Investigations of the Physical and Mechanical Properties of OPTIFER Alloys

Abstract

Within the framework of the development of low-activation structural materials to be used in fusion reactors, martensitic Fe 9.5 Cr alloys, called OPTIFER, have been conceived and meanwhile studied in further detail under a European long-term program. The alloy variants contain varying amounts of W, Ta, Ge and/or Ce as elements substituting the radiologically unfavorable alloying elements of Mo, Ni, Nb and Al. In three production series, thirteen different batches have been produced and analyzed so far. Up to now, the variant of 9.5 Cr-1W+Mn, V, Ta has proved to be the most promising one. For radiological reasons, a W-free (+Ge) version is studied as an alternative.

The behavior during transformation and heat treatment more or less corresponds to that of conventional martensitic 9-12% Cr steels. Maximum hardness is achieved as of an austenitization temperature of 950°C. The mechanical properties are strongly dependent on the hardening temperature. Optimum tensile and creep strengths are reached at the reference hardening temperature of 1075°C (+750°C annealed) with the notch impact toughness exceeding that of conventional steels. A decrease in the hardening temperature to 1030-950°C for a further improvement of the notch impact toughness, however, causes the tensile and creep strength values to be reduced.

In several variants, the influence of the O₂ content on notch impact toughness was found to be rather strong.

The major characteristics of the OPTIFER alloys are compared with those of the Japanese 2%W steel, F82H-mod., which is currently being investigated at European laboratories.