The high temperature decrease of the critical resolved shear stress in nickel-base superalloys

Nickel-base superalloys show a typical decrease in the critical resolved shear stress (CRSS) around 750–800°C. In situ tensile tests of the alloys NIMONIC 105 and NIMONIC PE 16 carried out in a high voltage transmission electron microscope revealed the onset of a new deformation mechanism at 780°C. Above 780°C the deformation is achieved by the viscous motion of pairs of identical γ-matrix dislocations with Burgers vector of type a/6〈112〉 (a: lattice constant). The dislocations are dragging behind long intrinsic stacking faults and thus establish microtwins. Along the trace of the moving dislocations the γ’-particles are sheared. A crystallography/diffusion-based model is presented which explains the (i) unusual pairing of a/6〈112〉 dislocations and (ii) the restriction of the viscous dislocation movement to high temperatures.