2017 Volume 103 Issue 3 Pages 157-165
The constant stress cyclic loading test was conducted in order to elucidate the influence of crystal orientation on cyclic deformation behavior in bcc Fe-3 mass%Al single crystal, where the stress axes are [001] and [011]. When the stress amplitude is larger than the cyclic yield strength, materials commonly showed a decrease in cyclic plastic strain followed by an increase and saturation at a certain plastic strain. The amount of saturated plastic strain is independent of stress level; however it depends on crystal orientation. In the case that the stress axis is [001], the abrupt increase in plastic strain took place at 2.0*105 cycles accompanied by extrusion along {211} plane resulting in fatigue fracture, whereas no fracture occurred when the stress axis is [011]. TEM observation into the specimen stressed along [001] revealed that straight dislocations introduced by the very early cyclic load pair-annihilate resulting in a characteristic dislocation structure with heterogeneous distribution of point-like sessile dislocations followed by their increase in density together with the new formation of short dislocations presumably due to the dislocation-dislocation interaction. Meanwhile, in the case the stress axis is [011], point-like sessile dislocations homogeneously distribute with lower density. Future work is needed to clarify the relationship between this unique dislocation structure and the fatigue crack initiation.
