Nanostructuring of pure metals by severe plastic deformation at cryogenic temperatures

Abstract

In the present study the effect of severe plastic deformation by high-pressure torsion in liquid nitrogen on the structure of Cu, Ni and Nb and its thermal stability is analyzed. It is demonstrated that the room-temperature HPT of Cu is accompanied by its dynamic recrystallization. The latter can be suppressed if the deformation temperature is decreased to cryogenic, and the nanocrystalline structure can be obtained, but it degrades at room temperature due to the post-dynamic recrystallization. The HPT of Ni in liquid nitrogen results in nanocrystalline structure with average grain sizes of 80 nm and microhardness of 6200 MPa, this structure being stable at room temperature. In Nb subjected to low temperature HPT the average crystallite sizes are 75 nm, and the microhardness is 4800 MPa. The thermal stability of nanocrystalline structure in Ni and Nb, obtained by HPT in liquid nitrogen, is considerably lower than after the room temperature HPT. The grain growth starts in Ni at as low as 200°C and in Nb at 300°C. Thus, the HPT at cryogenic temperature enables to refine the structure and increase microhardness considerably, but the thermal stability of the as-obtained structures is quite low.