LaRC Shape Memory Polymer Composites for Aerospace Applications
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Unoccupied electronic structure of Ni 2 MnGa ferromagnetic shape memory alloy
Momentum resolved inverse photoemission spectroscopy measurements show that the dispersion of the unoccupied bands of Ni 2 MnGa is significant in the austenite phase. Furthermore, in the martensite phase, it is markedly reduced, which is possibly related to the structural transition to an incommensurate modulated state in the martensite phase. Finally, based on the first principle calculations of the electronic structure of Ni–Mn–Ga, we show that the modification of the spectral shape with surface composition is related to change in the hybridization between the Mn 3d and Ni 3d-like states that dominate the unoccupied conduction band.
36 MATERIALS SCIENCE↗
Responsive lignin for shape memory applications
Here, we analyze thermally unstable lignin linkages that generate reactive sites and promote crosslinking reactions during thermal annealing at 180 °C. This results in significantly enhanced glass transition temperature (Tg). We use these characteristics of lignin to enhance interfacial reactivity of a lignin-based multiphase polymer synthesized by a dynamic shear process. An approximately 18 °C increase in Tg and more than 230% increase in storage modulus (E') are achieved by thermally annealing the 50 wt% hardwood lignin-nitrile rubber composite for 5 h at 180 °C. Tunable chemical and physical crosslinks within lignin and rubber resulted in shape programmability demonstrating excellent strain recovery of the synthesized renewable materials. The substantial improvement of the elastic work density of the lignin-based elastomers allows a broader applicable stress window for stress/strain sensing. The renewable materials also exhibit a better strain recovery after thermal annealing.
36 MATERIALS SCIENCE↗
Two–Photon Polymerized Shape Memory Microfibers: A New Mechanical Characterization Method in Liquid
Two-photon polymerization (TPP) is widely used to create 3D micro- and nanoscale scaffolds for biological and mechanobiological studies, which often require the mechanical characterization of the TPP fabricated structures. To satisfy physiological requirements, most of the mechanical characterizations need to be conducted in liquid. However, previous characterizations of TPP fabricated structures are all conducted in air due to the limitation of conventional micro- and nanoscale mechanical testing methods. In this study, a new experimental method is reported for testing the mechanical properties of TPP-printed microfibers in liquid. The experiments show that the mechanical behaviors of the microfibers tested in liquid are significantly different from those tested in air. By controlling the TPP writing parameters, the mechanical properties of the microfibers can be tailored over a wide range to meet a variety of mechanobiology applications. In addition, it is found that, in water, the plasticly deformed microfibers can return to their predeformed shape after tensile strain is released. The shape recovery time is dependent on the size of microfibers. The experimental method represents a significant advancement in mechanical testing of TPP fabricated structures and may help release the full potential of TPP fabricated 3D tissue scaffolds for mechanobiological studies.
36 MATERIALS SCIENCE↗
Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys
The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.
36 MATERIALS SCIENCE↗
In-situ, microscale characterization of heterogeneous deformation around notch in martensitic Shape Memory Alloy
Deformation characterization of low-symmetry phases such as martensite in SMAs is difficult due to a fine-scale hierarchical microstructure. With X-ray microLaue diffraction, in-situ deformation of martensite is examined in a notched NiTi specimen. The local deformation is influenced by the notch stress field, initial martensite microstructure, and interaction between notch stress field and the external load. The microstructure evolves heterogeneously and inelastically, with detwinning and twin nucleation occurring simultaneously through loading. These results contrast with the traditional view of martensite deformation that is partitioned in three distinct regimes: elasticity, reorientation and de-twinning.
36 MATERIALS SCIENCE↗
Stability of Phase Transformation Temperatures During Cycling of Ti-15.8Nb-4.94Al-0.06Sc Shape Memory Alloy
The active ω phase formation decreases the martensitic transformation temperatures sharply and can even suppress the martensitic transformation during thermal cycling in Ti-Nb-based alloys. No ω phase is observed in the present Ti-15.8Nb-4.94Al-0.06Sc alloy when it is annealed at 650 °C for 4 h or subsequently aged at 200 and 250 °C for 2 h. The exclusion of ω phase remarkably improves the stability of phase transformation temperatures during thermal cycling. The martensitic transformation start temperature of the present alloy decreases by 30 °C but tends to be stable gradually during the 20 thermal cycles, owing to the saturation of lattice defects.
36 MATERIALS SCIENCE↗
Deformation behavior of Nb nanowires in TiNiCu shape memory alloy matrix
An in-situ nanowire Nb/TiNiCu composite is fabricated based on the concept of strain under-matching between a phase transforming matrix and high strength nanomaterials. The deformation behavior of the Nb nanowire was investigated by means of in-situ synchrotron X-ray diffraction when the TiNiCu matrix underwent different deformation modes. The maximum lattice strain of the Nb nanowires was about 5% when the matrix deformed via martensitic transformation or 1% when deforming plastically by dislocation slip. As a result, the Nb nanowires showed a lattice strain of 3.5% when the matrix deformed in the mixed mode of plastic deformation and martensitic transformation, which means that the occurrence of plastic deformation does not impede load transfer from the matrix to the nanowires.
36 MATERIALS SCIENCE↗
Features of Crystallization of Rapidly Quenched Ni{sub 45}Ti{sub 32}Hf{sub 18}Cu{sub 5} and Ni{sub 25}Ti{sub 32}Hf{sub 18}Cu{sub 25} Alloys from Melt with High-Temperature Shape Memory Effect
A comparative study of the structure and the chemical and phase composition of Ni{sub 45}Ti{sub 32}Hf{sub 18}Cu{sub 5} and Ni{sub 25}Ti{sub 32}Hf{sub 18}Cu{sub 25} amorphous alloys obtained by fast-quenching of melt stream by spinning has been carried out by transmission and scanning electron microscopy and X-ray diffraction. The critical temperatures of their devitrification were determined by the data of temperatures measurements of electrical resistance. The features of the formation of ultrafine structure and the phase transformation at the vitrification depending on the regimes of heat treatment and chemical composition of alloy have been established.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Magnetic structure and phase transition of Ni 2 Mn 1.48 Sb 0.52 magnetic shape memory compound
Here, a comprehensive study of the structural and magnetic properties of the Ni 2 Mn 1.48 Sb 0.52 compound is presente using neutron diffraction and magnetic measurements. It is found that Mn atoms on 4a and 4 sites are ferrimagnetic coupled in the austenitic phase. At 300 K, the magnetic moments of Mn (4a) an Mn (4b) are determined to be 2.72(3) μ B and -2.67(5) μ B , respectively. From 260 K to 4 K, the compound is in martensite phase and the magnetic moments of Mn (2a), Mn (2f), Mn (2b), Mn (2e) at 4 K ar 2.2(5) μ B , 2.3(5) μ B , -2.1(5) μ B , and -2.6(5) μ B , respectively.
36 MATERIALS SCIENCE↗
Influence of Cobalt and Molybdenum Additives on the Structure and Shape Memory Parameters of Reaction-Sintered Porous Nickel Titanium Alloys
We have studied the structure and properties of porous nickel titanium (TiNi) alloys obtained upon reaction sintering of Ti and Ni powders with Co and Mo additives. It is established that Co and Mo doping additives retain the compaction of Ni powder achieved at the initial stage of sintering. The maximum deformation of porous samples loaded in the austenite state was observed upon adding Co, while the addition of Mo resulted in minimum deformation. The addition of Co leads to single-stage martensitic transformation in TiNi phase, while the addition of Mo leads to the two-stage transformation that is more homogeneous over the volume. Both Co and Mo additives lead to increase in the maximum accumulated strain due to the formation of favorably oriented stress-induced martensite and reoriented quench-induced martensite.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Formation of microcraters and hierarchically-organized surface structures in TiNi shape memory alloy irradiated with a low-energy, high-current electron beam
The regularities of surface cratering in TiNi alloy irradiated with a low-energy, high-current electron beam (LEHCEB) in dependence on energy density and number of pulses are studied. LEHCEB processing of TiNi samples was carried out using RITM-SP facility. Energy density E{sub s} was varied from 1 to 5 J/cm{sup 2}, pulse duration was 2.5–3.0 μs, the number of pulses n = 1–128. The dominant role of non-metallic inclusions [mainly, TiC(O)] in the nucleation of microcraters was found. It was revealed that at small number of pulses (n = 2), an increase in energy density leads both to increasing average diameter and density of microcraters. An increase in the number of pulses leads to a monotonic decrease in density of microcraters, and, therefore, that of the proportion of the area occupied by microcraters, as well as a decrease in the surface roughness. The multiple LEHCEB melting of TiNi alloy in crater-free modes enables to form quasi-periodical, hierarchically-organized microsized surface structures.
36 MATERIALS SCIENCE↗
Effects of Heat Input on Weld-Bead Geometry, Surface Chemical Composition, Corrosion Behavior and Thermal Properties of Fiber Laser-Welded Nitinol Shape Memory Alloy
Bead-on-plate laser welding was carried out on 1-mm-thick Nitinol sheets at different heat input values. The variations in weld-bead geometry and variations in microstructures across the bead were studied. The effects of heat input on surface layer composition of the welded samples, corrosion properties and phase transformation temperature were examined. From x-ray photoelectron spectroscopy (XPS), it was clear that relative intensity of elemental Ni peak was higher in parent material and the oxidation state of Ti was Ti{sup 4+}. It was further observed from Auger electron spectroscopy (AES) that the Ti to Ni ratio on the top surface of the welded samples was more than 1, and as a result of this the stability of TiO{sub 2} layer was enhanced, which eventually helped to improve the corrosion property of the welded samples. The polarization resistance of the welded samples was increased by more than ten times compared to that of the un-welded samples. From differential scanning calorimetry (DSC), it became evident that phase transformation temperatures got changed due to welding.
36 MATERIALS SCIENCE↗
Stacking faults and alternate crystal structures for the shape-memory alloy NiTi
Here, we use ab initio calculations to study the role of stacking faults in connecting the high-temperature B2 and the theoretically predicted low-temperature B33 NiTi phases. In contrast with prior work, we describe the B 2 → B 33 phase transformation in terms of alternate bilayer shifts by 1 2 [100] on the ( 011 ) B 2 plane, obtaining a viable pathway; the same mechanism could also work with the B19 parent phase. We then examine B33-like structures built from alternate stacking sequences of B19 bilayers, constructed to have monoclinic tilt angles close to the experimentally reported NiTi B19' martensite, and find four low-energy stacking-fault variants with energies 5.8–8.5 meV/atom above the calculated B19' martensite structure, suggesting that such structures might appear as a part of the NiTi martensite phase at low temperatures. Investigating further the occurrence of specific coordinated planar shifts in NiTi systems, we report a dynamically stable NiTi B27 phase and find that it is only 1.2 meV/atom above the calculated B33 ground-state structure, thus having a potential to also play a role in NiTi martensitic phase transformation.
36 MATERIALS SCIENCE↗
Protective Properties of Aluminum Nitride Coatings by DC-PVD Sputtering on Cu-10Al-13Mn Shape Memory Alloy
The AlN coating was applied on the surface of a Cu-10Al-13Mn alloy using the DC-PVD sputtering method. The substrate was set at three different temperatures which were ambient, 100 and 200 °C. The effect of the substrate temperature on the quality of AlN coating was modeled by simulating stress on the coating and substrate. The preparation, microstructure, and corrosion resistance of the coating were studied. The surface morphology and element distribution of the coating were investigated by scanning electron microscopy (SEM) and energy distribution spectroscopy (EDS), respectively. In addition, the corrosion resistance of coating was measured using electrochemical impedance spectroscopy (EIS) method in 1 M NaCl solution. The results showed that the AlN coating was formed on the Cu-10Al-13Mn alloy by DC-PVD sputtering method and the AlN coating considerably improved the corrosion resistance of Cu-10Al-13Mn alloy in the given corrosive media. According to EIS test, polarization resistance increased from approximately 1.6 to 115 KΩ, at 200 °C for uncoated and coated samples, respectively. Thus, corrosion resistance of AlN coating improved with the increase in the substrate temperature during the DC-PVD sputtering process.
36 MATERIALS SCIENCE↗
Empirical Study of the Multiaxial, Thermomechanical Behavior of NiTiHf Shape Memory Alloys
An empirical study was conducted to characterize the multiaxial, thermomechanical responses of new high temperature NiTiHf alloys. The experimentation included loading thin walled tube Ni(sub 50.3)Ti(sub 29.7)Hf(sub 20) alloy samples along both proportional and nonproportional axial-torsion paths at different temperatures while measuring surface strains using stereo digital image correlation. A Ni(sub 50.3)Ti(sub 33.7)Hf(sub 16) alloy was also studied in tension and compression to document the effect of slightly depleting the Hf content on the constitutive responses of NiTiHf alloys. Samples of both alloys were made from nearly texture free polycrystalline material processed by hot extrusion. Analysis of the data shows that very small changes in composition significantly alter NiTiHf alloy properties, as the austenite finish (Af) temperature of the 16-at Hf alloy was found to be approximately 60 C less than the 20-at Hf alloy (approximately 120 C vs. 180 C). In addition, the 16-at Hf alloy exhibited smaller compressive transformation strains (2 vs. 2.5 percent). Multi-axial characterization of the 20-at % Hf alloy showed that while the random polycrystal transformation strains in tension (4 percent) and compression (2.5 percent) are modest in comparison with binary NiTi (6 percent, 4 percent), the torsion performance is superior (7 vs. 4 shear strain width to the pseudoelastic plateau).
tensile deformation↗
On the fast kinetics of B2–L2 1 ordering in Ni-Co-Mn-In metamagnetic shape memory alloys
B2–L2 1 configurational order resulting from thermal processing of Ni-Co-Mn-In Heusler alloys is one of the main factors defining the characteristics of the martensitic transformation experienced by these alloys. While the effects of the quenched-in disorder on the martensitic transformation have been extensively investigated, the kinetics of the ordering process is not well known and mostly inferred from the indirect results such as the dependence of the martensitic transformation or magnetic transition temperatures to thermal processing. However, recent results suggested that the kinetics of B2– L 2 1 ordering could have a more significant role than expected in the final state of the material and its functional properties. In the current work, an attempt has been made to investigate the kinetics of B2–L2 1 ordering in order to evaluate the dependence of the quenched-in disorder on the different variables of the thermal procedures. Results showed that ordering rate has a strong dependence on annealing temperature. Below 600 K, slow kinetics is forcing the crystal to always contain a certain amount of quenched-in disorder at room temperature. Between 600 K and 750 K, kinetics is fast enough to reach thermal equilibrium state within tens of minutes during the annealing process. Above 750 K, kinetics is very fast and thermal equilibrium can be reached during heating, so no isothermal holding is required. Furthermore, these predictions have been confirmed using differential scanning calorimetry and in-situ high-energy X-Ray diffraction measurements for non-direct estimations of the degree of order.
36 MATERIALS SCIENCE↗
Shape memory in self-adapting colloidal crystals
We report reconfigurable, mechanically responsive crystalline materials are central components in many sensing, soft robotic, and energy conversion and storage devices1,2,3,4. Crystalline materials can readily deform under various stimuli and the extent of recoverable deformation is highly dependent upon bond type1,2,5,6,7,8,9,10. Indeed, for structures held together via simple electrostatic interactions, minimal deformations are tolerated. By contrast, structures held together by molecular bonds can, in principle, sustain much larger deformations and more easily recover their original configurations. In this report we study the deformation properties of well-faceted colloidal crystals engineered with DNA. These crystals are large in size (greater than 100 µm) and have a body-centred cubic (bcc) structure with a high viscoelastic volume fraction (of more than 97%). Therefore, they can be compressed into irregular shapes with wrinkles and creases, and, notably, these deformed crystals, upon rehydration, assume their initial well-formed crystalline morphology and internal nanoscale order within seconds. For most crystals, such compression and deformation would lead to permanent, irreversible damage. The substantial structural changes to the colloidal crystals are accompanied by notable and reversible optical property changes. For example, whereas the original and structurally recovered crystals exhibit near-perfect (over 98%) broadband absorption in the ultraviolet–visible region, the deformed crystals exhibit significantly increased reflection (up to 50% of incident light at certain wavelengths), mainly because of increases in their refractive index and inhomogeneity.
36 MATERIALS SCIENCE↗