High-cycle fatigue induced twinning in CoCrFeNi high-entropy alloy processed by laser powder bed fusion additive manufacturing

High-cycle fatigue (R=0.1, room temperature) induced microstructural evolution in a laser powder bed fusion (L-PBF) additively manufactured quaternary CoCrFeNi high-entropy alloy (HEA) was studied. The as-built material exhibited a combined < 001 > and < 110 > texture and high proportion of low-angle boundaries. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed that deformation twinning occurred under the high-cycle fatigue of σmax= 450 MPa (Nf=1.06 ×105), but not for the stress level of 300 MPa and 200 MPa. The deformation twins led to the cyclic softening, as manifested by the continuous increase of maximum strain under the stress-controlled fatigue, and the hardness increased by ∼80 HV0.2 in the post-fatigued condition. EBSD revealed that both the < 001 > and < 110 > orientations were favorable for the twin formation. Given that the size of grains with the < 001 > and < 110 > orientations was twice larger than those of the other orientations, the grain size effect on twin formation could play a certain role. High-resolution TEM revealed that the full dislocations, lattice distortion, stacking faults, and partial dislocations were associated with the twin, cellular and labyrinth wall-like dislocation structures. The underlying mechanism for the formation of nano-twins during high-stress fatigue involved the dissociation of 1/2 < 110 > full dislocations to 1/6 < 112 > partial ones. Moreover, the dislocation cell structure as observed in the as-built condition evolved into sub-grains after the high-cycle fatigue loading, with the immensely dense dislocations at the sub-grain boundary.