Characterization of surface microstructure in pure copper after severe plastic deformation by sliding

Sergey Suslov, Purdue University

Abstract

This study concentrates on characterization of the surface microstructure evolution in pure copper after severe plastic deformation in the form of "wave" sliding. This deformation process is based on conventional machining with modifications in the deformation geometry which leads to a drastic change in the output. Instead of the formation of a severely deformed chip, all the deformation goes into the workpiece, forming a severely deformed region in the workpiece subsurface. The amount of strain imposed into the subsurface of the material depends on the tool rake angle and the strain rate depends on the tool speed. In the present work, this deformation method is used to study the evolution of surface microstructure with strain and strain rate, and also the development of subsurface microstructure with depth from the surface. Three rake angles of -60°, -70° and -80° and two speeds of 1 and 200 mm/s were used in "wave" sliding, and two rake angles of 0 and +50° and speed of 10 mm/s were used in machining (chip formation) to study the surface microstructure evolution. Surface and subsurface microstructures of the deformed specimens were characterized with a range of techniques including specimen preparation and imaging in the Focused Ion Beam/Scanning Electron Microscope (FIB/SEM), Transmission Electron Microscopy (TEM), Electron Backscatter Diffraction (EBSD) and Vickers microhardness. Microstructural characteristics addressed in this dissertation involve boundary spacing and misorientation of the dislocation structures formed after the deformation. Texture evolution was also addressed. It was shown that the material flow during "wave" sliding at -70° and -80° rake angle was laminar while at -60° was rotational; which resulted in subsurface homogeneous (∼1) at -70°, (∼0.5) at -80° degree and heterogeneous (∼2-3.5) at -60° effective strain distributions, respectively. It was also shown that simple shear was the main deformation mode in "wave" sliding. Deformation textures developed in machining and "wave" sliding were proven to be of simple shear type and similar those developed in torsion and ECAP. The microstructures and textures developed at the surface of a chip rake face and corresponding machined surface were similar reflecting that the same effective strain was imposed in both surfaces. It was observed that in "wave" sliding, the material strain-hardened to a larger depth with one pass at a more positive rake angle, and with each pass (1-8) at -70° rake angle. Subsurface microstructure reached steady state after four passes in -70° "wave" sliding. It was confirmed that high speed (strain rate) "wave" sliding at -80° rake angle did not result in the formation of deformation twins.

Degree

Ph.D.

Advisors

Trumble, Purdue University.

Subject Area

Engineering|Nanotechnology

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