ROLE OF GRAIN BOUNDARIES IN HIGH TEMPERATURE FATIGUE.

A common observation in low cycle fatigue at elevated temperature is the severe degradation in fatigue resistance with decreasing cyclic frequency. This loss in life is associated with a transition from transgranular failure to intergranular failure. The problem is examined in detail in terms of the effect of heat treatment, grain orientation, and environment (air vs. high vacuum) in notched bar fatigue tests of the high temperature resistant iron base superalloy, A286. A consistent model framework is presented to account for the loss in fatigue life at low freqencies, and it is demonstrated that the major part of this loss can be attributed to a grain boundary environment interaction at the crack tip. The low frequency fatigue resistance in air is improved by heat treatment modifications which are believed to alter the deformation characteristics of the alloy. A further improvement is obtained in directionally solidified specimens by eliminating the transverse grain boundaries at which localized reaction with the environment normally occurs.