Porous Shape Memory Research Articles

Fabrication of porous Ni-Ti shape memory alloy via binder jet additive manufacturing and solid-state sintering

This paper explores the additive manufacturing of nickel-titanium (Ni-Ti) shape memory alloys (SMAs) using binder jetting and subsequent solid-state sintering under an Ar atmosphere. The consolidated 3D-printed Ni-Ti parts exhibit a correlation between pore morphology, pore fraction, and phase formation at different solid-state sintering temperatures. At a sintering temperature of 1175 °C, NiTi grains with TiC precipitates along grain boundaries are observed, accompanied by irregular, interconnected pores constituting ∼15 % of the volume. Increasing the sintering temperature to 1185 °C leads to the formation of a minor phase of Ni4Ti3 within NiTi grains, along with grain boundary TiC precipitates, and isolated pores with a volume fraction of ∼5 %. The higher sintering temperature corresponds to a higher average nanohardness value (8.1 GPa or 763 Hv compared to 6.9 GPa or 654 Hv), indicating the presence of a greater abundance of secondary phases and higher densification at the elevated sintering temperature. While the transformation temperatures (TTs) were undetectable in the bulk-printed samples, a different structure featuring designed channels and thin struts, sintered under similar conditions, exhibited detectable TTs, with a martensite start temperature of 34 °C. Biocompatibility tests demonstrated cell spreading and attachment on both sintered Ni-Ti samples. These findings offer valuable insights into the potential use of binder jetted Ni-Ti for medical implants and tissue engineering applications.

Programming-via-spinning: Electrospun shape memory polymer fibers with simultaneous fabrication and programming

Porous shape memory polymer (SMP) scaffolds are promising ‘smart’ materials for potential use in a wide range of biomedical applications. Electrospinning provides an approach to produce fibrous SMP scaffolds to enhance their porosity, mass transfer, and flexibility. Here, we studied the effects of electrospinning parameters (rotating collector rotational speed and solvent) on shape memory and mechanical properties of a biostable thermoplastic polyurethane (PUr) SMP. Scanning electron microscopy confirmed that fiber diameter and tortuosity could be tuned using varied collector rotation speeds and/or solvents. Mechanical properties, including modulus, tensile strength, and ultimate elongation, were tuned independently of chemistry based on variations in fiber architectures. All scaffolds demonstrated shape memory properties. Additionally, due to strains that are trapped in the fibers during the electrospinning process, SMP fibers are programmed into a strained, temporary shape during the fabrication step. These fibers can be immediately triggered to recover to a non-strained primary shape after fabrication to reduce sample preparation time and complexity. As a proof-of-concept, bacterial protease-responsive SMPs were electrospun and exposed to S. aureus in programmed secondary shapes. Upon exposure to bacteria, these SMPs underwent shape recovery, which resulted in reduced bacterial attachment and biofilm formation. These materials could be employed as bacteria-responsive wound dressings in future work. Overall, electrospinning provides a valuable tool for tuning mechanical and shape memory properties independently from chemistry and for programming SMPs during fabrication to enable scale-up of electrospun SMP scaffolds.

Gradient Hierarchically Porous Ionic-Junction Fibers of Wet-Spun Carboxymethyl Cellulose Coagulated with Copper Sulfate.

Polyelectrolyte-based ionic-junction fibers newly serve as signal transmission and translation media between electronic devices and biological systems, facilitating ion transport within organic matrices. In this work, we fabricated gel filaments of carboxymethyl cellulose (CMC) chelated with Cu(II) ions through wet-spinning, using a saturated coagulant of CuSO4. Interestingly, the as-spun fibers exhibited dramatic 3D porous frameworks that varied with the temperature and precursor concentration. At 20 °C, the Cu(II) chelation networks favored the formation of well-organized cellular chambers or corrugated channels, displaying dense stacking patterns. However, critical transitions from cellular chambers to corrugated channels occurred at precursor dope concentrations of approximately 2 and 7 wt %, with the porous structure diminishing beyond 8 wt %. We have proposed schematic diagrams to mimic the 3D pore structure, dense porous stacking, and formation mechanism, according to electronic micrographs. Our investigations revealed that the distinct ion-junction channels or chambers are under the control of axial drawing extension as well as the outside-inside penetration of Cu(II) ions into the dope and inside-outside diffusion of water into coagulants. Therefore, controlling the metal chelation-water diffusion process at specific temperatures and concentrations will offer valuable insights for tailoring ionic-junction soft filaments with gradient hierarchically porous structures and shape memory properties.

Shape memory behavior of polyethylene-foam-based nanocomposite for sustainable triboelectric nanogenerators

Durable self-powering electronic devices need high-performance triboelectric nanogenerators (TENGs) with shape memory capabilities. A novel porous shape memory polymer nanocomposite that includes polyethylene foam (PEF) with switching molecules and graphitic carbon nitride nanosheets is designed to create a robust, high-performance TENG. This nanocomposite shows outstanding thermal-actuated shape recovery capabilities by heating at 70 °C. Although the TENG initially excels in output performance (1840 V voltage, 79.66 mW.cm‐2 power density), prolonged harsh conditions cause nanocomposite deformation and performance decline. However, heating the nanocomposite to 70 °C fully restores the TENG’s original performance. Besides, the COMSOL Multiphysics simulation of TENG aligns with the experimental results. Furthermore, two TENG application scenarios are demonstrated: self-charging humidity sensing (where TENG’s output performance is humidity-sensitive and decreases as humidity rises) and powering electronic devices (where TENG turns on commercial LEDs, powers a smartwatch, and charges capacitors). Therefore, this TENG may open new possibilities for future self-powered electrical devices and sensors.

Porous shape memory NiTi-entangled material with incredible recoverable strain for downhole sand control in oil/gas production

Porous shape memory NiTi-entangled material with incredible recoverable strain for downhole sand control in oil/gas production

Synergetic enhancement of ultimate strength and damping properties by pore structure in novel porous Cu-Al-Mn shape memory alloy

To meet the higher device vibration reduction requirements, novel porous Cu71Al18Mn11 shape memory alloys (SMAs) with controllable porosity were prepared using Cu, Al, and Mn elemental powders as raw materials. To further enhance the strength and damping performance of SMAs, a low-temperature reactive synthesis combined with spark plasma sintering (LTRS+SPS) process was used. In this paper, the effects of LTRS+SPS process on pore structure and texture, compressive strength, and damping properties of porous Cu71Al18Mn11 alloy were studied. The outcomes indicated that LTRS+SPS process can effectively improve both compressive strength and damping properties. The associated closed pore structure caused by this process and the second phase γ2 phase formed by element diffusion result in a variety in compressive strength from 324.6 MPa to 617.9 MPa, which were 3.9 times that of the comparable porosity alloy with alloy powder as raw material by vacuum sintering. Meanwhile, the damping value changed from 0.27 to 0.10, which was 4.5 times that of the homogeneous dense alloy. The increase in damping property is caused by the increase of motionable interfaces in the matrix for the introduction of pores, which leads to the increase of energy dissipation. Besides. the interaction between the second phase γ2 and dislocation contributes to the improvement of damping, as well as the compressive strength. These findings in this paper provide new insights and references for the structural design and microstructure design of high-performance Cu-based alloys.

A low-density polymer/CrMnFeCoNi composite with high strength and high damping capacity

A novel damping composite was successfully prepared by taking a porous CrMnFeCoNi high-entropy shape memory alloy as the skeleton and filling its pores with the composite composed of carbon nanotubes and polyurethane/epoxy interpenetrating polymer networks. When the porosity, pore size and CNT loading are 80 %, 1.2 mm and 2 wt%, respectively, the compressive strength, elastic modulus and energy absorption capacity of the composite are 35.7 MPa, 1.31 GPa and 23.1 MJ/m3 (ε = 65 %), respectively. Furthermore, it has a mere density of 2.525 g/cm3. Its loss factor is greater than 0.093 and can reach a maximum of 0.145 within the temperature and frequency range of 20 ∼ 150 ℃ and 0.1 ∼ 200 Hz. A triple-phase micromechanical model was utilized to explore the damping mechanism of composites. Results indicate the coupling of multiple damping mechanisms is the reason for the high ground-state damping of composites, and interface damping is the primary damping mechanism. The superposition of the ε → γ reverse martensite transformation peak of the CrMnFeCoNi HESMA skeleton and the glass transition peak of the CNTs/polymer composite matrix realizes the wide damping temperature range of the composite.

Effect of pore on the deformation behaviors of NiTi shape memory alloys: A crystal-plasticity-based phase field modeling

Porous NiTi shape memory alloys (SMA) have been developed into important biomedical and damping materials. However, the effect of pore on the martensitic microstructure evolution, functional properties, and plastic deformation has not been fully clear yet. In this work, to reveal the functional properties including superelasticity (SE), elastocaloric effect (eCE), one-way shape memory effect (OWSME), and stress-assisted two-way shape memory effect (SATWSME), as well as plastic deformation of porous NiTi SMAs, the polycrystalline SMA system is considered as a composite containing the grain-interior (GI) and grain-boundary (GB) phases, a thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed for the GI phase, and an elasto-viscoplastic constitutive model is established for the GB phase. The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are comprehensively revealed and discussed. The simulations demonstrate that the pores cause a heterogeneous stress field, and the interaction among pores and that between pores and grain boundaries can enhance such heterogeneity, which can promote the initiation of martensitic transformation and reorientation. Moreover, the geometrical constraints caused by the pores can significantly affect the morphology and evolution of martensite microstructures. However, due to the diverse microstructure evolutions during SE, OWSME, and SATWSME, their interactions with the pores are quite different, resulting in various dependencies of each functional property and plastic deformation on the pore, and such dependencies are strongly influenced by porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution. The simulated results in this work are helpful to comprehensively and deeply understand the pore effect and its microscopic mechanism.

Mechanical and Magnetic Properties of Porous Ni50Mn28Ga22 Shape Memory Alloy

A porous Ni50Mn28Ga22 alloy was produced using powder metallurgy, with NaCl serving as the pore-forming agent. The phase structure, mechanical properties, and magnetic properties of annealed bulk alloys and porous alloys with different pore sizes were analyzed. Vacuum sintering for mixed green billets in a tube furnace was employed, which facilitated the direct evaporation of NaCl, resulting in the formation of porous alloys characterized by a complete sinter neck, uniform pore distribution, and consistent pore size. The study found that porous alloys within this size range exhibit a recoverable shape memory performance of 3.5%, as well as a notable decrease in the critical stress required for martensitic twin shear when compared to that of bulk alloys. Additionally, porous alloys demonstrated a 2% superelastic strain when exposed to 353 K. Notably, under a 1.5 T magnetic field, the porous Ni50Mn28Ga22 alloy with a pore size ranging from 20 to 30 μm exhibited a peak saturation magnetization of 62.60 emu/g and a maximum magnetic entropy of 1.93 J/kg·K.

Geometrically adaptive porous shape memory polymers towards personalized biomedical devices

Porous shape memory polymers (SMPs) can execute large volume expansion from their compressed temporary shapes to porous permanent shapes, which present promising practical potentials in biomedical and aerospace fields. While constructing sophisticated geometries are essential for achieving the corresponding functions of the foams, the original shapes which are typically determined by molds cannot be post-tuned in a customized fashion. Here we propose an easy but effective emulsion lyophilization method to fabricate porous SMPs, the original shapes of which can be post-regulated via a two-stage curing mechanism. A thermo-curable acrylate emulsion and a photo-curable polyurethane emulsion were prepared individually and then mixed in proportion. After lyophilization of the mixed emulsion, porous SMPs were prepared due to the first thermo-curing stage between epoxy and amine groups from the acrylate emulsion. The foams can be temporarily programmed into alternative shapes with relatively complex geometries which can afterwards be permanently fixed through the secondary photo-curing stage. In this way, simple permanent shapes of the porous SMPs determined by planar molds were converted into customized three-dimensional ones taking advantage of various deformation such as bending, imprinting, and twisting. Accordingly, customized biomedical devices were fabricated via the two-stage curing process, inspiring further application opportunities for porous SMPs.

Switchable surface and loading/release of target molecules in hierarchically porous PLA nonwovens based on shape memory effect.

In this work, hierarchically porous PLA (polylactic acid) shape memory nonwovens were prepared by electrospinning its blend solution with PEO (polyethylene oxide) and subsequent water etching. Based on shape memory effect resulting from tiny crystals and the amorphous matrix of PLA, the switch between compact and porous surfaces has been achieved via cyclical hot-pressing and recovery in a hot water bath. After hot-pressing, the disappearance of hierarchical pores contributes to compact surface, enabling embedding of the target molecule in PLA nonwoven (i.e., CLOSE state). Upon exposure to heat, PLA nonwoven recovers to its permanent shape and exhibits a porous surface, providing a penetrative diffusion pathway for small molecules (i.e., OPEN state). The hierarchically porous structure and shape memory effect endow PLA nonwoven with the capability of rapid release. Our results provide a good candidate for some potential applications, such as temperature-controlled quick-release of catalysts and drugs.

Direct ink writing of porous shape memory polyesters

In this study, the direct ink write (DIW) additive manufacturing technique is employed to print “self-fitting” shape memory polymer (SMP) scaffolds with requisite porosity from biodegradable poly(ε-caprolactone)-diacrylate (PCL-DA)-based polymers.

Non-negligible role of gradient porous structure in superelasticity deterioration and improvement of NiTi shape memory alloys

Bone-mimicking gradient porous NiTi shape memory alloys (SMAs) are promising for orthopedic implants due to their distinctive superelastic functional properties. However, premature plastic deformation in weak areas such as thinner struts, nodes, and sharp corners severely deteriorates the superelasticity of gradient porous NiTi SMAs. In this work, we prepared gradient porous NiTi SMAs with a porosity of 50% by additive manufacturing (AM) and achieved a remarkable improvement of superelasticity by a simple solution treatment regime. After solution treatment, phase transformation temperatures dropped significantly, the dislocation density decreased, and partial intergranular Ti-rich precipitates were transferred into the grain. Compared to as-built samples, the strain recovery rate of solution-treated samples was nearly doubled at a pre-strain of 6% (up to 90%), and all obtained a stable recoverable strain of more than 4%. The remarkable superelasticity improvement was attributed to lower phase transformation temperatures, fewer dislocations, and the synergistic strengthening effect of intragranular multi-scale Ti-Ni precipitates. Notably, the gradient porous structure played a non-negligible role in both superelasticity deterioration and improvement. The microstructure evolution of the solution-treated central strut after constant 10 cycles and the origin of the stable superelastic response of gradient porous NiTi SMAs were revealed. This work provides an accessible strategy for improving the superelastic performance of gradient porous NiTi SMAs and proposes a key strategy for achieving such high-performance architectured materials.

Microstructure Characteristics of Porous NiTi Shape Memory Alloy Synthesized by Powder Metallurgy during Compressive Deformation at Room Temperature

Porous NiTi shape memory alloys (SMAs) possess compatible mechanical properties with human bones and can effectively reduce the risk of stress shielding and stress concentration; therefore, they have been termed promising candidates for orthopedic implants. However, microstructure characteristics of porous NiTi SMAs during plastic deformation have rarely been investigated. The present study aims to specifically investigate microstructure characteristics and the corresponding underlying mechanisms of fabricated porous NiTi SMAs via a conventional sintering (CS) process with NaCl space holder during compressive deformation at room temperature. To realize the aforementioned target, X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) are applied in the present study. The results show that the fabricated porous NiTi SMA is 51.8% for porosity, 181.65 μm for the average pore size, and 0.78 μm for the average grain size. Many Ni4Ti3 and NiTi2 phases are formed in the mixed matrix with dominant B2 (NiTi) and some B19′ (NiTi). Severe inhomogeneous deformation happens within compressed specimens, leading to the occurrence of tangled dislocation and shear bands. Microcracks occur within fabricated porous NiTi SMAs at a deformation degree of 9.2%; then, they extend quickly to form macrocracks, which finally results in the failure of regions between pores. The observed nanocrystallization and amorphization around microcrack tips within the 12.5%-deformed sample can be attributed to the relatively small grain size and the grain segmentation effect via statistically stored dislocation (SSD) and geometrically necessary dislocation (GND).

Effects of porosity and cyclic deformation on phase transformation of porous nanocrystalline NiTi shape memory alloy: An atomistic simulation

Utilizing molecular dynamics simulation, this study aims to explore the phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) when subjected to cyclic deformation. The influences of porosity and cyclic deformation on the phase transformation of NC NiTi SMAs are examined and discussed. The simulation results show that the increase in the porosity and number of cycles leads to a decrease in both the critical phase transformation stress and peak stress whereas an increase in the residual martensite, phase boundary, and interstitial atoms; the related results can be supported by previous experiments. After cyclic deformation, the reduction in the potential energy for the entire system during the tensile phase occurs at an earlier stage, indicating that the martensitic transformation occurs earlier as the number of cycles increases. Notably, the dissipated energy demonstrates a decrease with an increasing number of cycles, and the potential energy during the austenite elastic unloading stage undergoes a transition from a decreasing to an increasing trend due to the presence of residual martensite increasing with the number of cycles.

Superelastic properties of biomedical Ti–Zr–Nb–Sn highly porous shape memory alloys prepared by fiber metallurgy

Artificial bone substitutes to satisfy the specific properties of cancellous bone for the repair of bone defects are highly desirable. In this study, combining the unique superelasticity and high porosity, biomedical Ti–Zr–Nb–Sn highly porous shape memory alloys (HPSMAs) were prepared by fiber metallurgy for the potential repair of damaged cancellous bone. Three-dimensional and open porous networks constructed by fiber-fiber bonds were obtained in the HPSMAs with a porosity of 79.5%. The recovery strain at room temperature was improved from 2.3% in the as-sintered specimen to 3.8% in the 700 °C solution-treated one by adjusting the transformation temperature. The mechanical performances in the Ti–Zr–Nb–Sn HPSMAs were similar to those of cancellous bone. Stable superelastic behaviors with a recovery strain of 4% were achieved at room temperature after cycle training. The Ti–Zr–Nb–Sn HPSMAs satisfy those specific properties of cancellous bone and show great potential in the repair of cancellous bone defects.

Constitutive modeling of porosity and grain size effects on superelasticity of porous nanocrystalline NiTi shape memory alloy

Constitutive modeling of porosity and grain size effects on superelasticity of porous nanocrystalline NiTi shape memory alloy

High-strength porous NiTi shape memory alloys with stable cyclic recovery properties fabricated using elemental powders

Based on previous research regarding the high-vacuum sintering of elemental powder (EP) NiTi alloys, this study investigates the preparation of high-strength, highly cyclically stable porous NiTi shape memory alloys. The microstructures, martensitic transformation behaviors, compressive and compressive recovery properties, and cyclic stabilities of the sintered and aged porous NiTi alloys were compared and analyzed. The porosity and average pore size increased with the addition of NaCl, with the oxygen content increasing slightly. The sintered alloy displayed an inhomogeneous distribution of the Ni4Ti3 precipitate, which exhibited a dispersed and homogeneous distribution after aging. The sintered porous NiTi alloy exhibited a multistage phase transformation. The martensitic transformation was suppressed after aging owing to changes in the size and distribution of the precipitates, and only R-phase transformation was observed. Porosity and aging precipitation influenced the dual yield points and recovery properties of porous NiTi. The pinning effect of aging precipitation increased the yield point and reduced the residual strain in the recovery. However, at a high porosity, the volume fraction of NiTi matrix decreased, resulting in a small strengthening contribution of precipitation. The disruption of porous structure was the dominant causes of irreversible strain. As the porosity increased, the porous structure became increasingly susceptible to damage during cyclic loading. Overall, the EP porous NiTi alloy exhibited a higher compressive strength, enhanced compression recovery properties and cyclic stability compared to those reported in previous studies. This is because of the optimization of high-temperature homogenization via sintering at 1250 °C and enhancement effects of aging precipitation. This study provides a novel approach for developing porous EP NiTi alloys.

The Influence of Ultrasonic Activation on Microstructure, Phase Transformation and Mechanical Properties of Porous Ni-Ti Shape Memory Alloys via Self-Propagating High-Temperature Synthesis.

Porous Ni-Ti shape memory alloys (SMAs) have been widely studied in biomedical and engineering applications. Porous Ni-Ti SMAs were obtained via self-propagating high-temperature synthesis (SHS), and their microstructure, phase transformation, and mechanical properties were investigated. This article presents the results of a study of changes in the microstructure, phase transformation, and mechanical properties of porous Ni-Ti SMAs when Ni and Ti metal powders were preliminarily subjected to ultrasonic activation at various periods. It was determined that the porosity of the obtained alloy samples was 62-68 vol%. The microstructure was composed of the main matrix Ni-Ti phase and the accompanying Ti and Ti2-Ni phases. The results show that the hardness 34.1-86.8 HB and elastic modulus 4.2-10.8 GPa increased with an increase in the ultrasonic activation time of the samples. The phase transformation temperature of the Ni-Ti shape memory alloy remained almost unchanged under the influence of ultrasonic treatment.

Effects of Thermal Cycling and Porosity on Phase Transformation of Porous Nanocrystalline NiTi Shape Memory Alloy: An Atomistic Simulation

Herein, the effects of thermal cycling and porosity on the martensitic phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) at the atomic scale are investigated by molecular dynamics simulation. The simulation results show that thermal cycling causes the NC NiTi SMAs to repeatedly undergo martensitic phase transformation and inverse phase transformation processes. The martensitic phase transformation capability is suppressed with increasing porosity and number of cycles, but the suppression effect of thermal cycling is weakened with increasing porosity. Moreover, the start temperature of martensitic transformation, residual martensitic phase, and the fraction of interstitial atoms increase with increasing porosity. Thermal cycling causes the disordered structures and shear plastic deformation to accumulate as cycling increases. Recoverable shear deformation is generated within the grain, but shear plastic deformation is mainly concentrated at both grain boundaries and pore surfaces. Residual martensitic phase increases after thermal cycling. These results provide important explanations and references for a deeper understanding of phase transformation behavior and pore effects caused by thermal cycling.