Nano-twinning enhanced room temperature fatigue crack growth in single crystalline CoCrFeMnNi high entropy alloy

Abstract

The equiatomic high entropy alloy CoCrFeMnNi is known for its unique strain hardening behavior which extends to cryogenic temperatures and gives remarkable monotonic toughness. Studies show that activation of twinning at such low temperatures is the key factor for the exceptional mechanical properties of this alloy. However, room temperature deformation has been assumed to be slip dominated. If deformation twinning develops during room temperature fatigue crack growth (FCG) it could impart beneficial properties and the current paper focuses on single crystals exhibiting twinning at increasing crack lengths. The results are demonstrated for <001> single crystals and compared to the <111> single crystals. The various activated slip/twin systems influencing FCG were identified using high resolution digital image correlation (DIC) and electron backscattered diffraction (EBSD). Threshold stress intensity value for crack initiation and the Paris slope were unambiguously established. Kmax for <001> and <111> samples extracted from precise displacement measurements turn out to be as high as 230 MPa-m0.5 and 195 MPa-m0.5 respectively, which far exceeds most materials. Concurrently, high resolution EBSD revealed extensive nano-twinning at the crack tip for the <001> case. EBSD results also elucidate slip-twin interactions and dislocation induced lattice rotation inside twinned volumes. In conjunction, TEM images from the crack tip confirm the presence of deformation twins and additionally reveal twin-twin interactions. Moreover, the local crack tip R-ratio (Kmin/Kmax) turns out to be much higher (>0.6) than the global R-ratio (σmin/σmax) due to the accumulation of large crack tip strains (>20%). Synchronously, the crack tip displacement profiles from DIC point to the absence of crack closure. Thus, extensive strain hardening of the material ahead of the crack tip without the loss of ductility due to activation of twinning results in the remarkable intrinsic damage tolerance for CoCrFeMnNi high entropy alloys.