An investigation of the influence of creep-exposure on the mechanical properties at room temperature of Rene' 41 alloy is in progress. The objestives are to delineate the conditions causing changes in these properties and to develop general principles for predicting the influence of creep on such properties of Ni base superalloys. The results for exposures at 1200 to 1800 F and for 10 to 200 hr show that thermally-induced structural changes reduce room temperature strength and ductility when the exposure temperatures are 1500 F or higher. Limited data indicate that surface reactions also reduce properties with the temperature range of the effect extending to lower temperatures than 1500 F.

Investigation of the influence of creep-exposure on the mechanical properties of Rene' 41 at room temperature was continued. The results showed that thermally-induced structural changes reduced the strength and ductility for exposures at temperatures from 1400 to 1800 F. Reduced yield strength was due mainly to a decrease in the measured volume fraction of gamma prime precipitate and secondarily to an increase in the gamma prime particle size. For the conditions studied, gamma prime was sufficiently stable up to 1400 F so that yield strength was not affected. Ductility was reduced by thermally-induced carbide precipitation in the grain boundaries in the temperature range from 1400 to 1800 F. Ductility reached a minimum after about 100 hr at 1700 to 1800 F and was not further decreased by creep. Creep had little effect on the gamma prime reactions.

An investigation was made of the influence of creep-exposure at 1000 to 1800 F on mechanical properties of a structurally stable 80Ni - 20Cr alloy. Strain hardening and residual stresses raised strength and lowered ductility for creep at the lower temperatures. With increasing temperature of creep, this factor diminished. Internal microcracking in the grain boundaries occurred increasingly with exposure temperature and creep strain. A large amount of microcracking was required to reduce ductility and, when more severe, to reduce ultimate tensile strength. Two grain sizes were investigated. The fine grained material was less damaged by cracking than the coarse grain material for a given exposure. It was, however, subject to grain growth at intermediate temperatures with a loss in strength. This indicated that internal cracking had to be extensive for appreciable damage. This is due to the low strength and high ductility of the alloy. (Author).