Grain refinement and enhanced diffusion of W-Cu gradient material processed by high pressure torsion with floating cavity

High-pressure torsion (HPT)processing with floating cavity was carried out under the applied pressure of 1.5 GPa with the revolution from 5 to 20 turns at the temperature of 300 degrees C, and the W-Cu gradient material with noble bonding interface and significant grain refinement was successfully obtained. The microstructure evolution and atomic diffusion around the bonding interface were analyzed by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) in a field emission scanning electron microscope (SEM). The mechanism of large shear strain on the microstructure evolution and diffusion ability of tungsten and copper at the interface was investigated. The results show that HPT processing with floating cavity can effectively refine microstructure of tungsten and copper to submicron. After 5 turns of HPT deformation, the grain size of copper was significantly refined to 0.59 mu m but experiences slight increases to 0.8 mu m with the additional revolution number to 20 turns, and the ultrafine equiaxed grains with relatively straight grain boundaries and few dislocations within grains were formed under the continuous dynamic recrystallization mechanism. However, the microstructure of tungsten was continuously refined within 20 turns of HPT processing due to the accumulation shear deformation, and a lamellar microstructure was formed along the W-Cu bonding interface with a gradient distribution in the width of banded grains from 0.06 mu m at the interface to 0.33 mu m towards the tungsten matrix. In addition, with the increase of torsion strain, the diffusion lengths of tungsten and copper gradually increasing to 1.6 mu m and 6.2 mu m, the interdiffusion coefficient of tungsten in copper remained basically constant, while the value of copper in tungsten increases continuously. The high density of defects and ultrafine grains with high angle boundaries introduced by large shear deformation of 20 turns of HPT processing play roles in the rapid diffusion paths to accelerate the atom diffusion and the interdiffusion coefficient of tungsten and copper is enhanced by 0.48- 1.79x 108 times compared to the lattice diffusion.