Theoretical Transformation Strain Research Articles

On the crystallographic texture and torsional behavior of NiTi shape memory alloy processed by laser powder bed fusion: Effect of build orientation

There is a growing industrial and academic interest in additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) both for producing parts with complex shapes and for tailoring the microstructure and properties of the resulting parts. This interest when it comes to NiTi-based shape memory alloys (SMA) has a third reason due to the poor machinability and need for several steps of additional treatments to create functional parts from these alloys. Generating the printability map and achieving fully dense parts is one of the priorities in AM communities, despite the aforementioned benefits of AM that open new doors for controlling the microstructure and thereby enhancing thermomechanical properties of additively manufactured components. Motivated by crystallographic texture-dependence of NiTi thermomechanical properties, a build orientation approach is utilized for controlling the crystallographic texture of LPBF-processed NiTi specimens. The microstructural and phase analyses of parts fabricated at varying angles show the significant effect of building orientation on the crystallographic texture of the as-printed samples. The mechanism of the effect of the proposed approach for controlling texture is discussed. SMA rotary actuator is a potential application of this methodology. We assessed the thermomechanical properties of the as-printed rod specimens under simple torsion using 3D digital image correlation (DIC) to capture the shear strain field. The recovery shear strains of the samples are then correlated to the crystallographic textures. It is shown that the recovery strain of the textured NiTi samples is in good agreement with the theoretical transformation strains obtained from lattice deformation theory.

Orientation dependence and tension-compression asymmetry of shape memory effect in Ti50Ni40Cu10 single crystals

Transformation strain under stress in Ti50Ni40Cu10 alloy single crystals, with a two-step B2–B19–B19′ martensitic transformation (MT), was investigated along three different crystallographic orientations, i.e., [1‾17], [011] and [1‾23], in order to reveal the orientation dependence and tension-compression asymmetry of the shape memory effect (SME) at different stress levels. It was shown that the SME depends on the crystal orientation and the deformation method (tension and compression). The maximum value of the SME εSME under tensile stresses turned out to be higher than the theoretical transformation strain resource ε0. In [1‾17]-oriented crystals, εSME = 6.1% exceeded ε0 = 3.4% by 2.7%; in [011]-oriented crystals εSME = 9.1% exceeded ε0 = 7.14% by 1.96%; in [1‾23]-oriented crystals εSME = 8.2% exceeded ε0 = 7.22% by 0.98%. Higher experimental values for εSME than the theoretical transformation strain resource ε0 at B2–B19–B19′ MT was found to be associated with elastic mechanical twinning along the {110}В19 planes of B19-martensite. Under compression, in the studied orientations, εSME was less than ε0 for the B2–B19–B19′ MT. This was due to lower stresses in the initial B2-phase and local plastic flow during development of an MT under stress at stresses of more than 200 MPa.

On the Decrease in Transformation Stress in a Bicrystal Cu-Al-Mn Shape-Memory Alloy during Cyclic Compressive Deformation.

The evolution of the inhomogeneous distribution of the transformation stress () and strain fields with an increasing number of cycles in two differently orientated grains is investigated for the first time using a combined technique of digital image correlation and data-driven identification. The theoretical transformation strains () of these two grains with crystal orientations and along the loading direction are 10.1% and 7.1%, respectively. The grain with lower has a higher initially and a faster decrease in compared with the grain with higher . The results show that the grains with higher might trigger more dislocations during the martensite transformation, and thus result in greater residual strain and a larger decrease in during subsequent cycles. Grain boundary kinking in bicrystal induces an additional decrease in transformation stress. We conclude that a grain with crystal orientation that has high transformation strain and low transformation stress (with respect to loading direction) will exhibit stable transformation stress, and thus lead to higher functional performance in Cu-based shape memory alloys.

Martensite aging in 〈001〉 oriented Co49Ni21Ga30 single crystals in tension

Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stress-induced martensite under compressive loads transformation temperatures can be increased significantly, qualifying Co–Ni–Ga for HT-actuation. The increase in transformation temperatures is related to a change in chemical order recently validated via neutron diffraction experiments. Since SMAs show distinct tension–compression asymmetry in terms of theoretical transformation strains and bearable stresses, understanding the impact of martensite aging in tension is crucial for future applications. The current results indicate that martensite aging in tension provides for a further improvement in functional properties.

In Situ Neutron Diffraction Analyzing Stress-Induced Phase Transformation and Martensite Elasticity in [001]-Oriented Co49Ni21Ga30 Shape Memory Alloy Single Crystals

Recent studies demonstrated excellent pseudoelastic behavior and cyclic stability under compressive loads in [001]-oriented Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs). A narrow stress hysteresis was related to suppression of detwinning at RT and low defect formation during phase transformation due to the absence of a favorable slip system. Eventually, this behavior makes Co–Ni–Ga HT-SMAs promising candidates for several industrial applications. However, deformation behavior of Co–Ni–Ga has only been studied in the range of theoretical transformation strain in depth so far. Thus, the current study focuses not only on the activity of elementary deformation mechanisms in the pseudoelastic regime up to maximum theoretical transformation strains but far beyond. It is shown that the martensite phase is able to withstand about 5% elastic strain, which significantly increases the overall deformation capability of this alloy system. In situ neutron diffraction experiments were carried out using a newly installed testing setup on Co–Ni–Ga single crystals in order to reveal the nature of the stress–strain response seen in the deformation curves up to 10% macroscopic strain.

Unusual reversible twinning modes and giant superelastic strains in FeNiCoAlNb single crystals

The superelastic response of the Fe41Ni28Co17Al11.5Nb2.5 (at. %) single crystals oriented along the [001] direction was investigated in tension at various temperatures after aging at 973K for 0.5h. The aging treatment led to the formation of ordered γ' particles with particle sizes of less than 3nm. Abnormally large reversible strain of up to 15.3% was observed at 77K, the highest ever reported for an iron-based shape memory alloys. It has been revealed that this giant superelastic strain, significantly higher than the theoretical transformation strain, is associated with γ-α' martensitic transformation and elastic mechanical twinning of <011>{110} type.

Effects of crystallographic orientation on the superelastic response of FeMnAlNi single crystals

The effects of crystal orientation on the tensile superelastic response of the Fe43.5Mn34Al15Ni7.5 single crystalline shape memory alloy, oriented along the <100> and <123> directions were investigated. The single crystals aged at 200°C for 3h demonstrated superelastic strains of 3.6% along the <100> orientation and 7.8% along the <123> orientation. Although the theoretical transformation strain in the <100> orientation is higher than that in the <123> orientation, the <123> oriented single crystals demonstrate better superelastic recovery in tension due to a higher number of active martensite variants, which promote better accommodation of internal strains and help minimize defect formation during transformation.

High-temperature functional behavior of single crystal Ni51.2Ti23.4Hf25.4 shape memory alloy

In this work the functional behavior of the new Ni51.2Ti23.4Hf25.4 high temperature shape memory alloy is investigated along three crystal orientations in compression and compared with the polycrystal behavior. Transformation temperatures were measured following specific aging treatments with the aim to optimize the shape memory and pseudoelastic behaviors. Two aging treatments were then selected in order to fulfill the optimum pseudoelastic behavior (500°C/4 h), and the maximum transformation temperatures (550°C/10 h) without compromising the alloy functionality. High temperature X-ray diffraction was utilized to determine the crystal orientations in the austenite phase for the single crystal specimens. The martensite structure was found to be the B19 orthorhombic, in contrast with monoclinic structure for lower Hf contents. Digital image correlation was successively used during isobaric strain-temperature experiments on the single crystal and polycrystal specimens. Large strain heterogeneities were found for the single crystals which pair with X-ray diffraction data that show un-transformed austenite below the martensite finish temperature. This reveals the heterogeneity of the austenite–to–martensite transformation for the present NiTiHf alloy which explains the divergence between the theoretical and experimental transformation strains. In addition, using digital image correlation it is possible to capture the local strain fields associated with fully transformed specimen regions. The transformation strains calculated locally are in close agreement with the theoretical transformation strains calculated with lattice deformation theory for the cubic austenite to the orthorhombic martensite phase transformation.

Shape Memory Response of Polycrystalline NiTi12.5Hf Alloy: Transformation at Small Scales

The transformation behavior of NiTiHf alloys is intriguing. In NiTiHf alloys, the experimental transformation strains have been reported to be considerably lower than theoretical transformation strains. In this study, the transformation strain is established with very careful strain measurements at small scales in isobaric and isothermal experiments. Because of the heterogeneity of strain distributions, the results depend on the sub-region considered. The measured local transformation strain can be as high as 6.0 % in compression which is in very good agreement with theoretical calculations for NiTi12.5Hf. The comprehension of NiTi12.5Hf alloy was furthered upon extensive microstructural characterization including high-resolution electron microscopy, establishing the volume fractions of precipitates and twin type. The volume fraction of precipitates is similar to that of Ni-rich binary NiTi alloys. Meanwhile, the twinning modes in the martensite are compound and Type I twins which were used in the theoretical calculations of transformation strains. This material also generates a high work output and represents a foundation for understanding higher Hf compositions.

The superelastic anisotropy in a NiTi shape memory alloy thin sheet

NiTi shape memory alloys are used in practice in their polycrystalline form and usually consist of strong textures. The present research aims to understand the anisotropy in the stress–strain curve of a superelastic NiTi thin sheet. Tensile tests were conducted along various directions between rolling and transverse directions. It was observed for the first time that, although the shape of the stress–strain curve is strongly orientation dependent, the shape recovery strain which is as high as 9% is nearly orientation independent. Through texture and crystallographic analysis, the most favorably oriented martensite variants with respect to each deformation direction were determined and the corresponding theoretical transformation strains were calculated. The relation between the theoretical transformation strain and the shape of the stress–strain curve was further analyzed. Results show that the magnitude of the plateau-strain in the stress–strain curve is determined by the formation of the most favorably oriented martensite variants, while the magnitude of shape recovery strain which is independent of the plateau-strain is determined by the reverse transformation of both favorably and less-favorably oriented martensite variants.Two approaches were taken to determine the most favorable martensite variants and the corresponding theoretical transformation strains. One is based on the resulted transformation strain according to austenite–martensite lattice correspondence, and the other is based on the Schmid factor. Interestingly, both methods have led to the same sets of martensite variant hence the same values of theoretical transformation strain. This is discussed based on recent observations of the stress-induced martensite in NiTi.

Effects of aging on the shape memory behavior of Ni-rich Ni50.3Ti29.7Hf20 single crystals

The shape memory properties of solutionized and aged Ni-rich Ni50.3Ti29.7Hf20 single crystals were investigated along the [001], [011] and [111] orientations in compression. The effects of crystal orientation and aging temperature on the transformation strain, thermal hysteresis and Clausius–Clapeyron (CC) relation were determined. Aging at 550°C for 3h introduced coherent 10–20nm precipitates in the matrix, which substantially improved the shape memory and mechanical properties of the Ni50.3Ti29.7Hf20 crystals. [001]-oriented single crystals showed high dimensional stability under stress levels as high as 1500MPa in both the solutionized and aged conditions, but with transformation strains of <2%. Thermal treatments can be used to tailor the transformation temperatures over a wide range with the martensite start temperature varying from −25°C in the solutionized case to 123°C by aging at 650°C for 3h. Compared to the solutionized condition, thermal hysteresis was reduced after aging at 550°C/3h, but increased with aging at 650°C. Perfect superelasticity with recoverable strain of >4% was observed for solutionized and 550°C/3h aged single crystals along the [011] and [111] orientations, and general superelastic behavior was observed over a wide temperature range. In contrast, aged [001]-oriented single crystals have a very high CC slope, in the range of 30–40MPa°C−1, which results in a lack of superelasticity. Theoretical transformation strains were calculated by using the energy minimization method and lattice deformation theory. The calculated transformation strains were higher than the experimentally observed strains since the calculated strains could not capture the formation of martensite plates with (001) compound twins.

Shape memory behavior and tension–compression asymmetry of a FeNiCoAlTa single-crystalline shape memory alloy

A 〈100〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe–28Ni–17Co–11.5Al–2.5Ta (at.%) single-crystalline material oriented along the 〈100〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed.

Magnetic Field‐Induced Phase Transformation in NiMnCoIn Magnetic Shape‐Memory Alloys—A New Actuation Mechanism with Large Work Output

AbstractMagnetic shape memory alloys (MSMAs) have recently been developed into a new class of functional materials that are capable of magnetic‐field‐induced actuation, mechanical sensing, magnetic refrigeration, and energy harvesting. In the present work, the magnetic &amp;!hyphen;field‐induced martensitic phase transformation (FIPT) in Ni45Mn36.5Co5In13.5 MSMA single crystals is characterized as a new actuation mechanism with potential to result in ultra‐high actuation work outputs. The effects of the applied magnetic field on the transformation temperatures, magnetization, and superelastic response are investigated. The magnetic work output of NiMnCoIn alloys is determined to be more than 1 MJ m−3 per Tesla, which is one order of magnitude higher than that of the most well‐known MSMAs, i.e., NiMnGa alloys. In addition, the work output of NiMnCoIn alloys is orientation independent, potentially surpassing the need for single crystals, and not limited by a saturation magnetic field, as opposed to NiMnGa MSMAs. Experimental and theoretical transformation strains and magnetostress levels are determined as a function of crystal orientation. It is found that [111]‐oriented crystals can demonstrate a magnetostress level of 140 MPa T−1 with 1.2% axial strain under compression. These field‐induced stress and strain levels are significantly higher than those from existing piezoelectric and magnetostrictive actuators. A thermodynamical framework is introduced to comprehend the magnetic energy contributions during FIPT. The present work reveals that the magnetic FIPT mechanism is promising for magnetic actuation applications and provides new opportunities for applications requiring high actuation work‐outputs with relatively large actuation frequencies. One potential issue is the requirement for relatively high critical magnetic fields and field intervals (1.5–3 T) for the onset of FIPT and for reversible FIPT, respectively.

Cyclic deformation behavior of single crystal NiTi

Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.