THE EFFECT OF DEFECTS ON THE FATIGUE CRACK INITIATION PROCESS IN TWO P/M SUPERALLOYS

A metallurgical investigation was performed to determine the effect of defect size, shape, and population on the fatigue initiation process in two high strength P/M superalloys, AF-115 and AF2-1DA. The specific alloy heats tested had contrasting defect populations the AF-115 alloy contained a large population of spherical pores, and a lesser number of elliptical ceramic inclusions and plate-like hafnium oxide inclusions, and the AF2-1DA material contained only a small population of the elliptically shaped inclusions. Strain controlled fatigue tests were performed on uniform section specimens at 760(DEGREES)C, 649(DEGREES)C, and 22(DEGREES)C using both continuously cycling and cyclic dwell waveforms.It is also shown that the room temperature fatigue tests failed in a different manner than the high temperature tests and with no surface-subsurface transition in initiation site. This observation is explained in terms of the differing slip behavior of the superalloys at room and elevated temperature.Finally, a significant creep-fatigue interaction is shown to occur for specimens tested at elevated temperature with a tensile dwell cycle while those specimens tested with a compressive dwell cycle failed in the same manner as the continuously cycling tests. The fatigue data is analyzed to show the effects of cyclic dwell cycles on both the crack initiation and propagation phases of the fatigue process.Microscopic examination of the failed specimens showed that for the high temperature tests there was a transition in the nucleation site of the cracks that caused failure from a surface to a subsurface location as the strain range was reduced. This surface-subsurface transition (STT) occurred at approximately the same strain range for both alloys. An explanation for this phenomenon is developed by first considering individually the crack initiation and crack propagation phases of the fatigue process, and hypothesizing that crack initiation is controlled by localization of plastic strain and crack propagation is a function of the environment and stress intensity at the crack tip. The results indicated that at high strain ranges the crack propagation phase of fatigue life was most critical and, therefore, defect size and population were most important. At the lower strain ranges, the initiation phase of life was more critical in the overall fatigue process and defect shape was shown to be of primary concern.