Effects of Pressure and Number of Turns on Microstructural Homogeneity Developed in High-Pressure Double Torsion

With electron backscatter diffraction and transmission electron microscopy, we study the rate of grain refinement and the uniformity in the evolution of microstructure in commercial purity Cu samples during high-pressure double torsion (HPDT). We aim to identify the processing conditions that would produce a microstructure that is both refined and uniform across the sample in grain size, texture, and intra-granular misorientation with minimal energy input. Two processing variables, pressure and number of turns, are probed. To provide a reference for HPDT, the investigation is also carried out using the standard high-pressure torsion (HPT) technique. For both processes, grain sizes decrease with the number of turns and applied pressure. Under pressure of 600 MPa and 4 torsional turns, HPDT provided a more homogeneous grain structure than HPT. Likewise, we also demonstrate that for the same processing condition, HPDT again produces the more homogeneous grain structure. It is found that a more homogeneous grain structure is achieved after doubling number of turns than doubling the pressure amount to 1.2 GPa. However, the rate of grain refinement substantially increases with doubling the pressure. Considering these results, the HPDT process, compared to HPT, takes better advantage of the role that high pressure plays in shear strain-induced grain refinement and homogenizing the microstructure. Last, analysis of the applied work finds that the least amount of work required for achieving fine and homogeneous microstructure occurs when the applied pressure is maximized and number of turns is minimized.