1.04.7.1 Introduction

The class of ferritic-martensitic alloys with chro­mium concentrations in the range of 9-12% has attracted interest in the fast reactor programs because of its radiation resistance, in particular, very low swelling and low irradiation creep. Alloys such as Sandvik HT-9 (12Cr1Mo.6Mn.1Si.5W.3V)) and other alloys of this class were irradiated in the EBR-II,24 in research reactors25 and with heavy ions.26 The quantitative results from the ion irradia­tions in this class of alloys and the low neutron absorption cross section led to inclusion of ferritic alloys into the fast reactor alloy development pro­grams, in particular in the United States in the mid-1970s. The radiation resistance has been con­firmed to displacement levels of 70 dpa.12,14

Further interest in this class of alloys was initiated by the fusion reactor programs in Europe and the United States when the necessity for low neutron activation structural materials was realized. Further
research on martensitic alloys by fusion programs in Europe, the United States, and Japan led to the devel­opment of low-activation alloys by replacing elements that result in long-term activation products. Molyb­denum and niobium, both of which result in long — lived activation products, were replaced by tungsten and tantalum. This research led to radiation-resistant alloys with a fracture toughness superior to that of the commercial alloys even in the unirradiated condi­tion.27 The compositions of representative members of this class of alloys referred to in this chapter are presented in Table 1. An excellent review of irradia­tion behavior of this class of alloys has been published by Klueh and Harries.27 Details of the metallurgy of martensitic alloys appears in Chapter 4.03, Ferritic Steels and Advanced Ferritic-Martensitic Steels.