Investigation into the Characteristics of White Layers Produced in a Nickel-Based Superalloy from Drilling Operations

The metallurgical nature/structure of white layers generated during metal cutting operations has for sometime been the object of scientific controversy. This research studies the structure of the white layer generated from nonproduction abusive drilling parameter in a nickel-based superalloy at macro and micro scales. This was achieved using (1) a Focus Ion Beam (FIB) to mill a sample for Transmission Electron Microscopy (TEM), (2) Scanning Electron Microscopy (SEM) and (3) Nano-indentation to evaluate material within and under this layer. In-depth analysis of this layer from non-standard drilling parameters in alloy RR1000 showed the layer is Face Centre Cubic (FCC) in structure and is the first to reveal it is polycrystalline with an average grain size of 50 nm compared to the bulk material of 22-63 μm (ASTM 8-5). Nano indentation testing has shown the white layer has a 45% higher hardness than the bulk material.