Effect of y' size on Strain Localization at Micro-Scale in Nickel-Base Superalloy RR1000

Polycrystalline nickel base superalloys used for turbine disc applications are being required to operate at higher thermal and mechanical stresses due to the efficiency gained from increasing turbine entry temperatures. The performance of these materials relies heavily upon the strengthening gained from the ordered phase Ni3Al (y’). In order for the mechanical properties of these materials to be optimised, it is key to improve our fundamental understanding of the effect of y’ size and its distribution on the deformation mechanisms and deformation characteristics. Fatigue resistance is an important requirement for these materials. In single crystal alloys, fatigue failure has been linked to deformation localization. Recent work [1] has suggested that the degree of localization is influenced by y’ size, which has important implications for the fatigue resistance of these alloys. The aim of this work is to quantify the level of strain localization, at the microstructural scale, in samples of polycrystalline disc alloy RR1000 with different y’ sizes.

Recent developments [2] have made it possible to map strain localisation with sub-micron resolution using digital image correlation (DIC) of images acquired with an electron microscope. DIC is a computational image analysis technique, which allows the calculation of in-plane strains at different stages during deformation from changes in a pattern deposited on the sample surface.

The results indicate that the fine y’ microstructure shows significant amounts of strain localization which is displayed by the presence of very defined slip bands caused by y’ shearing.  The coarse y’ microstructure however, displays far more homogeneous deformation with no apparent y’ shearing.  Electron backscatter diffraction was then used to determine the relationship between slip bands and crystallographic orientations.  Although the majority of slip bands are aligned with {111} slip traces, a significant amount are not.  Furthermore, the degree of strain localization does not seem to be related to the Schmid factor calculated by resolving the macroscopic applied stress along the likely slip direction.