Microstructural insights into creep of Ni-based alloy 617 at 700° C provided by electron microscopy and modelling

Abstract

In this work, microstructural changes during creep of Ni-based alloy 617 at 700 °C and 165 MPa have been investigated by electron microscopy, and complementarily modelled. Precipitate types, sizes and chemistry were determined by transmission- (TEM) and scanning electron microscopy (SEM). Apart from γ’ particles, MX and carbides, coarse μ-phase was found. Grain size, frequency of twins, deformation patterns and geometrically necessary dislocations were characterized by electron backscatter diffraction (EBSD). Based on measurements and literature data, creep behavior and a time-to-rupture (TTR) diagram of A617 have been numerically simulated at 700 °C in a range of 165 to 215 MPa with a new physical model. Our new creep model achieved excellent agreement with measured data and literature in terms of predicted creep life, times to 1% strain, minimum creep rate and microstructural evolution. We also succeeded in considering the varying ductility of the material in a novel damage law by implementing the reduction of area from fractured creep samples. Diffusion creep (coble creep) is considered in addition to dislocation creep in the model. The impact of diffusion creep is mostly visible at low stresses, leading to significant improvements within the TTR diagram.