Microstructure evolution in nanocrystalline NiTi alloy produced by HPT

A slightly Ni-rich nano-NiTi alloy, deformed by high-pressure torsion (HPT) was investigated. By HPT, almost complete amorphization of initial B2 NiTi is obtained. Crystallization and microstructure evolution during isothermal treatment were investigated by differential scanning calorimetry (DSC) and transmission electron microscopy.The DSC signals observed during continuous heating experiments indicate an unusually large separation between the crystallization and the growth stages. A detailed analysis of the evolution of the enthalpy release upon annealing reveals reproducibly non-monotonous trends with annealing time that cannot be explained solely by growth of crystalline volume fractions. The size of nanocrystals increases dramatically after annealing for 5 h. The effective activation enthalpies for stress relaxation (along with crystallization) and grain growth were estimated at 115 and 289 kJ/mol, respectively. The results are discussed with respect to the intricate interdependencies between synthesis and thermal processing pathways and the evolution of the nanoscale microstructure.

Research highlights
► The nanocrystallization of HPT deformed NiTi is studied during isothermal annealing at 563 K.
► A transient time exists which indicates a concurrent process to grain growth.
► The existence of reverse amorphization is highlighted.
► The effective activation energy related to stress relaxation (along with crystallization) and grain growth is 115 kJ/mol and 289 kJ/mol, respectively.