Papers by Author: Vincent Demers

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Authors: I. Khmelevskaya, Sergey Prokoshkin, Vladimir Brailovski, K.E. Inaekyan, Vincent Demers, Irina Gurtovaya, Andrey Korotitskiy, Sergey V. Dobatkin
Abstract: 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.
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Authors: K.E. Inaekyan, Sergey Prokoshkin, Vladimir Brailovski, I. Khmelevskaya, Vincent Demers, Sergey V. Dobatkin, E.V. Tatyanin, E. Bastarache
Abstract: 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.
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Authors: Vladimir Brailovski, Sergey Prokoshkin, E. Bastarash, Vincent Demers, K.E. Inaekyan, I. Khmelevskaya
Abstract: 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.
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