Effective Temperature of High Pressure Torsion in Zr-Nb Alloys

Severe plastic deformation by the high pressure torsion (HPT) leads to the phase transitions and strong grain refinement. The starting alpha Zr-phase in Zr alloyed by 2.5 and 8 mass% Nb transforms into beta + omega mixture. This beta + omega phase mixture can be found in the equilibrium phase diagram at higher (effective) temperature (T-eff = 620 degrees C for Zr-2.5 mass% Nb and T-eff = 550 degrees C for Zr-8 mass% Nb). The published papers on phase transitions during HPT are analysed and the values of effective temperature are estimated. Contrary to the increasing temperature, the increasing pressure slows down the diffusion and grain boundary migration. Therefore, the forced atomic movement during HPT produces the states equivalent to higher temperature, but not to the higher pressure.