Superelastic Strain Research Articles

Superelasticity and tensile strength of Ti-Zr-Nb-Sn alloys with high Zr content for biomedical applications

In this study, Ti-xZr-8Nb-2Sn (x = 40, 45, 50) (at.%) alloys were fabricated by arc melting method and then microstructures, martensitic transformation behavior, superelasticity and mechanical properties were investigated by means of optical m icroscopy, electron microscopy, X-ray diffraction, tensile test and Vickers hardness test. The alloys consisted of β phase only at room temperature and exhibited strong (200)β texture after solution treated at 1173 K for 1.8 ks. Athermal ω phase was not observed from transmission electron microscopy in the alloys. The stress induced β→α″ transformation and the reverse transformation occurred on loading and unloading, respectively. Transformation temperature (Ms(C-C)) decreased from 188 K to 77 K with increasing Zr content from 40 at.% to 50 at.%. The maximum recoverable strains for 40Zr, 45Zr and 50Zr alloys were 7.1%, 7.5% and 7.3%, respectively, which was due to a combined effect of the large lattice deformation and a strong recrystallization texture. All alloys were fractured in ductile manner with fracture strains larger than 25%. Ultimate tensile strength of 835 MPa and superelastic recovery strain of 5.5% were obtained at room temperature in the solution treated Ti-40Zr-8Nb-2Sn alloy.

Large superelastic recovery and elastocaloric effect in as-deposited additive manufactured Ni50.8Ti49.2 alloy

A Ni50.8Ti49.2 alloy is fabricated by the Laser Solid Forming (LSF) technique by using prealloyed powder in this work. Characterization of the microstructure, phase transformation, and shape recovery of the as-deposited alloy are explored. Columnar grains with ⟨110⟩ preferred orientation are well aligned with the building direction, and the martensitic transformation (MT) behavior shows spatial distribution characteristics. With the increase in the deposited height, MT varies from the two-stage B2 →R→ B19′ transformation to the one-stage B2 → B19′ transformation. Shape recovery properties are studied by compression tests. The as-deposited material without any further heat treatment shows a remarkably large superelastic recovery strain of 9.2% at room temperature, which is the largest value in the additive manufactured NiTi alloy. A negative adiabatic temperature change of –18.6 K induced by the elastocaloric effect during the unloading process is obtained. The shape memory effect (SME) of the as-deposited LSF Ni50.8Ti49.2 alloy is not only dependent on the detwinning of martensite but also associated with the partial superelasticity of the parent phase in the present work, demonstrating a unique two-stage yielding behavior. A residual strain up to 6% is recovered while heating, indicating an excellent SME.

Room temperature superelasticity in a lightweight shape memory Mg alloy

To realize a superelastic Mg-Sc lightweight alloy at room temperature, we investigated the stress–strain behavior of Mg-18.7 at.% Sc alloy sheets at room temperature with martensitic transformation starting temperature of approximately −90 °C. We found that the superelasticity of the alloy strongly depends on the grain size and a distinct room temperature superelasticity (~3% superelastic strain) was obtained in the alloy with a large grain size. The obtained maximum superelastic strain was smaller than that expected from the orientation dependence of the transformation strain. This result suggests that the stress-induced martensite phase was stabilized because of the introduction of slip defects.

Abnormal grain growth in annealing cast Cu–Al–Mn–V alloys and their superelasticity

Metals are usually difficult to fabricate bulk materials with a large grain size over cm-scale only through heat treatment with neither directional solidification nor macroscopic deformation. This paper reports a novel method to obtain large grains and single crystals over cm-scale only through simply annealing polycrystalline cast Cu–Al–Mn–V shape memory alloys. The results show that the abnormal grain growth (AGG) directly occurs when cast alloys are annealed at 1173 K, and ultra-large grains and single crystals are obtained after quenching. The AGG results from unique microstructure of cast Cu–Al–Mn–V alloys. All cast alloys consist of L21-Cu2AlMn parent and completely coherent bcc A2(V) nanoparticles due to bcc phase separation. The results further show that the dissolution of A2(V) nanoparticles into the matrix during annealing results in a continuous misorientation gradient within the matrix grains. It may provide the driving force for the rapid migration of grain boundaries, leading to the AGG. Furthermore, the single crystals close to exhibit good superelasticity. Cu-13.7Al-8.6Mn-1.4 V single crystal shows the largest full superelasticity of 7.0%, and Cu-13.2Al-9.8Mn-0.7 V single crystal exhibits the largest superelastic strain of 5.4%.

Inelasticity of nanocomposite based on ferromagnetic Fe–Ni–Co–Ti alloy after thermomechanical treatment

This paper presents the results of mechanical tests under a uniaxial tension of a nanocomposite obtained as a result precipitation hardening in the process of ageing of quenched ferromagnetic Fe-27.2%Ni-17.4%Co-5.2%Ti (wt%) alloy, having the martensitic, austenitic, or mixed structure of matrix after cooling and/or applied stress. To obtain the nanocomposite, the alloy samples were subjected to a preliminary thermomechanical treatment (TMT), which consisted of multiple drawing operations followed by quenching in water from 1373 K and ageing at 923 K for 5, 10, and 20 min. As a result of such TMT, a sufficiently high level of superelastic deformation and shape memory effect were achieved. It was experimentally established that the preliminary TMT, consisting of drawing with compression degree 22.5% followed by quenching and ageing at 923 K for 5–10 min, favors the deformation of the nanocomposite along the channels of phase and twinning plasticity in the range of testing temperature Ms < Tt < Af, and corresponds to the optimal combination of maximum superelastic strain and shape memory effect. A further increase in the degree of deformation results in inhibition of martensitic transformation and reducing of superelastic deformation by decreasing the austenite grain size and increasing the density of crystal defects.

Orientation dependence of superelasticity in FeMnAlNi single crystals under compression

The orientation dependence of the martensitic transformation characteristics and superelasticity were investigated in the Fe43.5Mn34Al15Ni7.5 single crystalline shape memory alloy along the [100], [111] and [123] orientations under compression. Solution heat treated samples exhibited a strong orientation dependence of the slopes for the stress vs. temperature phase diagrams (i.e. Clausius-Clapeyron slopes), while peak-aged samples demonstrated reduced orientation dependence. Maximum superelastic strains determined in the experiments along various single crystal orientations match well with the theoretical predictions.

Excellent superelasticity of Cu-Al-Mn-Cr shape memory single crystal obtained only through annealing cast polycrystalline alloy

The polycrystalline Cu-13.0Al-7.7Mn-0.6Cr cast alloy consists of the Heusler L21 (Cu2AlMn) parent and the disordered body-centered cubic A2(Cr) precipitate due to bcc phase separation. These two phases are completely coherent. When annealing cast alloy, the abnormal grain growth occurs. It is closely related to the dissolution of A2(Cr) particles and the formation of a continuous misorientation gradient within the matrix grains. The results show that the single crystal close to [100] orientation shows large superelastic strain of 8.5%. Furthermore, the single crystal almost remains full superelastic characteristic after 1.2 × 103 cycles, showing enhanced fatigue resistance.

Excellent superelasticity and fatigue resistance of Cu-Al-Mn-W shape memory single crystal obtained only through annealing polycrystalline cast alloy

In this study, the cast Cu-13.96Al-9.84Mn-0.51W shape memory alloy consisted of L21-Cu2AlMn parent with completely coherent bcc A2(W) nanoparticles due to bcc phase separation. The abnormal grain growth happened only through annealing cast polycrystalline alloy. The results showed that a continuous misorientation gradient within the matrix grains with a deviation of below 2° formed after the dissolution of A2(W) nanoparticles during annealing. This continuous misorientation gradient may result in the abnormal grain growth. The single crystal specimens were directly obtained from one of large grains. The single crystal close to [100] orientation exhibited very excellent superelasticity and fatigue resistance. Here full superelasticity up to 12% and the largest superelastic strain up to 8.5% were displayed, being comparable to commercial Ni-Ti shape memory alloys. Furthermore, the single crystal almost remained full superelasticity during 5 × 103 cycles. From the points of raw material cost, preparation technology and functional properties, the Cu-Al-Mn-W single crystal shows good potential as new superelastic material.

Superelasticity Evaluation of the Biocompatible Ti-17Nb-6Ta Alloy.

Recently, studying the shape memory effect of the biocompatible Ti alloys takes much attention in the biomedical and healthcare applications. This study concerns about characterizing the superelasticity of the new biocompatible Ti-17Nb-6Ta (TNT) alloy. Microstructure of TNT was observed using optical and confocal microscopes. The alloy consists of two phases: β (predominant phase) and α″ martensite phase. The influence of cold rolling deformation on the microstructure was illustrated in which the martensitic-induced transformation appeared by cold rolling. The alloy is ductile as only the fracture dimples appeared in its fracture surface. Multicyclic loading and deloading tensile testing was applied to TNT specimens (flat and wire shapes) in order to evaluate the superelasticity. A superelastic strain as high as 3.5% was recorded for this TNT alloy. Therefore, TNT alloy has high potential for many biomedical and healthcare applications.

Low-cost Cu-based shape memory single crystals obtained by abnormal grain growth showing excellent superelasticity

Metal single crystals are generally obtained by directional solidification with sophisticated equipment and process. This method is not only very expensive but also very time-consuming. In this study, we report a new approach to produce Cu-Al-Mn-Mo shape memory alloys with single crystals through annealing cast polycrystalline alloys. The microstructure of the cast alloys has a common characteristic that the L21-Cu2AlMn parent is completely coherent with bcc A2(Mo) precipitates due to bcc phase separation. The dissolution of bcc A2(Mo) nanoparticles during annealing process results in abnormal grain growth because of the formation of a continuous misorientation gradient within the grains. The single crystals obtained from large grains exhibit excellent superelasticity. When the orientation of single crystals is close to [−210], the largest full superelasticity is up to 10%, and the largest superelastic strain is up to 6.7%. It is expected that such microstructural design can also be applied to the production of large grains and even single crystals of other alloy systems with phase separation.

Strain-magnetization effect in superelastic Ni-Mn-Ga microfiber

Ni-Mn-Ga microfiber with a diameter of 80 μm was produced by glass-coated melt spinning method. The martensitic transformation temperature, TM, and the reverse transformation temperature, TA, is 279 K and 290 K, respectively. The as-cast microfiber shows nearly fully recoverable superelastic strain as large as ~10%, and perfect strain-magnetization effect, as the stress-induced martensite has lower initial susceptibility and higher saturation magnetization than the austenite. It will be a potential non-destructively and noncontactually monitor sensor material.

Selective electron beam melting of NiTi: Microstructure, phase transformation and mechanical properties

Recent years have witnessed extensive investigations of additively manufactured NiTi alloys using laser-source techniques. This study reports a selective electron beam melting process to make Ni-rich NiTi parts using the plasma rotating electrode processed pre-alloyed NiTi powder. Microstructures of the as-printed NiTi samples were investigated, including micro-morphology, phase constituent and texture. Chemical compositions (primarily oxygen and nickel) and martensitic transformation temperatures were compared between the in house made NiTi powders and as-printed NiTi samples. X-ray diffraction results show that the as-printed samples exhibit a dominant austenitic B2 phase and a minor in situ formed Ni4Ti3 phase. The as-printed samples demonstrate a strong B2 (001) orientation texture along the building direction. Differential scanning calorimetry results show asymmetric phase transformations: a two-step of B2 → R → B19′ phase transformation during cooling but a one-step B19′ → B2 transformation during heating. Excellent superelasticity and large reversible strain have been observed. The as-printed NiTi samples achieve a fracture tensile stress of 1411.0 ± 59.3 MPa and an elongation of 11.8 ± 0.9%. The observed tension-compression asymmetry has been also discussed.

The effect of superelastic strain on the damping capacity in columnar-grained Cu-Al-Mn shape memory alloy

Columnar-grained Cu-Al-Mn shape memory alloy (SMA) can experience high superelastic strain exceeding 7% and exhibits a nearly 100% superelastic recovery ratio due to its straight grain boundaries and strong texture along the solidification direction, which allows it to reach the strain level of a single crystalline Cu-based or Ni-Ti SMAs and possesses a potential for damping applications. In this paper, the damping capacity of Cu-Al-Mn SMA in various deformation conditions during a strain-controlled superelastic cyclic tensile process was studied. The anisotropy characteristics and the cyclic effect of damping capacity were analysed. The damping capacity of columnar-grained Cu-Al-Mn SMA increases as the loading strain increases in superelastic tensile processes, and a large damping capacity anisotropy occurs. At low loading strain (<6%), T-45° sample (45° is the angle between the loading direction and the axis of columnar grains) has the largest energy dissipation per unit recovery strain, ΔW1%, due to its high-stress level. At high loading strain (>6%), T-0° and T-15° samples can achieve a good damping capacity because of high superelasticity and plasticity. In addition, the damping capacity of columnar-grained Cu-Al-Mn SMA along the solidification direction has a significant cyclic effect. At the same loading strain (4%–10%), ΔW1%, and loss angle, ΔW/2πW first decrease to the minimum, then increase to the maximum and, finally, decreases as the number of cycles increases. The minima and maxima of ΔW1% and ΔW/2πW at different strain amplitudes correspond to the similar residual strain ratio, εr/εt. The change curves of damping capacity related to εr/εt can be divided into three stages: the stable damping capacity stage with low residual strain; the rapidly increasing damping capacity stage; the sharp cumulative stage of residual martensite with limited values of strain damping capacity.

Multifunctional Non‐Equiatomic High Entropy Alloys with Superelastic, High Damping, and Excellent Cryogenic Properties

A new class of non‐equiatomic FeNiCoAlTaB (NCATB) high entropy alloy (HEA) is introduced, which exhibits tunable properties from cryogenic/ambient superelasticity to ultra‐high strength through controlling the nature or type of martensite. In the current NCATB‐HEA alloy system, depending on the size of γ’‐Ni3Al (L12) precipitates, thin‐plate, lenticular, butterfly, and lath‐like martensite can form. When thin‐plate thermoelastic martensite is favored, a superelastic strain of about 0.025 (ambient) and ≈0.01 (cryogenic) is achieved with a high yield stress of ≈800 MPa and a high‐damping effect (10 times higher than Cu–Al–Ni superelastic alloy). While for butterfly and lath‐like martensite dominated NCATB‐HEA, an ultra‐high yield stress of around 1.1 GPa is achieved while no superelasticity is demonstrated. This current alloy system helps to expand the application domain of HEAs, for example, into high‐damping applications, robust actuators, space exploration, and other structural material applications.

A comparative study of a NiTi alloy subjected to uniaxial monotonic and cyclic loading-unloading in tension using digital image correlation: The grain size effect

The present digital image correlation study characterised the local axial and shear strain fields of a 56Ni44Ti wt.% shape memory alloy with an average grain size of 100 μm, under uniaxial monotonic and cyclic loading-unloading in tension. To elucidate the grain size effect, the results were compared with a previous investigation of the same alloy with an average grain size of 10 μm. The maximum local axial strain rate signified the direction and extent of the localised transformation. The widened single inclined transformation band and multiple criss-crossing patterns assist in straightening the sample edge by releasing an in-plane moment instigated by local shear strains. Electron back-scattering diffraction analyses showed that the plastic strain within the B2 grains and the remnant B19′ variants account for the residual strains after unloading. Smaller grain sizes correspond to greater constraint from grain boundaries, higher interfacial energy and higher elastic strain energy barrier for transformation, and smaller intragranular heterogeneity of plastic deformation. This is reflected in the increases to the transformation start stress, stress level and stress-strain slope within the macroscopic stress plateau region and smaller complete transformation strain, super-elastic and residual strains upon unloading.

The precipitation behaviors, martensite transformation and superelasticity in the aged Ni-rich Ti[sbnd]Ni alloy with the assist of super-high stress

The precipitation of Ti3Ni4 phase during aging under the assist of the super-high stress in the Ni-rich TiNi alloy was investigated. The Ti3Ni4 precipitated in the matrix, accompanied by the generation of dislocations as a consequence of the applied super-high stress of several GPa level. The martensitic transformation behaviors changed from one-stage martensitic transformation to multiple-stage martensitic transformation with the changing of the applied stress level. The external applied super-high stress can result in the plastic deformation during aging, which can serve as the work hardening and further increase the micro-hardness, with an exception of the precipitation strengthening. In addition, the accumulation of larger numbers of dislocations and the generation of the localized stress field around the Ti3Ni4 precipitate during aging due to the applied stress should be a certain responsible for the evolution of superelasticity strain.

Superelasticity and functional fatigue of single crystalline FeNiCoAlTi iron-based shape memory alloy

The development of Iron-based shape memory alloys is primarily motivated by the need for a cost-effective alternative to NiTi. This work explores the superelastic and functional fatigue properties of Fe42.5Ni30Co15Al10Ti2.5 SMA. Single crystalline samples were subjected to various heat treatments to optimize the precipitation content and achieve superelastic response. Precipitation heat treatments between 180 and 200 min at 600 °C were conducive to superelasticity with large recoverable strains (~7%) and high levels of recovery (>95% recovery). Heat treatments at lower temperatures altered the strength but without achieving superelasticity. A phenomenon which was attributed to plastic slip resulting from the critical transformation stress being higher than the slip resistance. Treatment times beyond 200 min induced a brittle response and premature fracture prior to transformation. Cyclic experiments were also conducted following different heat treatments and loading conditions to study the functional fatigue properties. In all cases, limited degradation of superelastic properties took place in the first 10 cycles. However, with continued loading, reduction of superelastic strains and loss of functionality was observed. This was attributed to the gradual buildup of local irrecoverable strains due to plasticity at the martensite boundary which acts to pin the interface and prevent reverse transformation.

Microstructure, superelasticity and shape memory effect by stress-induced martensite stabilization in Cu–Al–Mn–Ti shape memory alloys

In this study, the microstructure, martensitic transformation, superelasticity, stabilization of stress-induce martensite and the corresponding shape memory effect of Ti-doped Cu–Al–Mn shape memory alloys were investigated. The results show that the present Cu–Al–Mn–Ti alloys consist of two types of L21 phases, including Cu2AlMn parent and Cu2TiAl precipitate. The dispersive L21–Cu2TiAl precipitates strengthen the stabilization of stress-induced 2H(γ′1) martensite from L21–Cu2AlMn parent. Thus, all studied alloys show not only superelasticity at deformation, but also shape memory effect after unloading by heating. Consequently, the Cu–12.9Al–8.3Mn–0.5Ti alloy (wt%) possesses the best superelastic strain up to 4.6% and a shape memory effect of 1.5% under a pre-deformation of 11%, while Cu–12.8Al–7.7Mn–2.6Ti alloy (wt%) shows the best shape memory effect up to 2.5% simultaneously showing 1.9% of superelastic strain with a pre-deformation of 10%.

Superelastic response in 〈1 2 2〉-oriented single crystals of FeMnAlNi shape memory alloy in tension and compression

The superelastic response of the Fe43.5Mn34Al15Ni7.5 single crystalline shape memory alloy, oriented along the 〈122〉 direction was investigated. The quenched single crystals demonstrated superelastic strains of 4.0% in tension and 2.0% in compression. Ageing at 200 °C for 3 h led to the suppression of superelasticity in compression. An asymmetry in the tension and compression properties, probably associated with the ability of the martensite crystals to detwin under stress, was detected. It is likely that the detwinning of γ′-martensite in compression leads to an increase in the mechanical hysteresis Δσ and the stresses σcr required for the onset of strain associated with the stress-induced martensitic transformation in quenched crystals, suppressing superelasticity in the aged crystals. Tension without the detwinning of γ′-martensite result in a weak influence of coherent B2 (bcc/NiAl) precipitates on the mechanical hysteresis Δσ.

Special Features of L21–L10 Thermoelastic Martensitic Transformations in Ni54Fe19Ga27 Single Crystals in Tension

Special features of L21–(14М)–L10 martensitic transformations (MT) are experimentally established for [001]-oriented Ni54Fe19Ga27 (аt.%) single crystals under tensile stress, including staging of superelasticity curves, stress hysteresis, and strain level of martensite formation depending on the test temperature and MT sequence. An additional stage of inelastic strain with high strain hardening coefficient and a stress drop arise on the σ(e) curves in the interval of changes of the stress-induced MT sequence from L21–14M–L10 to L21–L10 (320–400 K) at superelasticity (SE). Special features and physical reasons of this stage and of the stress drop related with the processes of origin and detwinning of L10 martensite during stress-induced L21–L10 transformation are established. The stage of inelastic strain and the stress drop are absent at the L21–14М–L10 MT in the [001]-oriented single crystals or at the L21–L10 MT at other orientations, where detwinning of L10 martensite is suppressed by geometrical reasons.