Competitive strengthening between dislocation slip and twinning in cast-wrought and additively manufactured CrCoNi medium entropy alloys
Wanchuck Woo, Y.S. Kim, Hobyung Chae, S.Y. Lee, J.S. Jeong, C.M. Lee, Jong Woo Won, Young‐Sang Na, Takuro Kawasaki, Stefanus Harjo, Ke An
Abstract
In situ neutron diffraction experiments have been performed under loading in cast-wrought (CW) and additively manufactured (AM) equiatomic CoCrNi medium-entropy alloys. The diffraction line profile analysis correlated the faulting-embedded crystal structure to the dislocation density , stacking/twin fault probability, and stacking fault energy as a function of strain. The results showed the initial dislocation density of 1.8 × 10 13 m −2 in CW and 1.3 × 10 14 m −2 in AM. It significantly increased up to 1.3 × 10 15 m −2 in CW and 1.7 × 10 15 m −2 in AM near fracture. The dislocation density contributed to the flow stress of 470 MPa in CW and 600 MPa in AM, respectively. Meanwhile, the twin fault probability of CW (2.7%) was about two times higher than AM (1.3%) and the stacking fault probability showed the similar tendency. The twinning provided strengthening of 360 MPa in CW and 180 MPa in AM. Such a favorable strengthening via deformation twinning in CW and dislocation slip in AM was attributed to the stacking fault energy . It was estimated as 18.6 mJ/m 2 in CW and 37.5 mJ/m 2 in AM by the strain field of dislocations incorporated model. Dense dislocations, deformation twinning, and atomic-scale stacking structure were examined by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM).