Value Of Two-way Shape Memory Effect Research Articles

Comparative Study of Shape Memory Effects in Ni-Rich Ti–Ni Alloy After Training in Various Phase States

Six various training modes were used for a systematic study of the effect of the initial phase state when training procedure on shape memory effect (SME) and two-way SME (TWSME) in nanostructured Ti–50.7 at% Ni alloy. Two types of the B2 austenite structure were chosen: (1) the mixed structure consisting of nanosubgrained and nanograined structures (cold drawing with a true strain of e = 0.6 followed by aging 430 °C, 10 h,) and (2) recrystallized structure (annealing at 700 °C, 20 min followed by the same aging) with the average grain size of 10 μm. The SME and TWSME training procedure was performed in bending under load. Nanostructured material manifests the maximum recovery strain 14–14.7% as a result of using training modes involving R → B19′ transformation under load. The maximum TWSME value of 3.2–3.5% was realized in the material with both structure types.

Two-Way Shape Memory Effect in Rapidly-Quenched High-Copper TiNiCu Alloys Deformed in the Martensitic State

The effect of martensitic deformation on the two-way shape-memory effect (SME) of quasi-binary high-copper TiNi–TiCu alloys (with 25–40 at % Cu) is studied. The alloys are prepared by rapid quenching from the melt in an amorphous state in the form of ribbons 30–50 μm in thickness. The alloys crystallize during isothermal annealing, the time of which is varied from 100 to 300 s, and during a single current pulse 10 ms in duration. The relatively low bending strains (1.5–3%) are shown to cause two-way SME in TiNiCu alloys; as the strain of martensite increases, the reversible strain in the martensitic transformation range increases markedly. An increase in the copper content is found to cause a substantial decrease in the value of two-way SME, which is related to the formation of the brittle TiCu phase. At the same time, a decrease in the heat treatment time causes an increase in the reversible strain, which reaches the maximum value (2.35%) for the alloy with 25 at % Cu subjected to high-rate electric pulse treatment.

Degradation of two-way shape memory effect due to the relaxation of internal oriented stress in NiTi alloy on holding at 640÷700 K

This work aims to study the two-way shape memory effect degradation due to stress relaxation during holding pre-deformed Ni-50.0 at.% Ti alloy at temperatures from a range of 640–700 K. To initiate the two-way shape memory effect, samples were subjected to 12% preliminary isothermal deformation by torsion in the martensitic state (at room temperature). The influence of holding at 640 or 700 K for 0–60 min on the recoverable strain variation on cooling and heating of the pre-deformed samples was studied. The main degradation of the two-way shape memory effect was found after holding the sample up to 60 min; however, full degradation was not observed, even after holding at 700 K for 6 h. A decrease in the two-way shape memory effect value was believed to be caused by relaxation in the oriented internal stress and it was assumed that this process might be described by a first-order differential equation. This equation was used to simulate the influence of holding duration on the recoverable strain variation and a good correlation between the simulated and experimental data was obtained. The activation energy of the internal stress relaxation during holding at 640 or 700 K was calculated and found to be 0.84 ± 0.01 eV, which was close to the energy of vacancy migration.

Influence of maximum thermal cycle temperature on the two-way shape memory effect in TiNi Alloy

The influence of the maximum thermal cycle temperature on the value and temperatures of the two-way shape memory effect (TWSME) was studied in Ti50Ni50 alloy subjected to a preliminary deformation by two ways: through isothermal deformation up to 12% in the martensite state, or via cooling and heating under a constant stress of 80 MPa. Additionally, the samples were subjected to one or 10 thermal cycles within the temperature range of 440–300 K to change the dislocation structure formed during preliminary deformation. It was found that values of the TWSME and martensitic transformation temperatures varied in three temperature ranges on an increase in maximum thermal cycle temperature. In the first temperature range from 440–490 K, a variation in TWSME parameters was observed in the samples that were not subjected to a preliminary 10 thermal cycles. In the second temperature range from 490–640 K, a redistribution of the dislocations resulted in a decrease in the value of TWSME at a low rate, and hardly increased the Ms temperature. In the third temperature range from 640–700 K, a recrystallization led to a significant decrease in the value of TWSME and a strong increase in Ms temperature.

Influence of crystalline phase volume fraction on the two-way shape memory effect in amorphous–crystalline Ti40.7Hf9.5Ni44.8Cu5 alloy

The two-way shape memory effect (TWSME) has been studied in an amorphous–crystalline thin ribbon of Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy with different volume fractions of the crystalline phase. It is found that an increase in the volume fraction of the crystalline phase results in a non-monotonic variation in the value of the TWSME. A maximum strain of 1.1% is observed in the thin ribbon with a 50% crystalline phase. It is found that the TWSME in amorphous–crystalline Ti40.7Hf9.5Ni44.8Cu5 alloy is due to two mechanisms: the formation of high internal stresses on the amorphous/crystalline interfaces due to incompatible strain in amorphous and crystalline phases and the stress caused by defects induced in the crystalline phase during deformation. If the volume fraction of the crystalline phase is 50% or less, then the main contribution to the total TWSME is given by the formation of the internal stresses on the amorphous/crystalline interfaces. In this case, the TWSME value depends on the density of the amorphous/crystalline interfaces, plastic strain and volume fraction of the alloy undergoing a phase transition. If the volume fraction of the crystalline phase is higher than 50%, then the TWSME is due to the formation of the internal stresses on the defects in the crystalline phase and the TWSME value depends on the plastic strain in the crystalline phase and the volume fraction of the crystalline phase.

Nanostructured Titanium Nickelide: Realization of abnormally high recovery strain

The paper focuses on the study of the shape memory effect (SME) and two-way SME (TWSME) in nanostructured Ti-50.7 at.%Ni alloy. Two different types of structure were studied: nano-subgrained structure (annealing after the moderate deformation with true strain e = 0.6) and nanocrystalline structure (annealing after the severe plastic deformation with true strain e = 1.55). A homogenizing annealing at 700°C for 20 minutes served as a reference heat treatment (recrystallized structure of austenite). The SME training procedure was carried out in bending under load using eight various isothermal and non-isothermal modes covering all phase states and their combinations. The maximum recovery strain (εr = 14.7%) in Ti-50.7%Ni alloy is provided by the nano-subgrained structure with the grain size ≤ 5 pm obtained as a result of 10 h-aging after deformation with true strain e = 0.6, loading in the R-phase state with subsequent cooling in the loaded condition through R→B19' transformation under loading strain εt = 15,7 %. The maximum TWSME value εtw = 3.5% is provided by the loading in the R-phase state and further unloading.

Two-way shape memory effect of polycrystalline Ni–Mn–Ga–Gd high-temperature shape memory alloys

The two-way shape memory effect (TWSME) of polycrystalline Ni54Mn25Ga21−xGdx alloys is investigated. The largest TWSME value (2.9%) without training is observed in Ni54Mn25Ga20.7Gd0.3 alloy due to a small amount of Gd(Ni,Mn)4Ga phase formation, and this value increases to 3.5% after thermomechanical training. The TWSME rapidly decreases over the first few thermal cycles, and then slowly reaches a stable value as the number of cycles increases further. Finally, a 2.5% TWSME of as-trained Ni54Mn25Ga20.7Gd0.3 alloy is obtained after 50 thermal cycles.

Study of Surface State Influence on Functional Poperties of Ti-Ni Alloys

Binary TiNi alloys containing 50.0 at.% Ni and 50.7 at.% Ni were tested under bending. Shape memory effect (SME) and two-way SME (TWSME) parameters were measured. Low-temperature thermomechanical treatment (LTMT) by drawing with true strain e ≈ 0.6 and post-deformation annealing (PDA) was performed in the temperature range 300600С for 0.3  1 hr permitted obtaining different types of initial austenite structure. The wires with 0.3 and 0.45 mm in diameter were studied. The curves of element distribution through the surface layer were graphed. The SME training procedure was carried out in bending with the constrained strain 0.7 and 8 %. Thickness, structure and element composition of the oxide film strongly depend on PDA regimes. In all cases titanium quantity in the boundary layer correlates with the oxygen quantity, diffused from the surface. Evidently, it is caused by their chemical reaction and oxide formation. Nickel atoms remain in uncombined state, and their migration is directed from the surface deep into the sample. LTMT permits decreasing the film thickness more than by 10 times. Surface state strongly affects all studied parameters: characteristic temperatures, recovery strain and TWSME value. Increasing of wire diameter from 0.3 to 0.45 mm leads to significant changes of studied parameters. Forecasting of said influence is difficult because of its ambiguity. Therefore, the oxide film must be eliminated by etching in order to obtain the reliable results