Annealing behaviour of nanocrystalline NiTi (50 at% Ni) alloy produced by high-pressure torsion

Reeti Singh (1) , Sergiy Divinski (1) , Harald Rösner (1) , Egor Prokofyev (2) , R. Z. Valiev (2) , Gerhard Wilde (1)

1 Institute of Materials Physics
2 Institute of Physics of Advanced Materials, Ufa

An equiatomic nanocrystalline NiTi alloy, deformed by high-pressure torsion (HPT) was investigated. The as-prepared bulk NiTi alloy consists of both amorphous and nanocrystalline phases. Crystallization and structural changes during annealing were investigated by differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopy. The DSC thermograms and X-ray analyses reveal stress relaxation and partial crystallization below 500 K while grain growth of the nanocrystals occurs predominantly after heating to temperatures above 573 K. Along with the amorphous phase crystallization, a continuous growth of pre-existing nanocrystals that are retained after HPT is observed. 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 temperature that cannot be explained solely with nucleation and growth of crystalline volume fractions. Instead, the results can be rationalized by assuming a reverse amorphization process occuring during annealing at 523 K. This behavior, that also causes a large variation of the nanocrystals size after annealing at higher temperatures, is discussed with respect to the nanoscale microstructural heterogeneity after initial deformation processing.

Mots clés: Physical Sciences

Domaines: Matière Condensée [cond-mat]

Reeti Singh, Sergiy Divinski, Harald Rösner, Egor Prokofyev, R. Z. Valiev, et al.. Annealing behaviour of nanocrystalline NiTi (50 at% Ni) alloy produced by high-pressure torsion. Philosophical Magazine, 2011, pp.1. ⟨10.1080/14786435.2011.566228⟩. ⟨hal-00685380⟩