A TEM investigation on indentation plastic zones in Ni3Al(Cr,B) single crystals

Abstract

Nanoindentation has been carried out on (111) single crystals of Ni3Al with the maximum applied load ranging from 800 to 8000 μN. The indentation plasticity fields are found to consist of two regions, namely, a core with a very high dislocation density and a surrounding region where the dislocation density is much lower. In the zone with lower dislocation density, individual dislocations were discernible, and three sets of dislocations, with Burgers vector of the 〈11̄0〉 type and parallel to the sample surface, were identified in all specimens. The dislocation segments were curvy at small loads but long and straight screw dislocations appeared at large loads. Measurements of the radii of the plasticity zones from TEM images indicate that the core and the surrounding low dislocation density zone did not evolve in a self-similar manner with increasing load. From the dislocation structure observed just after the pop-in in the load-displacement curve, the strain rate in the indent core during pop-in is estimated to be about 170 s-1. The nominal hardness calculated using the Oliver-Pharr scheme exhibits a sharp and negative dependence with respect to load, with the hardness dropping from ∼ 12 GPa at 800 μN to about 6 GPa at 8000 μN. The value of P/πc2, where P is the load and c the radius of the surrounding dislocation zone, is however fairly constant with respect to load, with a variation of only about 10% in the same load range. The yield stress Y calculated from the equation Y = 1.5P/πc2 is about 356±32 MPa. These results suggest that, while nanohardness can still be expected to depend critically on the yield stress, there is an additional functional relationship that causes the hardness to vary with the indent size in the submicron regime. © 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.