Characterization of amorphous and nanocrystalline Ti-Ni-based shape memory alloys

Ti-50.0 and Ti-50.26 at.%Ni shape memory alloys are subjected to cold rolling (CR) with true strain varying from moderate (e = 0.3) to severe (e = 1.5-2). The Ti-50.0%Ni alloy was also subjected to high-pressure torsion (HPT) with a true strain of e = 6.16. A melt-spun fully amorphous ribbon of Ti-25 at.%Ni-25 at.%Cu alloy was used as a reference sample for DSC analysis. The study of the material structure and its thermal stability was performed using TEM and DSC. CR with true strains higher than 0.5 initiate grain refinement and austenite amorphization: the higher the cold work strain, the higher the grain refinement and the degree of amorphization. Parallel TEM/DSC analyses show that CR- and HPT-processed Ti-Ni alloys are strongly heterogeneous materials containing amorphous matrix with embedded nanocrystals. The volume fraction of the amorphous phase in such materials cannot be evaluated by DSC because exothermal peaks measured contain inseparable contributions from both crystallization and grain-growth phenomena. Evaluating this quantity therefore requires direct TEM observations in addition to the DSC. It is suggested that for a heterogeneous material, its apparent thermal stability depends on a trade-off between volume fractions of the amorphous phase and deformation-induced nanocrystals and activation energies for their respective transformations: for CR (e = 2), the Avrami exponent n = 1.25, which corresponds to annealing by grain-growth-dominated mechanism; for HPT (e = 6.16), n = 2.5, which corresponds to annealing by nucleation-and-growth mechanism. For fully amorphous melt-spun Ti-25 at.%Ni-25 at.%Cu alloy, the Avrami exponent reaches 4, which corresponds to the nucleation-dominated annealing. (C) 2008 Elsevier B.V. All rights reserved.