and irradiation defects. Additionally, also the effects of deformation temperature T, strain rate e` and a combined increase of ductile and dynamic fracture stresses sfd,f due to grain refinement have been considered which might result from a transition of dislocation glide to diffusion-induced grain-boundary sliding. A grain refinement of pure tantalum strongly decrease uniform ductility eu up to a minimum at ultra-fine (uf) grain sizes of dk>100 nm at T=300 K. Then it increases toward a saturation ductility of eu,sat at nc sizes up to achieve of a critical yield stress sy ?sL, where eu becomes limited by the strong reduction of fracture strain. Dispersion hardening of ODS-Ta via the Orowan- by-pass process reduces uniform ductility at larger dK>300 nm but increases it at uf and nc grain sizes, more pronounced at an optimum particle size dp*. This optimum size dp* increases with increasing fraction fv but strongly reduces with increasing strengthening by grain refinement, irradiation defects as well as weaker also by W alloying and pronounced cross-slip induced dislocation annihilation at higher T. A grain refinement of Ta-£40W alloys at constant fracture stresses increases DBTT and reduces USE and uniform strain more pronounced at higher tungsten contents. Dispersion hardening of ODS-Ta-W increases further DBTT and ductile energy reduction U=DUSE/USEo and might additionally reduce eu at low dp within the uf and nc regions. Thus, DBTT generally increases with decreasing uniform ductility more stronger within the uf, nc regions and lower dp. A combined increase of fracture stresses sfd ~kf dK-1/2, sf/sfd <1 due to grain refinement with the strength cf =kf/kHP>0, where kHP denotes the Hall-Petch constant, however, reduces DBTT and comparably weaker also shift U of ODS-Ta at uf grains and additionally grain size dk,c, where DBTT achieves room temperature. At ODS-Ta-W alloys then shift U becomes stronger reduced. Interestingly, also a distinct minimum in DBTT@40-120 K appears at an optimum uf grain size of 50–200 nm for higher cf>0.3 especially at Ta and ODS-Ta alloys. With increasing ratio cf more stronger DBTT than U is reduced due to grain refinement of ODS-Ta. At higher W contents of ODS-Ta-W alloys however, besides DBTT now more stronger shift U is decreased with increasing ratio cf which above cf>0.8 and dk?200 nm becomes negative corresponding to an increase of USE. Additionally, dK,c is increased more stronger for lower cf. The obtained analytical and numerical results, which compared with experimental data are used for optimisation of ductility and impact properties of ODS-Ta-W alloys, proposed as high-temperature (£1300°C) structural materials like the converter of fusion reactors by dispersion hardening and grain refinement. Especially the optimum particle and grain sizes in dependence of T, e` and <40at.% tungsten alloying are considered which additionally increases strength and strongly also resistance against hydrogen embrittlement.