Development of a Titanium-Modified Hastelloy N with Improved Resistance to Radiation Damage

Our studies of the effects of neutron irradiation on the hightemperature mechanical properties of Hastelloy N have shown generally that the creep-rupture life and ductility are reduced. The ductility is a strong function of the strain rate and shows a minimum at a minimum creep rate of about 0.1 percent per hour. The resistance of this material to radiation damage can be enhanced greatly by adding titanium. Laboratory and commercial melts have shown that the postirradiation creep-rupture ductility and strength rise sharply as the titanium content is increased above 0.3 weight percent. Although the degree of improvement is still rising as the titanium level is increased to 1 percent, most of our studies have been carried out on an alloy with about 0.5 weight percent titanium. Postirradiation creep-rupture tests at 650 C on specimens irradiated to a thermal fluence of 5 X 1020 neutrons cm2 indicate that a ductility minimum still exists as a function of strain rate. However, the minimum strain is 3 to 5 percent as compared with 0.5 percent for the standard alloy. In-reactor creep-rupture tests have also been run and the same improved properties are indicated. Our studies of the titanium-modified alloy have not progressed sufficiently to pinpoint the exact role of titanium in bringing about the improved properties. However, several observations have been made which allow us to make some postulations at this time.