Shape Memory Effect Behavior Research Articles

4D printing of PET-G via FDM including tailormade excess third shape

In this paper, for the first time, the 4D printing capability and shape memory effect of PETG as a novel shape memory thermoplastic has been investigated with an option of emerging tailormade excess curved third shape during the shape recovery step. A self-bending through the primary printed layers was observed due to entropic force relief caused by functionally-graded polymer chain extension in each layer. Implementation of printing temperature and speed of 200◦C and 70 mm/s, respectively, results in an opaque appearance and a more curved third shape. On the other hand, a transparent appearance with a gentle curved permanent shape was achieved by the parameters of 240◦C and 10 mm/s. The observations are attributed to the effect of printing parameters on the degree of polymer chains’ extension and its difference for each layer. Both samples show an exemplary shape memory effect behavior with higher than 96 % total shape recovery.

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses

Enhancement of mechanical properties and shape memory effect of Ti–Cr–based alloys via Au and Cu modifications

The requirements for biomedical materials have been raised greatly due to the rapidly aging global population. Shape memory alloys (SMAs) are indeed promising materials for biomedical applications due to their controllable shape deformation via the manipulation of temperature and/or stress. This study investigated the enhancement of the fundamental mechanical properties and the shape memory effect (SME) in the Ti–Cr–based alloys via the modification of Au and Cu. The quaternary Ti–Cr–Au–Cu alloys were successfully manufactured by physical metallurgy methods and their phase constitutions, mechanical properties, SME, and superelastic (SE) behaviors have been investigated in this study. Cold–workability, which was enhanced by the introduction of the Au element, was elaborated by the phase constitutions of the alloys. The β–parent phase was stabilized to around body temperature by the introduction of the β–stabilizers of Cr, Au, and Cu, and the functionalities of the specimens were revealed at the operating temperature. Perfect SME at the shape recovery rate of 100% was practiced by the substitution of Au by Cu and the mechanical properties, such as strength and ductility, were also enhanced. Functional mappings of the fundamental mechanical properties, which could be a helpful tool for the investigations of the quaternary Ti–Cr–Au–Cu alloys, were constructed in this work.

Yielding mechanisms for mechano-chemo-thermal couplings in amorphous shape memory polymer undergoing molecular entanglement

Shape memory polymers (SMPs) have good properties of recovering their original shape in the presence of an external stimulus. However, the thermodynamic behaviors behind the yielding effects on mechano-chemo-thermal shape-memory effect (SME) are yet to be explored. In this study, molecular entanglement theory is employed to develop a thermodynamic model in terms of glass transition temperature. Based on the Arrhenius equation and Adam–Gibbs theory, an extended yield model is formulated to investigate the synergistic coordination of yield stress and stored strain energy on mechano-chemo-thermal SME and relaxation behavior in the amorphous SMPs. Furthermore, the effectiveness of this model is demonstrated by applying it to predict thermomechanical and mechano-chemo-thermal shape-recovery behaviors of the SMP, and the theoretical results are well validated by the experimental data reported in the literature. This study explores the working principle of stored strain energy in terms of molecular entanglement and yield stress, and describes a constitutive relationship between molecular entanglement and mechano-chemo-thermal coupling thermodynamics in the amorphous SMP.

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

Characterization of NiTi Shape Memory Alloy Sintered at Different Temperatures in Reducing Environment

NiTi has received significant interest as medical implant materials due to its shape memory effect behavior apart from its good biocompatibility and mechanical properties. The formidable challenge of obtaining single phase NiTi from elemental powders via solid state is due to oxidation problem of elemental powders and the oxygen atoms dissolve in NiTi matrix as interstitial impurities forming stable oxygen-rich TiNiOx. This may deterioriate the shape memory behavior of NiTi. This research investigates the use of MgH2 in combination with CaH2 as in-situ reducing agent to eliminate oxidation of the specimen during sintering both at lower and higher sintering temperatures. Here, the effect of sintering temperature on phase formation and transformation behavior of NiTi in reducing environment was studied. The phase formation was characterized by using x-ray diffraction (XRD) where the morphology and elemental analysis were characterized by using the scanning electron microscope (SEM) equipped with EDS. The martensitic transformation behavior was analyzed using differential scanning calorimeter (DSC). The use of MgH2 and CaH2 as reducing agent has a significant influence on the phase formation of NiTi synthesized via solid state especially at 930 °C, where almost single phase NiTi was formed with good transformation behavior. This reducing agent creates a conducive environment for the production of single phase NiTi.

A ‘cross-relaxation effects’ model for dynamic exchange of water in amorphous polymer with thermochemical shape memory effect

Dynamic exchange of water has been shown to effectively trigger thermally responsive shape memory effect (SME) in shape memory polymer (SMP) at room temperature. However, the working mechanism of cross-relaxation effects which are generated simultaneously from two stimulus fields of water and temperature, has not been understood. In this study, a ‘cross-relaxation effects’ model was formulated for describing water-induced relaxation, thermomechanical behavior and strain recovery of amorphous SMPs. Based on Maxwell’s principle, the ‘cross-relaxation effects’ model was firstly proposed to quantitatively separate the effect of dynamic exchange of water on the thermochemical SME in SMP. Then the water-induced relaxation and strain recovery have been simulated and predicted using this model. Finally, experimental data were used to verify the proposed model, and the numerical results were found to fit well with the experimental ones. This ‘cross-relaxation effects’ model is expected to provide an effective tool for precise design and control of water-induced SME and shape memory behaviors by means of the dynamic exchange of water.

Shape memory effect of mycologically destroyed wood

Wood is a natural smart material which possesses shape memory effect (SME). The paper deals with SME behaviour of sound and mycologically destroyed wood and shows the influence of cellulose, hemicelluloses, and lignin content on the wood ability to remember the temporary and to recover the permanent shapes. The research was carried out on sliced veneer samples (beech Fagus sylvatica L. and pine Pinus sylvestris L.) and rotten samples (pocket rot, brown rot) to reveal the influence of on the deformative conversions and quantities of SME. Results of research are shown that at moisture drop the higher values of strain fixity rate for all wood species and types of rot are noted (0.82 – 0.97). The more remarkable difference for sound and rotten wood is observed for strain recovery rate at temperature change. Despite the change of chemical composition for pocket rot the speed of recovery of permanent shape was almost the same as for sound wood. Results of this research can be used for the developing of new biocomposite materials based on wood.

Effect of W Contents on Martensitic Transformation and Shape Memory Effect in Co-Al-W Alloys

To clarify the effect of W contents on the shape memory effect (SME) in the Co-Al alloys and its influencing mechanism, the SME, martensitic transformation, and deformation behavior were studied in the Co-7Al-xW (x = 0, 4, 6, 9 wt pct) alloys. The results showed that the additions of W all deteriorated the SME in Co-7Al alloy when deformed at room temperature. However, when deformed in liquid nitrogen, the SME in Co-7Al alloy could be remarkably improved from 43 to 78 pct after the addition of 4 pct W, above which the SME decreased rapidly with the increase of W content although the yield strength of the parent phase rose due to the solution strengthening of W. The deterioration in SME induced by the excessive addition of W could be ascribed to its resulting significant drop of the start temperature of martensitic transformation.

Highly biobased thermally-stimulated shape memory copolymeric elastomers derived from lignin and glycerol-adipic acid based hyperbranched prepolymer

Copolymers containing as high levels of bio-derived components were obtained mainly from two industrial byproducts, glycerol and lignin. A series of hyper branched prepolymers (HyBP) were prepared from in the first step by melt condensation of glycerol (Gly, B2B32) and adipic acid (AA, A2) and the prepolymer structure was also modified by the addition of commercially available diisopropanolamine (DIPA, DB42) and tris(hydroxymethyl) aminomethane (THAM, CB31) units. The HyBP were reacted with lignin to form thermally-stimulated shape memory copolymeric elastomers in the second step in the same pot by melt polycondensation. The prepolymers were characterized chemically by FTIR, NMR, and ESI-MS. Thermal and rheological behaviors were analyzed by DSC, DMA, TGA and dynamic rheometry. The higher branching crosslinkers, DIPA and THAM, were shown to influence the chemical and thermal properties of the prepolymers. The hyperbranched biobased lignin-copolymers demonstrated good shape memory and elastic properties. The shape transition temperature (Ttrans) could be tuned by variations of Gly, DIPA and THAM proportions for applications under different temperature circumstances. This study demonstrates that renewable Gly and AA based prepolymers could be incorporated into lignin based copolymeric systems with high biobased contents acting as soft segment to perform shape memory effect behavior.

Remarkable Improvement of Shape-Memory Effect in a Co-31Ni-3Si Alloy by Ausforming

In order to improve the shape-memory effect (SME) in Co-Ni alloys, the influence of ausforming temperature on the SME, microstructures, and mechanical behavior in a Co-31Ni-3Si alloy was studied. The results show that the ausforming at 1073 K (800 °C) could remarkably improve the SME in Co-31Ni-3Si alloy. A large recovery strain of 2.3 pct was obtained after bent by 3.7 pct at 77 K (−196 °C). The increase of yield strength and the decrease of the critical stress for the stress-induced gamma to epsilon martensitc transformation are responsible for the remarkable improvement of SME. The results indirectly showed that the SME in Co-Ni alloys results from the stress-induced gamma to epsilon martensitic transformation, and their low yield strength account for their poor SME. It can be expected that the strengthening of matrix by other methods, such as solution, dispersion, and grain refinement hardening, will improve the SME of Co-Ni alloys.

Analytical modelling of functionally graded NiTi shape memory alloy plates under tensile loading and recovery of deformation upon heating

This paper presents an analytical model describing the deformation behaviour of functionally graded NiTi plates under tensile loading and their shape recovery during heating. The property gradient of the plate is achieved by either a compositional gradient or microstructural gradient through the thickness. Closed-form solutions are obtained for nominal stress–strain variations of such plates under uniaxial loading at different deformation stages. It is observed that the martensitic transformation occurs partially over the nominal stress gradient, unlike typical NiTi shape memory alloys. The curvature–temperature relations are established for complex shape memory effect behaviour of such plates during the recovery period. The analytical solutions are validated with relevant experimental results.

Shape memory effect of TiNi-based springs trained by constraint annealing

This study investigated shape memory effect (SME) behaviors of TiNi-based shape memory alloys springs trained by constraint annealing treatments. The results indicated that SME behaviors were determined by constraint annealing sequences and the chemical composition of alloys. Springs made by Ni-rich TiNi binary alloys showed both single-way SME (SW-SME) and two-way SME (TW-SME) depending on the constraint annealing sequences, while those made by TiNiCu alloys only showed SW-SME. The mechanism and stability of TW-SME induced by constraint annealing were discussed. The occurrence of TW-SME was found to be associated with oriented internal coherent strain caused by preferential Ti3Ni4 precipitates. A special method, which could achieve a stable post-inherent TW-SME and size selectivity, was proposed to provide an effective approach for devising actuators in Ni-rich alloys.

High Temperature Martensitic Transformation and Shape Memory Behavior in Hfir Intermetallic Compound

The present paper is dedicated to the first observation of the martensitic transformation (MT) and associated shape memory effect (SME) in HfIr equiatomic intermetallic compound. Differential scanning calorimetry allowed determining MT temperatures, which are above 700 K. It was also shown that MT taking place at such high temperatures is associated with SME measured in 3 point bending. MT temperatures and crystal structure changes in the homogeneity range of HfIr compound were also studied. SME behavior changes in the homogeneity range of HfIr compound are discussed

A comparative analysis of some one-dimensional shape memory alloy constitutive models based on experimental tests

Recently, Shape Memory Alloys (SMAs) have been receiving more attention and further study, due to their ability to develop extremely large, recoverable strains and great forces. In this paper, three major models of SMA behavior, used in the literature, for studying the static performance of SMA components attributed to Tanaka, Liang and Rogers, and Brinson, have been analyzed and compared. The major differences and similarities between these models have also been emphasized and presented in this paper, based on the experimental data of the shape memory and superelastic behavior of an SMA wire. It is shown that these models all agree well in their prediction of the superelastic behavior of SMAs at higher temperatures, but the models developed by Tanaka, and Liang and Rogers cannot be used for predicting the shape memory effect behavior of SMAs. It is also shown analytically that the original evolution kinetics, proposed by Brinson, in a specified region, are inadmissible for some thermomechanical loading and initial conditions. Furthermore, corrected evolution kinetics is addressed here in detail, that is; admissible and valid in this region. According to this research, regarding the validation assessment of three major 1-D constitutive models with experimental data, it will be shown that the Brinson model with the corrected evolution kinetics developed by Chung et al. can be applied for the modeling of SMA smart structures, such as flexible SMA beam structures.

The Strain Energy Tuning of the Shape Memory Alloy on the Post-Buckling of Composite Plates Using Finite Element Method

This paper presents a geometric non-linear finite element model of shape memory alloy composite plates and its source code in order to determine critical loads and to trace post-buckling paths of the composite plates. A numerical study was conducted on symmetric and anti-symmetric angle-ply and cross-ply composite plates. Buckling and post-buckling improvements of composite plates due to the shape memory effect behaviour of shape memory alloy were carried out. The pre-strained shape memory alloy wires were embedded within laminated composite plates so that recovery stress could be induced with the heated wires. The methods of active property tuning and active strain energy tuning were applied to show the various effects of the shape memory alloy on the studied behaviour. The result showed that significant improvements occurred in the critical loads and the post-buckling paths of the symmetric and anti-symmetric angle-ply and the symmetric cross-ply composite plates due to the active strain energy tuning method. In the case of the anti-symmetric cross-ply composite plate where bifurcation point did not exist, the post-buckling path was substantially improved.

The Strain Energy Tuning of the Shape Memory Alloy on the Post-Buckling of Composite Plates Using Finite Element Method

This paper presents a geometric non-linear finite element model of shape memory alloy composite plates and its source code in order to determine critical loads and to trace post-buckling paths of the composite plates. A numerical study was conducted on symmetric and anti-symmetric angle-ply and cross-ply composite plates. Buckling and post-buckling improvements of composite plates due to the shape memory effect behaviour of shape memory alloy were carried out. The pre-strained shape memory alloy wires were embedded within laminated composite plates so that recovery stress could be induced with the heated wires. The methods of active property tuning and active strain energy tuning were applied to show the various effects of the shape memory alloy on the studied behaviour. The result showed that significant improvements occurred in the critical loads and the post-buckling paths of the symmetric and anti-symmetric angle-ply and the symmetric cross-ply composite plates due to the active strain energy tuning method. In the case of the anti-symmetric cross-ply composite plate where bifurcation point did not exist, the post-buckling path was substantially improved.

Infrared imaging of TiNi shape memory alloy subjected to deformation at various temperatures

The infrared imaging of stress-induced martensitic transformation occurring in TiNi shape memory alloy subjected to deformation in various conditions has been presented. During the loading and unloading processes the mechanical parameters and the infrared radiation from the specimen surface have been continuously recorded by infrared camera. Basing on the obtained stress-strain curves and the elaborated temperature changes it was found that onset of the martensitic transformation appears at the end of the elastic part of the stress-strain curve, since a temperature increase was noticed there. The uniform temperature distribution observed there means that the initial transformation started at this stage is macroscopically homogeneous. It is followed by the martensitic intense transformation, related to the significant temperature increase. The more advanced transformation, the higher temperature changes. For the shape memory alloy pseudoelastic behavior the narrow bands of higher temperature are observed, manifesting localized Luders-like deformation. For the shape memory effect behavior the process of transformation is much more homogeneous and characterized by uniform temperature distribution. Using a modern, sensitive and fast dynamic infrared camera enables to obtain new results of the start and development of the martensite forward and reverse transformation.

Shape memory and associated properties in Fe–Mn–Si-based ribbons produced by melt-spinning

Four Fe–Mn–Si alloys, Fe 62Mn 32Si 6, Fe 62Mn 20Si 5Cr 8Ni 5, Fe 62Mn 16Si 5Cr 12Ni 5 and Fe 65Mn 9Si 7Cr 10Ni 9, were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both “as quenched” and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases (α and γ), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data.

Shape memory effect and superelastic behavior of TiNi shape memory alloy processed by vacuum plasma spray method

Machining of TiNi shape memory alloy into a complicated three-dimensional (3D) shape is quite difficult, thus a near-net shape forming of 3D shaped TiNi alloy is attractive and cost-effective. Vacuum plasma spray (VPS) process is one of such near-net shape forming processes. In this paper, two kinds of thick TiNi layer, Ni-rich and Ti-rich compositions, were fabricated by VPS process and their shape memory effect (SME) and superelastic (SE) behaviors were characterized. As-VPS processed Ni-rich TiNi which was subjected to homogenization at 1163 K for 7.2 ks and subsequent aging at 773 K for 18 ks exhibited good martensitic transformation behavior. The recoverable strain of the TiNi alloys due to SME and SE behavior were measured to be 2.4% and 5.0%, respectively. The Ti-rich TiNi alloy also exhibited good martensitic transformation behavior and SME in the as-homogenized state.