Shape memory effect of NiTi alloy processed by equal-channel angular pressing followed by post deformation annealing

Processing by Equal-Channel Angular Pressing (ECAP) is generally considered superior to most other SPD techniques because it uses relatively large bulk samples. However, due to their low deformability it has proven almost impossible to successfully process NiTi alloys by ECAP at room temperature and therefore the processing is conducted at elevated temperatures. Recently, a new billet design was introduced and it was used to achieve the successful processing of NiTi shape memory alloys by ECAP. In this procedure, a NiTi alloy was inserted as a core within an Fe sheath to give a core-sheath billet. In this research, a NiTi was processed by one pass ECAP with this new billet design at room temperature. The structural evolution during annealing was investigated by X-ray diffraction (XRD) and microhardness measurements. Post deformation annealing (PDA) was carried out at 400°C for 5 to 300 min and the results indicate that the shape memory effect improves by PDA after ECA

Tailoring functional properties of a FeMnSi shape memory alloy through thermo-mechanical processing

The influence of thermo-mechanical processing on the microstructure and functional properties of a Fe–17Mn–5Si–10Cr–4Ni-1(V–C) (wt%) shape memory alloy was systematically investigated. The as-received material was subjected to 25 % cold rolling followed by a recrystallization at 925 °C and single or double aging treatments. Transmission electron microscopy revealed the formation of the ε-martensite and annealing twin boundaries and Shoji-Nishiyama orientation relationships of ε-martensite and γ-austenite in double aged specimen. Cyclic tensile testing demonstrated that the recrystallized and double aged alloy exhibited excellent pseudoelasticity. In the incremental strain test, the alloy achieved the highest peak stress and pseudoelasticity at each cycle. In the constant stresses test, the alloy accumulated a minimal residual strain of only 0.12 % over 50 cycles. This stability was attributed to a strong precipitation strengthening and the interactions between the martensite and the refined microstructural features. In addition, the recrystallized and double aged sample resulted in the greatest recovery stress of 450 MPa upon heating after pre-straining, because of its high yield strength suppressing new martensite formation during cooling process. The results of high-resolution transmission electron microscopy identified a non-Shoji-Nishiyama orientation relationship between the stress-induced ε-martensite after the stress recovery test and γ-austenite matrix, inducing additional irrecoverable strain and raising the recovery stress. Overall, the study can demonstrate that the tailored thermo-mechanical processing enables optimizing the functional performance of FeMnSi alloys

Effect of low-temperature precipitates on microstructure and pseudoelasticity of an Fe–Mn–Si-based shape memory alloy

Fe–Mn–Si-based shape memory alloys (Fe-SMAs) have attracted much research attention due to their potential applications for vibration mitigation, energy dissipation, and re-centering in the construction sector. Because of the crucial impact of precipitation on the pseudoelasticity (PE) behavior of Fe-SMAs, the equilibrium phase diagram of an Fe–17Mn–5Si–10Cr–4Ni–1(V-C) (wt%) SMA was used in this study to identify a low-temperature precipitate and study its effect on the microstructure and PE of the alloy after a low-temperature aging process. Transmission electron microscopy (TEM) studies revealed that aging at 485 °C for 6 h after aging at 750 °C for 6 h led to the precipitation of fresh, parallelogram-shaped, (Cr–V–C)-rich precipitates along with elliptical-shaped, V-rich precipitates in the austenite grains of the recrystallized samples. Numerous parallel stacking faults (SFs) were formed due to the presence of the precipitates within the austenite grains. It is postulated that such an arrangement of SFs can further improve the PE by reducing the activation energy for the nucleation of ɛ-martensite laths and inhibiting them from colliding with each other and consequent formation of α'-martensite, resulting in a residual strain reduction to 2.7% after 4.0% tensile straining

Diffusion evaluation of Cu in NiTi Bi-layer thin film interface

NiTi alloys are shape memory alloys (SMAs) which combine two very unique properties, shape memory effect and super-elasticity. These properties depend strongly on the composition and structure. In this research volume diffusion of Cu in bulk Ni50.8Ti49.2 alloy with B2 ordered structure has been measured and diffusion phenomenon on a bi-layer Ni-Ti thin film was also studied. The penetration profiles were determined by two techniques: secondary ion mass spectroscopy (SIMS) and energy dispersive X-ray analysis (EDS). Cu diffusivity in NiTi-Ni rich bulk was found to follow linear Arrhenius dependencies with the pre-exponential factor of 5.8 ± 1 x 10^-14 m2/s and the activation energy of 40 ± 4 kJ/mol. Anti-site defect diffusion mechanism is suggested to provide diffusion of copper in NiTi alloy. The diffusion coefficient of Cu in Ni-Ti Ni-rich was used to estimate the gradient composition in the bi-layer thin film (Ni-rich/NiTiCu) after annealing. The bi-layer thin film was deposited on Si (1 1 1) using DC magnetron sputtering and annealed at 773 K for 1 h. The crystallization temperature and microstructure of the thin film were characterized using differential scanning calorimeter (DSC) and X-ray diffractometer (GIXRD). The results depicted that estimation of the concentration gradient in the bi-layer thin film using the diffusion equation obtained from the NiTi bulk samples shows good agreement with the measured concentration gradient in the bi-layer

Evaluation of structure and mechanical properties of Ni-rich NiTi/Kapton composite film

NiTi thin films are usually sputtered on silicon wafers by magnetron sputtering. But the systems composed of thin film on flexible polymeric substrate are used in many applications such as micro electromechanical systems (MEMS). Investigation on mechanical properties of thin films has attracted much attention due to their widespread applications. In this paper, the mechanical properties of 1 mu m-thick crystallized Ni-49.2 at%Ti thin film alloy deposited by DC magnetron sputtering on Kapton substrate are investigated by using tensile test. The as-deposited thin films are in amorphous state, then for crystallization, the thin film was annealed at 450 degrees C for 30 min. Formation of the austenite phase after annealing was confirmed by X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). The surface morphology of as deposited and crystallized thin films were examined by scanning electron microscopy (SEM). Stress-strain curves of the NiTi alloy thin film were obtained by subtracting of the stress-strain Kapton curves from the corresponding curves of the NiTi/Kapton composite. The XRD results revealed that the NiTi thin film deposited on the Kapton is austenitic and presents super-elastic effect at room temperature. This pseudo elastic effect leads to more recoverable strain in NiTi/Kapton composite film compared with Kapton foils on loading/unloading test. Furthermore, it was concluded that nanostructure of the NiTi thin film is responsible for remarkable improvement of ultimate tensile strength (1.4 GPa) at a strain of 30% compared with the bulk material. (C) 2016 Elsevier B.V. All rights reserved

Effect of a minor titanium addition on the superplastic properties of a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

A CoCrFeNiMn high-entropy alloy (HEA) with an addition of 2 at.% Ti was processed by high-pressure torsion to produce a grain size of ~30 nm and then tested in tension at elevated temperatures from 873 to 1073 K using strain rates from 1.0 × 10-3 to 1.0 × 10-1 s-1. The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of 830% at a temperature of 973 K. It is shown that the Ti addition contributes to the formation of precipitates and, combined with the sluggish diffusion in the HEA, grain growth is inhibited to provide a reasonable stability in the fine-grained structure at elevated temperatures. By comparison with the conventional CoCrFeNiMn HEA, the results demonstrate that the addition of a minor amount of Ti produces a smaller grain size, a highervolume fraction of precipitates and a significant improvement in the superplastic properties

Evolution of microstructure and hardness in NiTi shape memory alloys processed by high-pressure torsion

Experiments were conducted on Ni-50.2 at.% Ti and Ni-50 at.% Ti alloys in order to examine the evolution of hardness and microstructure after processing by high-pressure torsion at room temperature. Disks were pressed through different numbers of revolutions up to a maximum of 40 using an applied pressure of 2.0 GPa. It is shown that there is a gradual evolution in both the hardness and the microstructure with increasing numbers of turns but even after 40 turns there is not full homogeneity. There is evidence that after 10 turns the edges of the disks achieve a well-defined saturation hardness and by further processing to 40 turns the hardness in the centers of the disks increases. The results show that a martensite-to-austenite transformation occurs during processing. The austenitic transformation around the edge of the disks achieves saturation after 5 and 10 turns in the Ni-50 at.% Ti and Ni-50.2 at.% Ti alloys, respectively