Characterization of creep deformation of Ni-Cr solid solution alloy Nimonic 75

Creep testing of Nimonic alloy 75, a simple Ni-Cr solid solution alloy was carried out over the temperature range 873-950 K (600-677 degrees C) at 160 MPa. The activation energy for creep was found to vary over this temperature range, and suggests that two deformation processes occur simultaneously, dislocation climb within the grains along with grain boundary (Coble) diffusional creep. Under these conditions the material also exhibits a sigmoidal rather than normal primary creep transient. Despite this, the characteristics of Class I solid solution creep are not observed. The sigmoidal primary transient is explained in terms of a transition from dislocation mobility-controlled (initial stage of sigmoidal primary transient) to substructure-controlled deformation (second stage of sigmoidal primary transient). A reduction in sample size has been found to reduce the early stage creep resistance of this material. This size effect is analyzed in terms of the sample volume to surface area ratio (V/S) and agrees well with size effect results reported in the literature.