Gigacycle fatigue properties of additively manufactured Hastelloy X

Gigacycle fatigue tests were conducted on additively manufactured (AM) Hastelloy X. Individual AM samples manufactured in the Z and X directions were tested at room temperature, and the results were compared with those for conventionally produced material. The samples were subjected to ultrasonic fatigue testing up to 109 cycles and to servo-hydraulic fatigue testing up to 107 cycles. The microstructures of the AM samples consisted of (101) texture that showed high internal plastic strain. The 0.2% proof stress levels of the AM samples were much higher than those of the conventional samples. Internal fractures were seen in both the AM and the conventional samples above 107 cycles, indicating conventional fatigue limits at 107 cycles to be absent in Hastelloy X. The gigacycle fatigue strengths were comparable between the AM and conventional samples and between the Z and X directions. The conventional fatigue test results for the AM samples revealed longer fatigue lives than those for the conventional samples because of the high 0.2% proof stress. These results show the ultrasonic fatigue test results of the AM samples to be continuously connected to the conventional fatigue test results, unlike the gap seen with conventional samples. Conventional fatigue limits at 107 cycles appear to be absent in Hastelloy X, with new fatigue limits possible in the gigacycle region. The internal fracture origins in the AM samples were solidification cracks or the matrix itself, meaning that fatigue failure was not caused by porosities or lack of fusion in these AM samples.