Interpretation of the creep behavior of nanocrystalline Ni in terms of dislocation accommodated boundary sliding

Abstract

Creep behavior and deformation-induced grain growth in electrodeposited (ED) nanocrystalline (nc) Ni with a grain size of about 20 nm were studied over more than five orders of magnitude of strain rate (10−9 s−1 to 2×10−4 s−1) at 393 K (0.23 Tm, where Tmis the melting point). In addition, the activation energy for the creep in ED nc-Ni was determined by using the temperature change procedure. The results show that the creep behavior of the material is characterized by (a) a stress exponent that increases continuously from about 4.5 to about 30 with increasing applied stress; (b) an apparent activation energy for creep in the range of 126 to 141 kJ/mol; (c) an activation volume of about 20 b3 where b is the Burgers vector; and (d) a grain size that upon loading, grows, attaining a constant value once steady-state creep is approached. The mechanical characteristics cannot be accounted for by current deformation processes. Analysis of the creep data along with consideration of available information leads to the suggestion that the creep behavior of nc-Ni arises from a deformation process that is based on the concept of dislocation-accommodated boundary sliding. By quantitatively developing this concept, a rate-controlling deformation process is formulated. It is shown that the predictions of this process, are in good agreement with experimental results and trends.