Superelastic Strain Research Articles

Effect of Equal-Channel Angular Pressing on the Microstructure and Superelastic Behavior of Ti-25 at. % Nb Shape Memory Alloy

Microstructure and mechanical behavior of Ti-25 at. % Nb shape memory alloy processed by equal-channel angular pressing (ECAP) at 673 K have been investigated. The effect of multi-passes ECAP on the martensitic transformation temperature and superelastic recovery strain of Ti-26 at. %Nb alloy have been analyzed. It is found that the Ti-25 at. % Nb alloy exhibits superelasticity with a recovery strain of 1.6 % after one pass ECAP process. As the ECAP pass numbers increases to four passes, the superelasticity increases a little. The yielding stress (σ0.2) of Ti-25 at. % Nb alloy increases to 430 MPa and the ultimate tensile strength (UTS) is near 678 MPa after one pass ECAP process at 673 K, but the yielding stress and ultimate tensile strength increases little after four pass ECAP. The grain size decreases sharply from 100 μm to no more than 500 nm after four passes ECAP process. Effect of ECAP on the microstructure evolution and Young's modulus has been analyzed.

Functional properties of nanocrystalline, submicrocrystalline and polygonized Ti–Ni alloys processed by cold rolling and post-deformation annealing

Thermomechanical processing consisting of cold rolling ( e = 0.3–2.0) and post-deformation annealing (300–450 °C, 1 h) was applied to binary Ti–Ni alloys to produce nanocrystalline structures (NS) or polygonized dislocation substructures (PDS), or their mixture. The evolution of the material structure and properties was studied using TEM, X-ray, microhardness, calorimetry and tensile testing techniques. Recovery stress and strain of the Ti–50.26 at.%Ni alloy and superelastic strain of the Ti–50.6 at.%Ni alloy were measured under static and fatigue conditions. It was found that higher true yield stress of NS alloys not only increases the recovery stress potential, but, since it is combined with a relatively low transformation yield stress; it increases the completely recoverable strain. NS alloys generate recovery stresses that are twice as high as those of PDS alloys (1200 MPa), completely recoverable strains that are 10% greater (up to 6% in tension), and they demonstrate a higher cyclic stability of shape memory and superelastic properties. This improvement comes with the cost of a lower NS alloy fatigue damage tolerance, aggravated by the presence of microcracks caused by cold working. Binary Ti–Ni alloys, processed by annealing of an intermediately cold-worked ( e = 0.75…1) alloy and containing mixed nanocrystalline structure and polygonized dislocation substructure, allow a high fatigue life combined with relatively high and cyclically stable functional properties.

Effect of nitrogen addition and annealing temperature on superelastic properties of Ti–Nb–Zr–Ta alloys

The composition dependence of the mechanical properties and martensitic transformation behavior of Ti–Nb–4Zr–2Ta–N alloys is investigated. The effect of annealing temperature on the microstructural evolution and superelastic properties in the N-added and N-free alloys is compared. The addition of N decreases M s of Ti–Nb–4Zr–2Ta alloys by about 200 K per 1 at.%N and improves the superelastic properties of Ti–Nb–4Zr–2Ta alloys. The dissolution of α phase increases the martensitic transformation start temperature and decreases the superelastic recovery strain for the N-free alloy, whereas it causes opposite effects for the N-added alloy. The different annealing temperature dependences of superelastic properties are discussed on the basis of microstructure observation.

Exploratory investigation on mechanical anchors for connecting SMA bars to steel or FRP bars

Using superelastic shape memory alloys (SMAs) as reinforcing bars in concrete structures proved to have a great potential in seismic areas because of its recentering capability. However, using them in an entire structure is generally not economically feasible due to their high cost. Therefore, it is more practical to limit their use to the plastic hinge zones, while regular steel can be used in the other regions of the structure. Connections between SMA and steel are critical, and need to be strong enough to transfer the full force from SMA bars to steel bars. Various mechanical couplers are available in the market to splice bars in reinforced concrete (RC) structures, each of which has several advantages and disadvantages. The efficiency of these couplers for connecting steel bars is tested and reported in this paper. Since these couplers are intended for connecting steel bars only, another experimental investigation has been performed to determine the suitability of these couplers for connecting SMA with steel bars. Commercially available screw-lock couplers are found to be unsuitable for connecting SMA to steel bars. An existing coupler has been modified for SMA–steel splicing to allow SMA bars to achieve their full superelastic strain. Additional tests have also been performed for connecting FRP bars to SMA bars. A new generation mechanical-adhesive type coupler has been developed for splicing FRP to SMA bars.

Ferrous Polycrystalline Shape-Memory Alloy Showing Huge Superelasticity

Shape-memory alloys, such as Ni-Ti and Cu-Zn-Al, show a large reversible strain of more than several percent due to superelasticity. In particular, the Ni-Ti-based alloy, which exhibits some ductility and excellent superelastic strain, is the only superelastic material available for practical applications at present. We herein describe a ferrous polycrystalline, high-strength, shape-memory alloy exhibiting a superelastic strain of more than 13%, with a tensile strength above 1 gigapascal, which is almost twice the maximum superelastic strain obtained in the Ni-Ti alloys. Furthermore, this ferrous alloy has a very large damping capacity and exhibits a large reversible change in magnetization during loading and unloading. This ferrous shape-memory alloy has great potential as a high-damping and sensor material.

Ferrous Shape Memory Alloy

So-called shape memory alloys “remember” the shape they are processed into, and can return to that shape after being deformed by heat. A limitation for most metal-based shape memory alloys is the extent to which they can be deformed elastically. Tanaka et al. (p. [1488][1]; see the Perspective by [Ma and Karaman][2] ) demonstrate an iron-based alloy that shows much higher levels of superelastic strain, surpassing the performance of nickel-titanium alloys. In addition to high superelastic strain, this ferrous shape memory alloy has much higher strength than NiTi and copper-based shape memory alloys and, consequently, a high-energy absorption capability. These properties may allow shape memory alloys to be exploited as strain sensors or energy dampers. [1]: /lookup/doi/10.1126/science.1183169 [2]: /lookup/doi/10.1126/science.1186766

The characterization of shape memory effect for low elastic modulus biomedical β-type titanium alloy

This work investigates the textures of biomedical TiNbTaZr alloy rolled by 99% cold reduction ratios in thickness. The relationship between textures and superelasticity of the specimens treated at 873 K and 1223 K for 1.2 ks is studied. The microstructure of tensile specimen is investigated by transmission electron microscopy. Textures of cold-rolled and heat-treated specimens are studied. During unloading, the anisotropy of superelastic strain and pure elastic strain in the heat-treated specimens is observed. Superelastic strain along rolling direction and transverse direction is larger than those along 45° from rolling direction while pure elastic strain shows the highest value along 45° from rolling direction in the specimen treated at 873 K. For the specimen treated at 1223 K, higher pure elastic strain is obtained along rolling direction. The maximum recovered strain around 2.11% is obtained along rolling direction.

Effect of α-precipitation on the superelastic behavior of Ti–40 wt.%Nb–0.3 wt.%O alloy processed by equal channel angular extrusion

A ultrafine-grain-structured Ti–40Nb–0.3O (wt.%) alloy has been fabricated via equal channel angular extrusion (ECAE) followed by annealing within an α + β two-phase field. The precipitation of α-phase was responsible for producing an annealed ultrafine-grain structure of about 1 μm. As-ECAEed specimen showed a large recovery strain of 3.5% at −125 °C, although no superelastic property was observed. Post-annealing of the ECAEed specimen led to a recovery of superelastic behavior. The superelastic strain first increased and then decreased with annealing time. A maximum superelastic strain of 2.5%, on subjecting 896 MPa, was observed at −125 °C. The increase of superelastic strain during the initial stage of annealing was due to the precipitation of α-phase leading to oxygen depletion in β-matrix. The precipitation of α-phase also led to a decrease of critical stress for stress induced martensitic transformation and to an increase of critical stress for slip.

Giant two-way shape memory effect in high-temperature Ni–Mn–Ga single crystal

A perfect two-way shape memory effect (TWSME) with reversible strain of about 9% has been found in the high-temperature Ni57.5Mn22.5Ga20.0 single crystal transforming into 2M non-modulated martensitic phase. Two thermal/mechanical treatment routes were utilized in the experimental procedures. The outstanding high-temperature TWSME is observed as a result of tensile stress-strain cycling along 〈100〉 axis involving huge superelastic strains due to stress-induced martensitic transformation. It is argued that the TWSME is related to the anisotropic internal stresses produced by a network of lattice defects. The defects are generated in the course of oriented growth/shrinkage of the dominating martensitic variant in the tensile sample.

High-temperature superelasticity and competing microstructural mechanisms in Co49Ni21Ga30 shape memory alloy single crystals under tension

A large temperature range of stress-induced martensitic transformation with >8% superelastic strain is demonstrated in [1 0 0]-oriented Co49Ni21Ga30 single crystals. The nature of the superelastic response is greatly affected by temperature and test history due to the temperature-dependent dominant microstructural mechanisms. We provide an insight into these mechanisms and divide the response into three temperature stages. Interestingly, a small stress hysteresis of only 2 MPa was achieved after the superelastic experiments at 300 °C compared to 20 MPa of the virgin sample.

Shape memory and superelasticity in polycrystalline Cu–Al–Ni microwires

We report a strategy to significantly improve the ductility and achieve large superelastic and shape memory strains in polycrystalline Cu–Al–Ni shape memory alloys that are normally brittle. We use a liquid-phase (Taylor) wire forming process to obtain microwires of 10–150 μm diameter with a bamboo grain structure. The reduction of grain boundary area, removal of triple junctions, and introduction of a high specific surface area in the wire decrease constraints on the martensitic transformation, and permit both superelasticity and stress-assisted two-way shape memory with recoverable strains as high as 6.8%.

Microstructure and superelasticity of in situ synthesized (TiB + La2O3)/Ti-alloy composites with different mass fraction of LaB6

In situ techniques have been developed to synthesize titanium matrix composites. The microstructures and the superelastic characterization of (TiB+La2O3)/(Ti–35Nb–2Ta–3Zr) TMC were examined and studied. The effects of TiB whiskers and La2O3 spherical particles on superelasticity were discussed. In the LaB6-additioned specimens, minimum size of grain around 12μm was obtained and critical stress for slip deformation was increased. Excellent superelastic strain and pure elastic strain were obtained in 0.1% and 0.2% LaB6-additioned specimens. The largest superelastic strain was observed in 0.5% LaB6-additioned specimen.

Effects of aging on stress-induced martensitic transformation in ductile Cu–Al–Mn-based shape memory alloys

Effects of aging at 473–573 K on stress-induced martensitic transformation for textured Cu 71.9Al 16.6Mn 9.3Ni 2B 0.2 and random-textured Cu 72.1Al 16.9Mn 10.5Co 0.5 shape memory alloy (SMA) wires with a large relative grain size d/ D = 6 were investigated by cyclic tensile testing at room temperature, where d and D indicate mean grain size and wire diameter, respectively. The random-textured Cu 72.1Al 16.9Mn 10.5Co 0.5 wire cannot be uniformly deformed and the ductility is drastically reduced by aging treatment. On the other hand, in the textured Cu 71.9Al 16.6Mn 9.3Ni 2B 0.2 SMA wire, the critical stress for martensitic transformation σ t and the tensile strength σ f are increased by aging without the associated loss of superelasticity (SE). Even in textured wire with a high σ t of over 750 MPa, an excellent SE strain of about 6% can be obtained due to the formation of a fine bainite phase. Moreover, it was confirmed by in situ observation that stress-induced martensite plates grow, accompanying distortion of the bainite plates.

Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire

Homogeneous 177-μm diameter nanocrystalline Nitinol wires were produced and compared to microcrystalline Nitinol from equivalent ingot stock. Property measurement was carried out using cyclic tension testing, strain-controlled fatigue testing and bend and stress-free recovery testing (BFR). A B2 cubic structure of 5- to 60-nm grain size was confirmed by transmission electron microscopy (TEM). The fatigue strain capability at 107 cycles is 30% greater in the nanocrystalline versus microcrystalline annealed wire. Grain size and residual strain are positively correlated in cyclic uniaxial tension testing within the superelastic temperature and strain regime. A transformation-induced loading plateau that is 40% longer than in standard superelastic polycrystalline wire is also reported for the first time.

Cyclic deformation behavior of a Ti–26 at.% Nb alloy

The effect of cyclic deformation on superelasticity was investigated in a Ti–26 at.% Nb alloy. Loading and unloading tensile tests with a constant maximum applied strain of 2.5% were carried out until the 500th cycle. The critical stress for inducing the martensitic transformation and superelastic strain decreased, while the accumulated residual strain increased with increasing number of cycles. The increase in the residual strain during cyclic deformation was due mainly to α′′ martensite phase stabilization. Both the residual strain and the residual α′′ martensite phase increased with increasing number of cycles. The stability of superelasticity was improved, i.e. the residual strain decreased and the superelastic strain increased, by intermediate-temperature annealing and/or aging. The specimen annealed at 873 K for 0.6 ks followed by aging at 573 K for 3.6 ks exhibited the most stabilized superelasticity, owing to the combination effect of work hardening and fine ω-phase precipitation.

Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys

Recently, the Ti-Zr-Nb alloys have been developed as Ni-free shape memory and superelastic alloys. In this study, the effects of Nb and nitrogen (N) contents on martensitic transformation behavior, shape memory effect and superelasticity in Ti-18Zr-(12∼16)Nb-(0∼1.0)N (at%) alloys are investigated using loading and unloading tensile tests, optical microscopy and X-ray diffractometry. The shape memory effect is observed in Ti-18Zr-(12∼13)Nb and Ti-18Zr-12Nb-0.5N alloys at room temperature. The superelastic behavior appears by the increase of Nb or N content. The Ti-18Zr-(14∼15)Nb, Ti-18Zr-(13∼14)Nb-0.5N and Ti-18Zr-(12∼14)Nb-1.0N alloys exhibit the superelasticity at room temperature. The martensitic transformation start temperature decreases by 75 K with 1 at% increase of N content for the Ti-18Zr-13Nb alloy. The critical stress for slip deformation and the stress for inducing the martensitic transformation increase with increasing N content. The superelastic recovery strain is also increased by adding N. The maximum recovery strain of 5.0% is obtained in the Ti-18Zr-14Nb-0.5N alloy.

Elastic and Superelastic Properties of NiFeCoGa Fibers Grown by Micro-Pulling-Down Method

Low-frequency elastic modulus, internal friction, tensile stress-strain loops, and thermally-induced strain recovery of single-crystal fibers obtained by pulling-down method from the melts of NiFeGaCo alloys were studied. A minimum of the elastic modulus and the maximum peak of the internal friction were obtained at the martensitic transition temperature. A large super-elastic strain of about 10% was obtained in the fiber of the Ni49Fe18Ga27Co6 alloy. Thermodynamic estimation by the Clausius-Clapeyron equation was consistent with the experimental results.

Texture and superelastic behavior of cold-rolled TiNbTaZr alloy

This work investigates the deformation texture and strain-induced α″ martensite texture of TiNbTaZr alloy during cold rolling. The alloy is rolled by 20% and 90% reductions without changing rolling direction. Textures of cold-rolled specimens are investigated by X-ray diffraction measurements. Besides {2 2 1} β〈1 1 4〉 β twinning texture, {1 0 0} β〈0 0 1〉 β texture is developed in the specimen with 20% reduction. In the 90% cold-rolled specimen, {1 0 0} β〈0 1 1〉 β texture appears along rolling direction and strain-induced α″ martensite texture tends to [0 1 0] and [0 0 1] directions along rolling direction (RD) and transverse direction (TD), respectively. Superelastic strain ( ɛ SE) exhibits higher value along RD and TD. Pure elastic strain ( ɛ E) shows higher value along RD and 45° from RD.

Superelastic Properties of Rapidly Solidified Fe-Pd Ribbons

The Fe-Pd ferromagnetic shape memory alloy is a multifunctional material. We investigated the behavior of the superelastic properties of rapidly solidified Fe-Xat%Pd (X ¼ 29:3, 29.6, 30.2 and 30.4) alloy ribbons to develop biomedical materials applicable to orthodontics and medical instrumentation. The appearance of a stress-induced martensite phase under the loading stress was confirmed by X-ray diffraction measurements. Stress-strain curves were drawn from results obtained using a tensile testing machine at temperature of 290 and 313 K. The FeXat%Pd (X ¼ 29:6, 30.2, 30.4) ribbons exhibited superelastic strain of 3.0–3.5%. Moreover, when these Fe-Pd alloy ribbons were treated in Hanks’ solution for 150 days, they exhibited good biocorrosion resistance. [doi:10.2320/matertrans.MRA2007216]

Evolution of crack-tip transformation zones in superelastic Nitinol subjected to in situ fatigue: A fracture mechanics and synchrotron X-ray microdiffraction analysis

The ultrahigh spatial resolution (∼1 μm 2) of synchrotron X-ray microdiffraction is combined with fracture mechanics techniques to directly measure in situ three-dimensional strains, phases and crystallographic alignment ahead of a growing fatigue crack (100 cycles in situ) in superelastic Nitinol. The results provide some surprising insights into the growth of cracks in phase-transforming material at the microscale. Specifically, despite a macroscopic superelastic strain recovery of 6–8% associated with the phase transformation, individual austenite grains experience local strains of less than 1.5%. This observation indicates that it is the localized process of the accommodation of the transformation and subsequent loading of the martensite that provide the main source of the large recoverable strains. Furthermore, the plastic region ahead of the crack is composed of deformed martensite. This micromechanical transformation process is dependent upon the material texture, and directly influences the transformation zone size/shape as well as the crack path.