粉末高温合金FGH4096的疲劳小裂纹扩展行为

Inevitable nonmetallic inclusions (NMIs) exist in powder metallurgy (PM) superalloys. These inclusions serve as preferred sites for crack initiation either by fracture of inclusions or inclusion/matrix decohesion. After initiating from NMIs, fatigue cracks will experience the small crack propagation phase. Small fatigue cracks could grow under the fatigue crack growth threshold and propagate at a vibrated rate. To investigate the propagation behavior and reveal the underlying mechanisms, small crack propagation experiments under fatigue loads of different maximum stresses were conducted on PM superalloy FGH4096 using the small fatigue crack-propagation experiment system. The characterization of microstructure was conducted and orientations of grains were calibrated using SEM integrated with EBSD. Focusing on the three-dimensional nature and the physical basis, the propagation and stagnation behavior of small cracks were revealed. Experimental results showed that the small cracks propagated along octahedral slip planes, from initiation to a length even longer than 1.0 mm. During the propagation in the grain-containing twin, small cracks grew along the direction parallel to the twin boundary. However, several twin boundaries impeded crack growth. Small cracks were stagnated at grain and twin boundaries of which M factors were lower than adjacent ones. Three behaviors were observed after the stagnation of small cracks due to the different properties of grain/twin boundaries and the applied load; first, stagnated small cracks could continue to propagate by consuming more cycles; second, small cracks could propagate by alternating to another slip plane in current or other grains on the crack path; third, secondary cracks would initiate within 1-2 grains from the tip of the fully stagnated cracks and connected to the main crack. This behavior was observed only in the specimen in which the maximum stress is close to the lower limit of the yield strength.