The use of fracture toughness in the development of the 9Ni-4Co alloy system is described in this paper. The primary concern during this study was the delineation of metallurgical trends rather than the generation of quantitative data. Thus, screening tests have been employed.

The particular test chosen for any study should have four characteristics: 1. be compatible with previous data 2. be reasonable in cost 3. reflect the section size anticipated for eventual service 4. have sufficient sensitivity to define the effects of the variables investigated.

High-strength steels were screened effectively with center fatigue precracked specimens. The thickness of the specimens was adjusted to fit a given toughness level. The sensitivity was greatest when the ratio of the nominal notch strength to the yield strength was between 0.4 and 0.8.

For tough materials at strength levels of 200 ksi and lower, the determination of valid plane-strain toughness values in the early stages of alloy and process development is impractical due to the large size of the required specimens. Precracked and standard Charpy impact specimens appeared to define usefully qualitative trends. However, in the final optimization programs, plane-strain fracture toughness specimens were used to obtain absolute fracture toughness numbers compatible with the best known state of the art.