V. Brailovski, V. Demers, A. Korotitskiy, Ecole de technologie superieure, Montreal, QC, Canada; S. Prokoshkin, Moscow Institute of Steel and Alloys, Moscow, Russia; K. Inaekyan, I. Khmelevskaya, Moscow State Institute of Steels and Alloys, Moscow, Russia; S. Dobatkin, Baikov Institute of Metallurgy and Material Science of RAS, Moscow, Russia; A. Glezer, G.V. Kurdyumov Institute of Physical Metallurgy, Moscow, Russia; E. Tatyanin, Institute for High Pressure Physics of RAS, Troitsk, Russia
Ti-50.0, 50.26 and 50.7at%Ni shape memory alloys are subjected to cold-rolling with deformation varying from moderate (true strain e=0.30) to severe (e=1.5-2.1). Post-deformation annealing of the cold-worked material allows obtaining of the polygonized dislocation substructure (after cold work of moderate intensity) or the nanocrystalline structure (after severe cold work). The material characterization is performed using TEM, DSC, microhardness and tensile testing as well as special techniques for the recovery stress and recoverable strain assessment.
Cold-rolling with true strain higher then 0.5 initiates the austenite amorphization. The fraction of the amorphous phase as a function of the cold work intensity is evaluated using the crystallization enthalpy measured by DSC. The crystallization enthalpy obtained with the same alloys but processed by High Pressure Torsion (HPT) technique is taken as a reference value. The DSC results are validated by direct TEM observations. The problem of the storage stability of the amorphous structure obtained by cold-working as well as of the nanocrystalline structure obtained after annealing is equally studied. For Ti-50.26at%Ni alloy, no significant changes in crystallization enthalpy and microhardness values are observed after 7-month storage at room temperature.
It is shown that binary Ti-Ni alloys with grain size of about 50-100 nm processed by amorphizing cold-work and nanocrystallizing annealing possess higher combination of functional properties as compared to the same alloys with conventional polygonized dislocation substructure: 6% of the maximum completely recoverable strain in tension (as compared to 5%) and 1420 MPa of the maximum recovery stress (as compared to 900 MPa). The former value is the highest recovery stress value ever reported for binary Ti-Ni alloys.
Summary: It is shown that binary Ti-Ni alloys with grain size of about 50-100 nm processed by amorphizing cold-work and nanocrystallizing annealing possess higher combination of functional properties as compared to the same alloys with conventional polygonized dislocation substructure: 6% of the maximum completely recoverable strain in tension (as compared to 5%) and 1420 MPa of the maximum recovery stress (as compared to 900 MPa).
