Segregation-assisted plasticity in Ni-based superalloys

Correlative high resolution transmission electron microscopy and energydispersive X-ray spectroscopy are used to study deformation-induced planar faults in the single crystal superalloy MD2 crept at 800 ◦C and 650 MPa. Segregation of Cr and Co at microtwins, anti-phase boundaries (APB) and complex/superlattice extrinsic and intrinsic stacking faults (CESF/SESF and CISF/SISF) is confirmed and quantified. The extent of this is found to depend upon the fault type, being most pronounced for the APB. The CESF/SESF is studied in detail due to its role as a precursor of the microtwins causing the majority of plasticity under these conditions. Quantitative modelling is carried out to rationalize the findings; the experimental results are consistent with a greater predicted velocity for the lengthening of the CESF/SESF – compared with the other types of fault – and hence confirm its role in the diffusion-assisted plasticity needed for the microtwinning mechanism to be operativ