Investigation of micro-indentation characteristics of P/M nickel-base superalloy FGH96 using dislocation-power theory

The effects of indentation size and loading rate on micro-hardness of the P/m nickel-base superalloy FGH96 were investigated by the micro-indentation tests performed in the indenter load rang from 100 mN and 450 mN and the loading rate range from 1.94 mN/s to 19.37 mN/s at room temperature. The results show that the micro-hardness obviously decreased with the increase of indentation depth, while the influence of indentation depth on elastic modulus was slight. The average value of elastic modulus was 193.97 GPa. Moreover, the rate-dependence phenomenon was prominent during indentation tests as well. It was found that the micro-hardness increased with the increasing loading rate. By means of Nix and Gao model, the size effect during indentation tests was studied. The discipline between H-2 and 1/h exhibited a better linear relationship. Further, the density of statistically stored dislocations (SSDs) obtained from the model exhibited a decreasing tendency with the increase of loading rate. In contrast, the density of geometrically necessary dislocations (GNDs) showed an inverse relationship. According to the dynamic material model, the equivalent strain rates during indentation tests were obtained. It was found that the equivalent strain rate decreased with the increase of indentation depth. This phenomenon was considered to be related with the plastic power density (PPD), which represented the kinetic energy of GNDs. By means of Zener Hollomon parameter, the relationship between activation energy and kinetic energy was established. Furthermore, the dislocation-power theory was developed and applied to interpret the deformation mechanism and the influence of indentation size and loading rate during indentation tests. It should be pointed that the decreasing kinetic energy caused by the decrease of PPD was closely related to the geometry changes, and the decrease of equivalent strain rate would lead to the increase of activation energy which exhibited ascending deformation resistance. In this paper, the influences of indentation size on micro-hardness could be attributed to the variation of equivalent strain rate. (C) 2011 Elsevier B.V. All rights reserved.