Anatoliy Klopotov; Victor Gunther; Ekaterina Marchenko; Gulsharat Baigonakova; Timofey Chekalkin; Ji-soon Kim; Ji-hoon Kang
Abstract
Within the wide family of shape memory alloys (SMAs), TiNi-based alloys are characterized by unique characteristics, with good workability in the martensite phase and good resistance to corrosion and fatigue. In the nearest future, TiNi-based SMAs are expected the second birth to begin regarding their ...
Read More
Within the wide family of shape memory alloys (SMAs), TiNi-based alloys are characterized by unique characteristics, with good workability in the martensite phase and good resistance to corrosion and fatigue. In the nearest future, TiNi-based SMAs are expected the second birth to begin regarding their practical application, especially in creating a new material generation showing enhanced characteristics for clinical goals. Such a kind of expectations is naturally supposed to make a search among alloying elements for TiNi-based SMAs, as well as studies of adjacent effects in order to improve material properties. The objective of the work is to investigate the effect of heat treatment on the structure and properties of the quaternary Ti50Ni47.7Mo0.3V2 SMA, as potentially promising for medical devices. Specimens were prepared and annealed at 723, 923, 1123 K for 1 h. It was found that the studied alloy was in a multiphase state: TiNi-based intermetallic in three crystallographic modifications (austenite B2-phase and martensitic R- and B19¢structures) and secondary Ti2Ni(V) phase. The increase of the annealing temperature doesn't affect the martensite transformation (MT) sequence B2«R«B19′, but leads to a growth in lattice parameter of the parent phase. The most remarkable effect on the studied alloy was at 723 K. Volume fraction of Ti2Ni(V) precipitates in the structure was also maximum. It owes their presence to the shift of the MT points toward the lower temperature range. The temperature vs resistivity r(T) curves show a characteristic shape, which is typical for TiNi-based SMAs with a two-step nature of the B2«R«B19′ MT.
Victor E. Gunther; Timofey L. Chekalkin; Ji-Soon Kim; Valentina N. Hodorenko
Abstract
Thermoelastic martensitic transformations play a fundamental role in the shape memory effect and related phenomena. Owing to their unique crystallographic and thermomechanical behaviour, martensitic transformations have generated considerable research in the areas of crystallography, thermodynamics and ...
Read More
Thermoelastic martensitic transformations play a fundamental role in the shape memory effect and related phenomena. Owing to their unique crystallographic and thermomechanical behaviour, martensitic transformations have generated considerable research in the areas of crystallography, thermodynamics and mechanical behaviour. In the area of thermodynamics a theoretical approach is now added which provides the basis for thermoelastic behaviour through consideration of the Gibbs energy change. In this paper, the interrelation of internal elastic stresses and martensite shear stresses in phase transitions has been defined. A thermoelastic stress equilibrium equation for a wide range of martensitic transformation temperatures has been presented. On the basis of the calculations made, an estimation of dislocation defects formation energy for the TiNi-based alloy has been made. For TiNi-based composition made of TN-10 brand alloy, commercially produced for medical goals, the energy of vacancy formation is about 0.06 kcal/mol. The study and calculations are shown to make clear, using a new approach, the considering of phase transitions in terms of external and internal stresses.
Victor E. Gunther; Timofey L. Chekalkin; Ji-Soon Kim; Valentina N. Hodorenko
Abstract
The brief overview of published articles on Nitinol studies reflects, in fact, a little misunderstanding of a thermodynamic importance since there is no whole concept or paradigm considering the martensite transition and related effects in terms of thermodynamic arguments. TiNi-based alloys are basically ...
Read More
The brief overview of published articles on Nitinol studies reflects, in fact, a little misunderstanding of a thermodynamic importance since there is no whole concept or paradigm considering the martensite transition and related effects in terms of thermodynamic arguments. TiNi-based alloys are basically functional materials. They’re more important for what they do than for what they are. In order to make it easier to understand complicated behavior of TiNi-based alloys under general and special conditions, a new approach is introduced. This work was aimed at studying the hysteretic behavior of TiNi-based alloys. Mechanism of hysteretic phenomenon in TiNi-based alloys showing shape memory effect, superelasticity and ferroelasticity has been considered. Hysteretic behavior in repeatable shape memory effect was thermodynamically analyzed with the aid of Helmholtz potential formalism. Correlation of deformation, temperature and latent heat of phase transition under the constant load was established. Maximum hysteresis width achieves when the product ??? during phase transition tends to ?H, and when the product ??? tends to zero the value of ?T approximates the difference (As – Mf) or (Af – Ms).
