Materials Science Forum Vols. 738-739

Abstract: In this work, a method for the identification of the transformation surface of Shape Memory Alloys based on full field measurements is presented. An inverse method coupled with a gradient-based algorithm has been developed to determine the characteristic parameters of the transformation surface. The constitutive equations of the chosen model that capture the macroscopic behavior of Shape Memory Alloys are first presented. The material parameters, to be identified, that are characteristic of the tension-compression asymmetry of the alloy are detailed. The identification algorithm, based on full field measurements obtained by Digital Image Correlation (DIC) and numerical simulation by Finite Element Analysis are introduced. The identification algorithm is validated using a numerically generated strain field on a Meuwissen-type specimen.

Abstract: This paper presents a time-resolved investigation of tensile loading induced temperature profiles in pseudoelastic NiTi thin film specimens. A finite element model for coupled mechanical and time dependent thermal analysis is presented that accounts for the different effects of local generation of latent heat and heat transfer. For strain rates larger than 0.24/s, the maximum temperature increases from room temperature to above 50 °C. Subsequent stress relaxation after temperature compensation results in a temperature decrease from room temperature down to -20 °C. The observed evolution of temperature profiles is in qualitative agreement with infrared thermography experiments on NiTi films fabricated by magnetron sputtering.

Abstract: The phenomena of appearance of thermo-emf at non-stationary heating in equiatomic titanium nickelide was examined. The peculiarities of kinetics of display of thermokinetic emf, also during thermocycling were determined. The possibility of generation of thermo-emf in amorphous-crystalline ribbon of Ti₅₀Ni₂₅Cu₂₅ during the transition to crystalline state was determined.

Abstract: Influence of the combined action severe deformation and a pulse current on a microstructure and mechanical behavior of long-size rods and strips with thin section out of Ti50Ni50 and Ti49.3Ni50.7 alloys is presented. Dependence of electroplastic effect on phase composition of the initial alloys is demonstrated. It is established, that electropulse postdeformation processing promotes acceleration of a stress relaxation at stages of recovery, polygonization and recrystallization in alloys.

Abstract: 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

Abstract: A Ti-50.8at.%Ni wire produced using a co-drawing method and a commercial Ti-50.8at.%Ni wire were annealed between 450°C and 700°C. Grains with diameter less than 100nm were revealed by TEM for both wires before annealing treatment. However, the microstructural heterogeneity of the co-drawn wire is more obvious than that of the commercial wire. The transformation behavior and mechanical properties of both wires were found to be sensitive to the annealing temperature. Multi-stage martensitic transformation was observed in the co-drawn wire, compared with the one-stage A↔M transformation in the commercial wire after annealing at 600°C for 30min. Moreover, the ultimate tensile stress and plateau stress of the commercial wire were found to be higher than that of the co-drawn wire under the same annealing conditions. The differences of total elongation, plateau strain and pseudoelastic recoverable strain between the commercial wire and the co-drawn wire were also observed. The differences of the transformation behavior and mechanical properties between the commercial wire and the co-drawn wire are attributed to the microstructural difference between these two wires.

Abstract: Applications of the functional, smart materials SMA need a careful evaluation of the working conditions and fatigue life. Structural fatigue failure in NiTi usually comes from a surface defect inducing crack growth, and this means that fatigue has to be studied for concrete applications, with the correct samples, as the state of the material presents size effects. Testing machine experiments on Ni-rich pseudoelastic wires indicate that the main parameter controlling the fatigue life in the traction-traction experiments is the effective stress on the NiTi wire. Long wire life (in the million cycle regime) can be achieved under limited stresses (under 170 MPa) in traction-traction tests. Also, experiments have been done on thermal actuation of NiTi wire under traction at constant load. Long actuator life (larger than 400000 cycles) can be achieved at low stresses (around 100 MPa), coherently with the results from the mechanical cycling.

Abstract: The influence of increased dislocation density; dispersity of Ni-rich (Ti₃Ni₄ and Ti₂Ni₃) particles and volume fraction of Ti-rich (Ti₂Ni) particles on the low-cycle (high amplitude) and high-cycle (low amplitude) fatigue resistance of nitinol has been considered in this paper. It was revealed that the fatigue resistance of nitinol in low-cycle conditions may be improved by increasing the part of deformation which is realized by martensitic mechanism. This part may be estimated by measuring ε_cr, which can reflect the influence of the structure parameter both on σ_M and σ_slip. It was found that in high-cycle fatigue conditions the substructure of nitinol predominantly determine its fatigue resistance, which is being the better in samples that had higher dislocation density or high dispersity of Ni-rich particles (up to 30 nm).

Abstract: Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase and chemical compositions and structure of Ti₂NiCu alloys. The alloys of the quasi-binary section TiNi–TiCu, which exhibit in the initial as-cast state thermoelastic martensitic transformations B2↔B19 and related shape memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti₂NiCu alloys was varied with respect to titanium and nickel within x ≤ ±1 at. %. The mechanical properties of the alloys have been measured in the initial state and after subsequent heat treatment. The kinetics of the crystallization from the amorphous state, devitrification processes and the forward and reverse thermoelastic martensitic transformations were investigated. Their characteristic temperatures have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition has been constructed.

Abstract: The review on effect of a high strain rate on the properties of TiNi-shape memory alloys is presented. The study of thermo-mechanical and functional properties of SMA after high strain rate loading was carried out. The object of study was an equiatomic TiNi shape memory alloy. The samples were tensioned at a strain rate of about 10³ s^-1 at various temperatures in martensitic, austenitic, and two-phase state, using the Split Hopkinson Pressure Bar technique. Two-way shape memory effects were investigated. Two-way shape memory after high strain rate loading was less than after the quasi-static one for all cases, except for straining in martensitic state.