Croatian

A three-dimensional phenomenological model for shape memory alloys including two-way shape memory effect and plasticity

In recent years, ferromagnetic shape memory alloys (FSMAs) have been widely investigated in many countries [1–4]. Of the developed FSMAs, NiMnGa alloys have been found to exhibit nearly 10% magnetic-fieldinduced strain which is the maximum obtained MFIS in these FSMAs [5]. Based on NiMnGa alloys, researchers have developed new ferromagnetic shape memory alloys, i.e., NiMnFeGa alloys [6, 7]. The structure, shape memory effect, and magnetic properties of Fe-doped NiMnGa alloys have been studied. It is assumed that the addition of Fe element enhances the alloy plasticity without sacrificing its magnetic and thermoelastic properties [6]. However, little work has been devoted to studying transformation behavior and the mechanical properties of polycrystalline Fe-doped NiMnGa alloys. The aim of the present letter is to study the effect of additions of Fe on transformation behavior and the hardness of NiMnGa alloys. A range of alloys with the nominal composition of Ni48.7Mn30.1−x Fex Ga21.2 (at%) (x = 0, 2, 5, 8, 11), each weighing about 80 g, were prepared. The alloys were fabricated by arc melting under an argon atmosphere using appropriate quantities of nickel (99.99% purity), manganese (99.9% purity), iron (99.9% purity) and gallium (99.99% purity). The ingots obtained were sealed in a quartz tube under a vacuum, followed by annealing at 1123 K for 24 hrs and quenched into ice water for homogeneity. Rectangular samples 4×4×6 mm were cut from the post-treated ingots for hardness measurements. Some 10 mg samples were taken from the ingots for DSC measurements. The transformation temperatures were determined by differential scanning calorimetry using a TA-2920. The heating and cooling rates were 10 K/min. The crystal structure at room temperature was confirmed by powder X-ray diffraction using D/MAX-RA X-ray diffractometer with Cu Kα radiation. Micro Vickers hardness (HV) of eight points was measured using a Akashi HM-102 Vickers Hardness Tester. The applied load and time were 200 g and 15 s, respectively. The HV values were averaged excluding the highest and lowest values [8]. Table I summarizes the transformation temperatures measured by DSC analysis in Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen, at room temperature, that the parent phase and martensite coexist in the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, whereas in the other three alloys only the parent phase exists. The results are in agreement with X-ray diffraction results in Fig. 1. X-ray patterns depict that the cubic parent phase and tetragonal martensite coexist when x = 0, 2, while only the cubic parent phase exists when x = 5, 8, 11. As shown in Table I, the thermal hysteresis of these five alloys are −1.38, −3.30, 0.46, 2.60 and 2.20 ◦C, respectively. For the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, the thermal hysteresis is negative. Hence Fe-doped NiMnGa alloys exhibit very narrow thermal hysteresis which means that the chemical energy of the phase transformation is very small. The narrow thermal hysteresis also shows that the driving force for inducing the martensitic transformation in Fe-doped NiMnGa alloy is very small. Wayman [9] divided thermal transformations into two types: the first is Af > As > Ms > Mf while the second is Af > Ms > As > Mf. From Table I it can be seen that the first two alloys belong to the second type of thermal transformation, while the other three alloys belong to the first type of thermal transformation. In addition, only single forward and reverse transformation are determined by DSC curves. Fig. 2 depicts the phase transformation temperatures vs. Fe content for Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen that with increase of Fe content all the transformation temperatures, Ms (martensitic transformation start temperature), Mf (martensitic transformation finish temperature), As (reverse martensitic transformation start temperature), and A f (reverse martensitic transformation finish temperature), decrease. The relationship between Ms temperature and Fe content is nearly linear, varing from 26.40 ◦C for Ni48.7Mn30.1Ga21.2 without Fe to −55.00 ◦C for Ni48.7Mn19.1Fe11Ga21.2 which has the maximum Fe content. Fig. 3 exhibits the relationship between transformation temperatures and valence electron numbers per atom of Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11) and the shape of the curves is similar to Fig. 2. The tendency is clear that all transformation temperatures decrease with increase of electronic concentration in these five alloys, and the increasing rate is also basically consistent with the change of the electronic concentration. Here, it is assumed that the number of valence electrons per atom for Ni, Mn, Fe, and Ga atoms is 10(3d84s2),7(3d54s2),8(3d64s2) and 3(3s24p1), respectively. It is believed that two factors, the conduction electronic concentrations [10] and the size factor [11],

The objective of this study is to evaluate the effect that mechanical training has on the properties of NiTi based shape memory alloys. The unique mechanical behavior of shape memory alloys, which allows the material to undergo large deformations while returning to their original undeformed shape through either the shape memory effect or superelastic effect, has shown potential for use in seismic design and retrofit applications for civil engineering structures. However, cyclic loading has been shown to degrade the energy dissipation capacity and decrease the recentering capability of the material due to fatigue effects. It has been recommended that mechanical training of superelastic shape memory alloys prior to use in applications can limit these fatigue effects. A factorial experimental design is employed to explore the optimal number of mechanical training cycles, strain level of training, and the effect of the loading rate after training in order to minimize the degradation in the loading plateau stress, residual strain, and equivalent viscous damping properties. The results presented can serve as a guide to optimizing the properties of NiTi shape memory alloys for seismic applications. The ability to obtain stable properties of shape memory alloys under a specified training schedule further supports the eventual implementation of the material into actual building and bridge systems as seismic design and retrofit devices.

The Influence of Long-Term Storage on the Functional Properties of Shape Memory Alloys

This study evaluates the properties of superelastic shape memory alloys under cyclical loading to asses their potential for applications in seismic resistant design and retrofit of civil engineering structures. Shape memory alloy bars are tested to evaluate the effect of bar size (diameter) and loading history on the strength, equivalent viscous damping, and recentering properties of the shape memory alloys in superelastic form. The bars are tested under both quasi-static and dynamic loading. The results show nearly ideal superelastic properties can be obtained in large diameter shape memory alloy bars. However, comparing these results to previous studies, the more common wire form of the shape memory alloys show higher strength and damping properties compared with the large bars. The recentering capabilities (based on residual strains) are not affected by the section size of the bar. Overall, the damping potential of superelastic shape memory alloys is low for large diameter bars, typically less than 7% equivalent viscous damping. Degradation of the superelastic properties of the shape memory alloys occurs for cyclical strain greater than 6%, leading to increased residual strains and reduction in energy dissipated. Finally, strain rate effects are evaluated by subjecting the shape memory alloys to loading rates representative of typical seismic loadings. The results show that increased loading rates lead to slight decreases in the equivalent damping, but have negligible effect on the recentering of the shape memory alloys.

Shape memory alloys (SMAs) exhibit the ability to absorb large dynamic loads and, therefore, are excellent candidates for structural components where impact loading is expected. Compared to the large amount of research on the shape memory effect and/or pseudoelasticity of polycrystalline SMAs under quasi-static loading conditions, studies on dynamic loading are limited. Experimental research shows an apparent difference between the quasi-static and high strain rate deformation of SMAs. Research reveals that the martensitic phase transformation is strain rate sensitive. The mechanism for the martensitic phase transformation in SMAs during high strain rate deformation is still unclear. Many of the existing high strain rate models assume that the latent heat generated during deformation contributes to the change in the stress-strain behavior during dynamic loading, which is insufficient to explain the large stress observed during phase transformation under high strain rate deformation. Meanwhile, the relationship between the phase front velocity and strain rate has been studied. In this dissertation, a new resistance to phase transformation during high strain rate deformation is discussed and the relationship between the driving force for phase transformation and phase front velocity is established. With consideration of the newly defined resistance to phase transformation, a new model for phase transformation of SMAs during high strain rate deformation is presented and validated based on experimental results from an austenitic NiTi SMA. Stress, strain, and martensitic volume fraction distribution during high strain rate deformation are simulated using finite element analysis software ABAQUS/standard. For the first time, this dissertation presents a theoretical study of the microscopic band structure during high strain rate compressive deformation. The microscopic transformation band is generated by the phase front and leads to minor fluctuations in sample deformation. The strain rate effect on phase transformation is studied using the model. Both the starting stress for transformation and the slope of the stress-strain curve during phase transformation increase with increasing strain rate.

Shape memory alloys (SMAs) exhibit unique properties, making them highly valuable in various industrial and medical applications. These materials are characterized by their ability to return to a predeformed shape upon exposure to specific thermal or mechanical stimuli. The critical properties of SMAs include the shape memory effect, pseudoelasticity, and high damping capacity. The shape memory effect is the ability of SMAs to undergo deformation at one temperature and recover their original shape upon heating. This effect is divided into one-way and two-way memory effects, allowing the material to remember and switch between two distinct shapes at different temperatures. Pseudoelasticity, or superelasticity, enables SMAs to resist substantial strains and revert to their original form upon unloading at a constant temperature. Additionally, SMAs exhibit high damping capacity, which makes them effective in absorbing and dissipating energy, thereby reducing vibrations and noise. These properties result from the reversible martensitic transformation within the material’s crystal structure. Common SMAs include Ni–Ti (Nitinol), Cu-based, and Fe-based alloys, each offering distinct advantages in terms of mechanical properties, corrosion resistance, and cost-effectiveness. The exceptional properties of SMAs have led to their widespread use in applications such as medical devices, actuators, aerospace components, and vibration dampers. This chapter highlights the fundamental properties of SMAs that underpin their diverse and critical applications in modern technology.

High temperature shape memory alloys offer numerous potential applications in industrial domains like aeronautics. Even if up to now, none of the studied alloys have found a place in airplane turbines, research in this field is still active. Starting from the well-known “room temperature” shape memory alloy NiTi, it has been demonstrated that the addition of a ternary element such as gold in substitution of nickel greatly enhances the temperatures of the martensitic transformation. In the binary TiAu compound, the martensite start temperature can attain 875 K with satisfying reversibility and cycling stability. From lattice parameters measurements, it has been shown that the maximum transformation strain can reach 10.75 % for Ti47Au53 alloy, which is comparable to that of the NiTi alloy. However, to the best of our knowledge, quantitative measurements of the recoverable strain by shape memory effect are not available in the literature. We present here some quantitative results of shape memory effect associated to this phase transformation in Titanium-Gold alloys measured after compression tests.

Shape memory polymers (SMPs) usually have a one-way shape memory effect. In this paper, an easy-operating method to realize a two-way shape memory effect was demonstrated in a ring-shaped bilayer structure where the two layers are SMPs with different thermal transition temperatures. By designing specific thermomechanical processes, the mismatched deformation between the two layers leads to a morphology change of ring-shaped bilayer structures from a smooth ring to a gear-like buckling shape under cooling and a reversible recovery to the smooth shape under heating. Such a morphology change is ascribed to occurrence and recovery of thermoelastic buckling. This method was validated by finite element simulation. We experimentally investigated the influence of pre-strain on buckling, and it was found that both the buckling occurrence and recovery temperature vary with pre-strain. Furthermore, considering a ring-shaped SMP–SMP bilayer structure, finite element analysis was conducted to study the influence of film thickness and modulus ratio of two layers on buckling behavior. The results showed that the critical buckling wavelength was greatly influenced by film thickness and modulus ratio. We made a theoretical analysis that accorded well with the numerical results.

One way Shape Memory Effect (SME) is not suitable mechanism for application to the repeated actuation of an Shape Memory Alloy(SMA) wire because the host structure does not return to its initial shape after it cools down. In the present study, the two-way SME under residual stress is considered. A structure using the two-way effect returns to its initial shape by increasing or decreasing temperature under an initially given residual stress. A thermo-mechanical constitutive equation of SMA proposed by Lagoudas et al. was employed in the present study. Laminated composite beams and plates are considered as simple morphing structural components. The modeling of beams and plates are based on first-order shear deformable laminated composite beam and plate theories with large deflections. Numerical results of fully coupled SMA-composite structures are presented. The proposed actuation mechanism based on the two-way SMA effect and a simulation algorithm can be used as a powerful morphing mechanism and simulation tool for structures.

Martensitic transformation in stoichiometric NiMn and Ni–Mn-X alloys: A first principles study

Evaluation of Shape Memory Effect and Wear Characteristics of Cu-Al-Be-Mn Quaternary Shape Memory Alloys

It is illustrated, that our method [1-3] (developed for the separation of pressure dependence of the elastic and dissipative energy terms) can also be applied for the determination of stress-dependence of the non-chemical (elastic and dissipative) free energy contributions to the martensitic phase transformations. For this the results of Tanaka et al. [4], on the effect of constant applied tensile stress on martensitic phase transformations, in Ti-51at%Ni shape memory alloys (up to 400 MPa) have been used. Supposing that the volume derivatives of the elastic energies differ from zero at the martensite start (and austenite finish) as well as at the martensite finish (and austenite start) temperatures, it was shown that the dissipative energy terms were approximately zero for the B2-R transformation. For the R-B19' transition these energies are not negligible and are of the order of 1 x 10 7 J/m 3 . The stress dependence of the elastic energy terms of the B2-R and R-B19' transitions have also been determined from the original data. The results are consistent with the effect of hydrostatic pressure on the martensitic transformation measured in a similar alloy [21.

Phase transformation and structure of functionally graded Ni–Ti bi-layer thin films with two-way shape memory effect

The stress created by the austenite transformation of mechanically constrained NiTi shape memory alloys (SMAs) significantly exceeds that required to deform (detwin) its low temperature martensitic phase. The one-way shape memory effect can therefore be used to create antagonistic, fully reversible flexural shape morphing structures by assembling opposing pairs of linear contractile SMA actuators. These structures possess two distinct inactive configurations, neither of which requires continuous power for shape retention making them potentially suitable for long-term applications. Here, the shape changes of a representative antagonistic flexural unit cell have been experimentally evaluated, and the effect of the pre-strain of near equiatomic NiTi alloys on the actuation strain has been analyzed and discussed. The predicted deformations are then successfully compared to the response of prototype actuators.