An Insight into Machining of Thermally Stable Bulk Nanocrystalline Metals

Recent advances have enabled the production of nano-grained metallic alloys with a thermally stable microstructure. Consequently, the production of bulk samples and subsequent machining and testing of standardized test specimens is now a real possibility. Therefore, for the first time, the authors report on the in-depth characterization and machinability of bulk nanocrystalline materials. In particular, this study addresses the feasibility of machining and to what extent, if any, the microstructure is altered due to the high stresses and temperature incurred during machining. Toward that goal, a series of copper-tantalum nanocrystalline threaded cylindrical tensile samples are machined from extruded rods. Advanced characterization techniques, such as transmission electron microscopy, are employed which indicated that the grain size of the Cu-Ta alloy was further reduced by approximately one-third. This reduction in grain size is quite noteworthy given an estimated total strain of 260% and moderate temperature increase resulting from the machining operation. This unexpected grain refinement is attributed to tantalum-based nanoclusters dispersed through the matrix which limit grain growth in the initial microstructure during machining. Overall, the authors observe a continuous chip formation and machinability of bulk nanocrystalline materials, which stems from stable nano-grains and having a near elastically perfectly plastic material behavior.