High-Ti inducing local Eta-phase transformation and creep-twinning in CoNi-based superalloys

Precipitate shearing mechanisms during compressive creep of L12-containing CoNi-base alloys with different Ti/Al ratio have been investigated in this work. Interrupted creep tests were conducted at 950 degree under air with constant load stress of 241 MPa. It was found that the creep resistance increases with Ti/Al ratio rising in CoNi-based alloys. In addition, we firstly found that the type of planar defects on (111) planes during precipitate shearing change from antiphase boundary (APB) towards superlattice extrinsic stacking fault (SESF) with Ti content increasing - shearing of the gamma' phase is mainly dominated by APB in Ti-free and low-Ti alloys but dominated by SESF in high-Ti alloys. By employing density functional theory (DFT), the APB energy was found be lower than complex stacking fault (CSF) energy in Ti-free and low-Ti alloys but this situation becomes opposite in high-Ti containing alloys. Additionally, the SESF energy is lower than SISF energy in L12-(Co,Ni)3Ti structure strongly supporting SESFs formation in high-Ti alloys. By energy dispersive X-ray spectroscopy analysis under the scanning transmission electron microscope mode, the observed chemical segregation enables APB becoming disordered gamma phase structure in Ti-free and low-Ti alloys and enables SESF becoming local ordered Eta phase structure in high-Ti alloys. However, the microtwins were found as well in high-Ti alloys which usually contributes higher creep strain than other planar defects, e.g. SESF and APB. This finding provides a new insight how to use Ti content reasonably in superalloy design.