Papers by Keyword: Pseudoelasticity

Abstract: Shape memory alloys (SMAs) are a type of smart material and have excellent engineering and medical applications. TiNi binary alloys possess remarkable shape recovery, mechanical properties, corrosion resistance, and excellent biocompatibility. By ternary elements addition just like Au, Pt, Pd, Hf, and Zr, increases transformation temperatures, leading to high-temperature shape memory alloys (more than 100°C) but other elements (Fe, Cu, Co, and Mo) form low-temperature shape memory alloys, (lower than 100°C). In the present work, it is reported that the effect of ternary element addition on microstructural properties, shape memory properties, mechanical properties, corrosion resistance, and biocompatibility of ternary shape memory alloys. Ag, Au, and Cu-based TiNi ternary alloys have excellent biocompatibility. The addition of ternary elements such as Ag and Nb increases corrosion resistance, Fe rises the hysteresis loop, Hf enhances thermal stability, and Mo raises workability.

Abstract: Conventional strain gauges made of constantan or CuCr for instance have a low value for structural health monitoring issues in plastic composites. These strain sensor materials exhibit small elastic regions and show fatigue when dynamically loaded with strain levels over 0.3 percent. For this reason, these sensors would break or fail before the composite life-time and thus cannot be integrated into this kind of composite materials. Pseudoelastic thermal shape memory alloys are therefore used as strain sensors and integrated into composites in order to allow piezoresistive strain measurement and structural health monitoring in such materials. Thermal treatments are used to create sensor structures out of shape memory alloy wires. Pseudoelastic shape memory wires can be strained up to 8 percent repeatedly. Their gauge factor is higher than 5. Shape memory strain sensors are successfully embedded into glass fibre reinforced plastics and show a significant and reproducible resistance change when the composite is strained. The dynamic strength is magnificently higher compared to conventional strain gauges. Shape memory strain sensors are an efficient alternative to fiber-bragg-grating sensors and can potentially be used for strain measurements in different plastics and textile materials. Shape memory sensor structures can be embedded or applied and are good candidates for structural characterisation and monitoring applications.

Abstract: This chapter is devoted to a study of structure, martensitic transformation and shape memory behaviour in TiNi foams produced by self-propagating high-temperature synthesis. The influence of the chemical composition of the Ti+Ni powder mixture as well as pre-heating temperature on the structure and properties of TiNi foams is studied. It is shown that the variation in Ni concentration in the powders mixture allows one to produce a porous TiNi alloy with properties close to the equiatomic cast Ti₅₀Ni₅₀ alloy or Ni-rich cast TiNi alloy. It is shown that the TiNi foams produced from a mixture where the Ni concentration is higher than 45 at.% should be subjected to post-production annealing to decrease the Ni concentration in the TiNi phase. The influence of annealing temperature and duration on the structure and martensitic transformation in TiNi foams produced by SHS is studied. The optimal conditions for annealing of the TiNi foams are found. It is shown that TiNi foams after optimal heat treatment demonstrate good shape memory properties.

Abstract: The pseudoelasticity (PE) and shape memory effect (SME) are the two main behaviors presented by the shape memory alloys (SMA's) and are associated respectively to mechanical and thermally induced martensitic transformations. The aim of this work is to investigate the effects of heat treatment temperature on the microstructure properties and phase transformation temperatures of a NiTi alloy with 57 w.t. % Ni. The X-ray diffraction (XRD) was carried out to obtain the phases present. The phase transformation temperatures were measured by differential scanning calorimetry (DSC). The alloy chemical composition and hardness were obtained by X-ray fluorescence (XRF), hardness (HRC) and microhardness (HV) tests, respectively. The analysis were performed in the state as received and after aging treatment at different temperatures between 350 °C and 600°C. The samples as received contained a fully austenitic microstructure at room temperature and the DSC analysis showed the presence of a phase transformation in multiple steps (B19'-R-B2). After aging at 350 °C the R phase was observed at room temperature with the austenite. With the aging treatment at 600 °C the R phase was solubilized and the alloy phase transformation occurred in a single step, ie, direct transformation from austenite to martensite and vice versa. The transformation temperatures Af, As, Ms and Mf changed with the aging treatment temperatures.

Abstract: Fe-10Ni-7Mn (wt. %) steel is a member of ultrahigh strength steels which shows good ductility in the solution annealed condition and excellent age hardenability. In the current research, this alloy was subjected to heavy cold rolling in which the reverse transformation of martensitic to austenite was brought about. From the XRD, DSC and dilatometric analyses, it is resulted that after 60 % cold rolling the austenite phase may be formed by displacive mechanism. Stability of austenite at room temperature is referred to the ultrafine/nanograin size of austenite after deformation which prevents the austenite to martensite transformation. The presence of ultrafine/nanoaustenite formed by displacive mechanism leads to the observation of new mechanical properties during cyclic tensile test. This behavior is known as pseudoelastic phenomenon. In this behavior, during loading-unloading tensile cycle, the shape of the specimen return to its original configuration with a hysteresis loop in its path to the zero strain point.

Abstract: Pseudoelastic NiTi-based devices are often required to recover their shape repeatedly and their working performance can be judged from the amount of residual deformation after use. The quality problem in this respect can even be removed from fatigue life or safety issues and impact on the functional and aesthetic value of the product. While linear deformation can be appreciated quantitatively in a straightforward manner, the bending strains are more difficult to assess directly. We devised a very simple digital image-based method to measure the residual bending deformation by comparison of the pristine shape of the device with the one resulting from bending and free recovery. The program was written in LabView and is capable of reporting about the deflection and location of strain concentration along slender pseudoelastic elements in a semi-quantitative way appropriate for quality sample checks. The method is semi-automatic and provides a user-friendly interface for the operator. Apart from simple shapes like straight wires and ribbons, the method was tested on devices as complex as spectacles frames. This application is particularly interesting, where shape recovery and functional and aesthetic value are tightly linked, and deformation by severe handling is a typical effect of use.

Abstract: Pseudoelasticity of Fe₃Ga polycrystals doped with third elements (Ti, V, Cr, Mn, Co, Ni, Si, Ge) was examined. Fe₃Ga polycrystals with the appropriate heat treatment were found to exhibit large pseudoelasticity based on reversible motion of dislocation dragging an antiphase boundary (APB). In Fe₃Ga crystals with the D0₃ superlattice structure, paired 1/4<111> superpartial dislocations mainly moved dragging the next-nearest-neighbor APB during loading. During unloading, the APB pulled back the superpartial dislocations resulting in the pseudoelasticity. The D0₃ ordered phase also developed in Fe₃Ga polycrystals with 2at% of the third elements. However, the strain recovery of Fe₃Ga polycrystals depended strongly on third element. Fe₃Ga polycrystals doped with 2at% of Mn, Cr and Co demonstrated large pseudoelasticity. In contrast, the other doped elements decreased the amount of strain recovery. The frictional stress of 1/4<111> superpartial dislocations and the back stress due to the APB, acting on the dislocations, changed by doping the third elements, which was closely related to the pseudoelastic behavior. It is also noted that there was a good correlation between the APB back stress and the ordering temperature from the B2 to D0₃ phase.

Abstract: In order to develop new nickel-free biomedical Ti-based alloys, effect of silver additions on mechanical properties of Ti-5Cr (mol%) alloy was investigated. Cold workability of Ti-5Cr alloy was 5% in thickness reduction and the cold rolling reduction was improved to be 38% by 2mol% Ag addition and 96% by 4mol%Ag addition. The improvement was due to β phase stabilization. From the XRD results, α’ martensite was the dominant phase in Ti-5Cr-2Ag alloy and β phase was the dominant phase in Ti-5Cr-4Ag alloy. By tensile tests, Ti-5Cr-4Ag alloy showed good strength of 447 MPa in ultimate tensile strength (UTS) and ductility of 13% in fracture strain. Ti-5Cr-4Ag showed higher hardness of HV398 than Ti-5Cr-2Ag with HV288. The hardening by increase of Ag is probably due to the solid solution strengthening. By the cyclic loading-unloading tensile tests with a constant strain increment, Ti-5Cr-4Ag showed pseoudoelastic behavior. Ti-5Cr-4Ag also showed shape memory effect with 57% in shape recovery ratio. It is concluded that Ti-5Cr-4Ag is hopeful as a new non-allergic shape memory material for biomedical applications.

Abstract: This paper provides an original experimental characterization of the shakedown state of a shape memory alloy structure under cyclic pseudo-elastic loading. This analysis is performed through the observation of the dissipated energy at a macroscopic scale as well as the temperature on the surface of the sample through infrared thermography measurement. Morevover, a deeper study is led thanks to acoustic emission to quantify the shifts between microscopic evolutions at a lower scale. The main conclusion is that these 3 quantities are correlated and enable us to identify different stages the structure crosses until the shakedown state.

Abstract: In Fe₃Ga single crystals with the D0₃ structure, three types of pseudoelasticities based on dislocation motion, martensitic transformation and twinning take place depending on the heat treatment, the loading axis and the stress sense. In this paper, we report the detail of the transformation and twinning pseudoelasticities in the crystals focusing on the crystallography and the temperature dependence. In particular, the driving force for the twinning pseudoelasticity was discussed, focusing on the atomic arrangement. In Fe₃Ga single crystals homogenized or solutionized in the α disordered region, the martensites with the 14M structure, containing numerous stacking faults were stress-induced during loading, while they disappeared during unloading by the reverse transformation, resulting in the transformation pseudoelasticity with small stress-strain hysteresis. In contrast, twinning pseudoelasticity caused by twinning and untwinning of 2.2^T-type pseudo-twins appeared in the well-ordered D0₃ crystals, accompanied by a serrated flow in the stress-strain curve. The contribution of the twinning pseudoelasticity to strain recovery became significant at low temperatures at which the dislocation motion was difficult. It should be noted that the formation of the pseudo-twins could be regarded as a certain displacive phase transformation since the crystal structure of the twins became orthorhombic due to the twin shear without atomic shuffling. The free energy difference between the D0₃ matrix and the pseudo-twins resulted in the twinning pseudoelasticity. Moreover, the pseudo-twins were transformed into the perfect twins by annealing at 300 °C where the atomic shuffling could occur. The perfect twins remained even after complete unloading due to their low driving force for the pseudoelasticity.