I. Khmelevskaya

Functional Properties of Nanostructured Ti-50.0 at % Ni Alloys

Ti-50 at % Ni alloy wire is subjected to cold-rolling (true strain e=0.3-1.9) and post-deformation annealing (200–700°C range). For all levels of cold work, the maxima of recovery strain and stress are obtained after annealing in the 350–400°C range. For the moderately-to-high cold-worked material (e=0.3-0.88), this annealing leads to polygonization, while for the severely cold-worked one (e=1.9), to the material nanocrystallization (grains of 50–80 nm in size). Nanocrystallized alloy generates 30 % higher recovery stresses (up to 1450 MPa) and 10% higher completely recoverable strains (more than 8 %) as compared to the polygonized alloy, while having comparable mechanical properties in tension.

Studies of Severe Plastic Deformation Conditions for Amorphous and Nanocrystalline Structures Formation in Ti-Ni Based Alloys

Structure formation in TiNi-based shape memory alloys depending on deformation temperature (-196 °C to 400 °C) and pressure (4 to 8 GPa) under conditions of high-pressure torsion (HPT) was studied using TEM and X-ray diffraction methods. The tendency to form an amorphous structure depends on relative positions of the deformation temperature and Ms temperature. Isothermal martensitic transformation is observed in the Ti – 48.5 % Ni alloy as a result of 10-year keeping at RT after HPT. Increasing of pressure suppresses the tendency to form an amorphous structure. The upper deformation temperature limits for amorphous and nanocrystalline structures formation are determined. The thermomechanical conditions of the equal-channel angular pressing for obtaining actual nanocrystalline structure are recommended.

Substructure and Nanocrystalline Structure Effects in Thermomechanically Treated Ti-Ni Alloys

Substructure and structure formation as well as functional properties of thermomechanically treated Ti-Ni wire have been studied using differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and mechanical. The low- temperature themomechanical treatment (LTMT) was carried out by rolling at room temperature in a true strain range e = 0.3 to 1.9. It was shown that severe plastic deformation (e=1.9) of Ti-50.0at.%Ni alloy results in partial amorphization and formation of nanocrystalline austenite structure during post-deformation annealings up to 400 °C. As a result, the fully recoverable strain and recovery stress become much higher than the values reachable after traditional LTMT (e=0.3 to 0.88) with post-deformation annealing which creates a poligonized dislocation substructure.

Thermal Stability and Nanocrystallization of Amorphous Ti-Ni Alloys Prepared by Cold Rolling and Post-Deformation Annealing

The thermomechanical processing consisting in severe cold rolling (true strain 0.7–1.9) followed by a post-deformation annealing (200-700oC) is applied to Ti-50.0 and 50.7at%Ni alloys. The thermal stability of the amorphous phase as well as the influence of post-deformation annealing on the structure, substructure and temperature range of martensitic transformations are studied using TEM and DSC techniques. For a given level of cold work, the equiatomic alloy has a higher volume fraction of amorphous phase than the nickel-rich one. For both alloys, the higher the volume fraction of the amorphous phase, the higher the thermal stability. For a given post-deformation annealing temperature, the DSC martensitic transformation peaks from the material subjected to amorphization cold work are sharper and the hysteresis between the direct and reverse transformations is narrower than those for a material subjected to strain hardening cold work. This observation confirms the absence of the well-developed dislocation substructure in the severely deformed alloy subjected to nanocrystallization heat treatment, which is consistent with TEM results.

Functional Properties of Ti-Ni-Based Shape Memory Alloys

The main functional properties (FP) of Ti-Ni Shape Memory Alloys (SMA) are their critical temperatures of martensitic transformations, their maximum completely recoverable strain (er,1 max) and maximum recovery stress (sr max). Control of the Ti-Ni-based SMA FP develops by forming well-developed dislocation substructures or ultrafine-grained structures using various modes of thermomechanical treatment (TMT), including severe plastic deformation (SPD). The present work shows that TMT, including SPD, under conditions of high pressure torsion (HPT), equal-channel angular pressing (ECAP) or severe cold rolling followed by post-deformation annealing (PDA), which creates nanocrystalline or submicrocrystalline structures, is more beneficial from SMA FP point of view than does traditional TMT creating well-developed dislocation substructure. ECAP and low-temperature TMT by cold rolling followed by PDA allows formation of submicrocrystalline or nanocrystalline structures with grain size from 20 to 300 nm in bulk, and long-size samples of Ti-50.0; 50.6; 50.7%Ni and Ti-47%Ni-3%Fe alloys. The best combination of FP: sr max =1400 MPa and er,1 max=8%, is reached in Ti-Ni SMA after LTMT with e=1.9 followed by annealing at 400°C which results in nanocrystalline (grain size of 50 to 80 nm) structure formation. Application of ultrafine-grained SMA results in decrease in metal consumption for various medical implants and devices based on shape memory and superelastiсity effects.

Metal Forming Aspects of Cyclic Severe Plastic Deformation of Ti-Ni Shape Memory Alloys Using MaxStrain Device

The effect of severe plastic deformation using MaxStrain (MS) device which is a part of the Gleeble thermo-mechanical simulator of rolling and forging processes on the structure and functional properties of Ti–50.0 at.% Ni shape memory alloy has been studied. The use of the MS module allows performing SPD of the material under isothermal conditions with precise control of the deformation parameters. The deformation temperature was lowered from 370 to 330 °C. The accumulated true strain varied from e=4.6 to 9.5. Structure features were studied by the transmission electron microscopy. The maximum completely recoverable strain was determined by a thermomechanical method using a bending mode for strain inducing. A mixed submicrocrystalline and nanosubgrained structure with average grain/subgrain size below 100 nm was formed using SPD at 330 °C. A very high completely reсoverable strain (9.3%) was obtained against a reference treatment (2.5%).

Structure and Functional Properties of Ti-Ni-Based Shape Memory Alloy after Electroplastic Deformation

Application of electroplastic deformation (EPD) by rolling to bulk long-sized samples of Ti-50.7 at.%Ni alloy allows increasing of the deformation strain without macrofailure by 1.5 to 3 times in comparison to cold rolling without electrical current application. Structure formation and functional properties were studied after various EPD regimes: current density (84 to 168 А/mm2) and impulse duration (80 and 160 ms). When the stage of mixed nanocrystalline and amorphous structure formation is reached as a result of EPD, a post-deformation annealing at 400 °С leads to a nanocrystalline structure formation in austenite and highest recovery stress values generation by the martensite.