Structural,physical and damping properties of melt-spun Ni–Mn–Ga wire-epoxy composites

Ni–Mn–Ga is a ferromagnetic shape memory alloy that exhibits large, magnetic-field- and stress-induced strains via energy dissipating twinning when processed into single crystals. Grain boundaries suppress twinning and render polycrystalline Ni–Mn–Ga brittle. Ni–Mn–Ga/polymer composites overcome the drawbacks of polycrystals and could thus provide a less expensive and easier to handle alternative to Ni–Mn–Ga single crystals for damping applications. Ni–Mn–Ga wires were produced by melt-spinning and were polycrystalline in the as-spun state. Annealed wires were ferromagnetic at room temperature with non-modulated martensite and a bamboo microstructure. The annealed wires displayed a hysteretic stress–strain behavior typical for twinning. Ni–Mn–Ga wire-epoxy matrix composites were fabricated with as-spun and annealed wires. The damping behavior of annealed Ni–Mn–Ga wire-epoxy matrix composites was higher than that of as-spun Ni–Mn–Ga wire-epoxy matrix composites and of pure epoxy.     

Martensitic transformation and microstructure of polycrystalline Ni–Mn–Ga and C-doped Ni–Mn–Ga Heusler alloys

Abstract

The martensitic transformations of Ni–21·7Mn–23·8Ga (at.-%) (NiMnGa) and Ni–19·4Mn–22·7Ga–1·6C (at.-%) (NiMnGaC) alloys were investigated by the measurement of resistivity. Two kinds of martensitic transformations occur in NiMnGa alloy. The first martensitic transformation is thermoelastic, which exhibits a steep increasing in resistivity. The second transformation exhibits a larger thermal hysteresis compared with the first transformation. NiMnGaC alloy only shows a single martensitic transformation and the C addition increases the first martensitic transformation temperatures. The first martensitic phase of NiMnGa alloy is of five layered structure while the martensitic phase of NiMnGaC alloy is of non-modulated structure. Combined with the observation of optical microscopy and TEM, NiMnGa alloy exhibits much wider martensite twins than NiMnGaC alloy does.     

Effect of enhanced interfacial reaction on the microstructure,phase transformation and mechanical property of Ni–Mn–Ga particles/Mg composites

This study investigated the microstructure, phase transformation and mechanical property of Ni–Mn–Ga particles/Mg composites with a strong interfacial reaction between the particles and the matrix. The strong interfacial reaction was related to the large surface area and energy per unit volume of the flaky shape Ni–Mn–Ga particles that favors the reaction between the particles and matrix. The martensitic transformation behavior was largely weakened due to the interfacial reactions and thus the reduced volume fraction of Ni–Mn–Ga particles. The composites exhibited a much improved compressive strength and ductility in comparison with that of the Ni–Mn–Ga alloy. The compressive plasticity of the composites was decreased when the Ni–Mn–Ga particle content exceeded 40 wt%. In comparison with the Mg-composites with large size Ni–Mn–Ga particles, the composites with small size particles would have a much stronger interfacial reactions, which was detrimental to the phase transformation and mechanical ductility of the composites. The investigation results in this article could provide a reference for the design and preparation of the particles reinforced metal matrix functional composites.     

Fabrication conditions and transformation behavior of epitaxial Ni–Mn–Ga thin films

Epitaxial Ni–Mn–Ga films have been grown onto heated substrates by sputtering. Their chemical composition depends on the sputtering argon pressure. Representative epitaxial films of Ni_52.3Mn_26.8Ga_20.9, 0.5 μm-thick, transform martensitically at about 120 °C, accompanied by sharp changes in the lattice parameter and resistivity, and orders ferromagnetically below 98°. The observed high transformation temperature, orthorhombic martensitic structure, twinning mode and film morphology, indicate a potential multifunctional behavior of the film, such as high-temperature shape-memory effect and magnetic field actuation.     

Self-accommodation in polycrystalline 10M Ni–Mn–Ga martensite

10M Ni–Mn–Ga polycrystals show a typical self-accommodated microstructure consisting of macro and micro twins. The macro twin lamellae separate micro twins creating a so-called “twins within twins” microstructure. Such a configuration allows the distribution of martensitic variants with no net change in shape of the sample. The arrangement of variants can occur on different length scales, from a few nanometers up to a few millimeters, not only depending on grain size but also on processing condition (e.g., extrusion, torsion). Small austenite grains do not completely transform to martensite giving rise to some residual austenite. Furthermore, characteristic branching of macro and micro twins is observed due to lowering of the elastic energy at grain and macro twin boundaries, respectively.     

Microstructure–toughness relationships in calcium aluminate cement–polymer composites using instrumented scratch testing

We investigate the influence of the microstructure on the fracture properties of calcium aluminate cement/polymer composites. We carry out microscopic scratch tests during which a Rockwell C diamond probe pushes across the surface of a polished specimen under a linearly increasing vertical force. We extend the scratch fracture method to heterogeneous materials. The scratch test induces a ductile-to-brittle transition as the penetration depth increases. Scanning electron microscopy imaging shows that the low porosity and the strong cement-binder interphase favor toughening mechanisms such as crack trapping and bridging. Nonlinear fracture mechanics theory yields the fracture toughness in the fracture-driven regime. The fracture toughness of macro-defect-free (MDF) cement is found to decrease as the polymer-to-cement ratio increases. This decrease in the fracture resistance can be explained by the decrease in anhydrous cement content and the increase in the inter-particle distance between cement grains. By evaluating the fracture toughness of the micro-constituents of MDF cement, we show that the high value of the fracture toughness at the composite level stems from tough calcium aluminate phases and a highly packed non-porous granular microstructure.     

Phase transition and mechanical properties of constraint-aged Ni–Mn–Ga–Ti magnetic shape memory alloy

Ni–Mn–Ga Heusler-type ferromagnetic shape memory alloys are attractive materials for micro-actuator, but the relatively poor ductility and low strength of Ni–Mn–Ga alloys have triggered a great deal of interest. In this study, we attempt to introduce some ductile second phase in the alloy by partially substituting Ti for Ga and constraint aging treatment. The results show that the martensitic transformation temperature first decreases and then increases slightly with the increasing of constraint-aging temperature, which can be attributed to the decrease of Ni content in the matrix and strengthening effect of the second particles. It is found that the amount of the Ni-rich precipitates by constraint-aged samples is more and the size of the second phase particle is smaller than that of the free-aged samples. The compressive stress and ductility can be significantly improved by the constraint-aging treatment, and the maximum compressive stress for constraint-aging alloy is about 1400 MPa, which is the highest value up to date compared with the 400 MPa in solution-treated Ni–Mn–Ga–Ti alloy and about 900 MPa in Ni–Mn–Ga–Ti alloy free-aged at 1073 K for 3 h. Scanning electron microscopy observations of fracture surfaces confirm that the Ni-rich second phase play a key role in improving the compression stress and ductility of Ni–Mn–Ga–Ti alloy.     

Compressive mechanical properties of closed-cell aluminum foam–polymer composites

The compressive mechanical properties of two kinds of closed-cell aluminum foam–polymer composites (aluminum–epoxy, aluminum–polyurethane) were studied. The nonhomogeneous deformation features of the composites are presented based on the deformation distributions measured by the digital image correlation (DIC) method. The strain fluctuations rapidly grow with an increase in the compressive load. The uneven level of the deformation for the aluminum–polyurethane composite is lower than that for the aluminum–epoxy composite. The region of the preferentially fractured aluminum cell wall can be predicted by the strain distributions in two directions. The mechanical properties of the composites are investigated and compared to those of the aluminum foams. The enhancement effect of the epoxy resin on the Young’s modulus, the Poisson’s ratio and the compressive strength of the aluminum foams is greater than that of the polyurethane resin.     

Particulate basalt–polymer composites characteristics investigation

Basalt has remarkable physical and mechanical characteristics. However, its use and further processing is limited due to mechanical characteristics, e.g., density, hardness, fraying resistance, etc. The basalt–polymer composite enable maintains of physical and mechanical characteristics, further processing, and broader basalt application in various industries. Consequently, composites characterization, and production process technological parameters influence on the composites properties need to be appraised.     

Processing and properties of 15–70 MHz 1–3 PZT fiber/polymer composites

This paper reports on the fabrication and characterization of fine scale piezoelectric composites with 1–3 connectivity using fibers derived from a metal alkoxide sol-gel process. Using this technique, pure thickness mode resonance for this type of composite has been increased from 15 MHz up to 70 MHz by maintaining pillar aspect ratio requirements. Piezoceramic fibers of Nb or La modified lead zirconate titanate (PZT) were produced with final diameters ranging from 15 to 50 μm. Composites having 1–3 connectivity were produced using the fibers as pillars. Composites could be fabricated with volume fractions from 10 to 45% allowing tailoring of both the dielectric constant and acoustic impedance without degrading coupling. Dielectric constant, polarization and coercive field values varied slightly from bulk values due to clamping by the polymer matrix, increasing as the fiber diameter decreased. Composites with resonance frequencies ranging from 15 to 70 MHz were studied. The thickness dependence of the properties gave indications to radial mode/thickness mode interactions at pillar aspect ratios near 1.7 to 1 thickness to diameter. Coupling coefficients (k_t) from 58% to 73% with mechanical quality factors <15 were detected. Received: 4 April 2000 / Reviewed and accepted: 8 June 2000     

Fibrillar polymer–polymer composites: morphology, properties and applications

Micro- or nano-fibrillar composites (MFCs or NFCs) are created by blending two homopolymers (virgin or recycled) with different melting temperatures such as polyethylene (PE) and poly(ethylene terephthalate) (PET), and processing the blend under certain thermo-mechanical conditions to create in situ fibrils of the polymer that has the higher-melting temperature. These resulting fibrillar composites have been reported to possess excellent mechanical properties and can have wide ranging applications with suitable processing under controlled conditions. However, the properties and applications very much depend on the morphology of created polymer fibrils and their thermal stability. The present paper develops an understanding of the mechanism of micro-/nano-fibril formation in PE/PET and polypropylene (PP)/PET blends by studying their morphology at various stages of extrusion and drawing. It is revealed that this subsequent mechanical processing stretches the polymer chains and creates fibrils of very high aspect ratios, thus resulting in superior mechanical performance of the composites compared to the raw blends. The study also identifies the primary mechanical properties of the main types of MFCs, as well as quantifying their enhanced resistance to oxygen permeability. Furthermore, the failure phenomena of these composites are studied via application of the modified Tsai–Hill criterion. In addition to their usage as input materials in different manufacturing processes, possible applications of these fibrillar composites in two different areas are also discussed, namely food packaging with controlled oxygen barrier properties and biomedical tissue scaffolding. Results indicate a significant scope for using these materials in both areas.     

Microstructure and martensitic transformation behavior of a constant-strain aged Ni–Mn–Ga–Ti magnetic shape memory alloy

The Ni₅₃Mn_23.5Ga_18.5Ti₅ ferromagnetic shape memory alloy has been aged under 2% constant-strain at various temperatures for 3 h, and the microstructure and martensitic transformation behaviors have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). It was found that after constant-strain aging, the amount of the Ni-rich precipitates with lenticular morphology is higher and the size of the second-phase particle is smaller when compared to that of the conventional aged samples. The martensitic transformation temperatures first decrease remarkably with the increase of aging temperature, and then increase when the aging temperature exceeds 973 K, which can be attributed to the change of the Ni-content in the matrix as well as the strengthening effect by fine Ni₃Ti precipitates.     

Effects of intermediate metal layer on the properties of Ga–Al doped ZnO/metal/Ga–Al doped ZnO multilayers deposited on polymer substrate

Ga–Al doped ZnO/metal/Ga–Al doped ZnO multilayer films were deposited on polyethersulfone (PES) substrate at room temperature. The multilayer films consisted of intermediate Ag metal layers, top and bottom Ga–Al doped ZnO layer. The multilayer with PES substrate had advantages such as low sheet resistance, high optical transmittance in visible range and stable mechanical properties. From the results, sheet resistances of multilayer showed 9 Ω/sq with 12 nm of Ag metal layer thickness. Average optical transmittance of multilayer film showed 84% in visible range (380–770 nm) with 12 nm of Ag metal layer thickness. Moreover the multilayers showed stable mechanical properties than single-layered Ga–Al doped ZnO sample during the bending test due to the existence of ductile Ag metal layer.     

Synthesis of cobalt nanoparticles to enhance magnetic permeability of metal–polymer composites

Metallic cobalt nanoparticles were synthesized by hydrogen reduction method. Particles were coated in situ with carbon by adding ethene to reaction flow. Particles were characterized by transmission electron microscopy, energy dispersive X-ray emission, X-ray diffraction, X-ray fluorescence and BET method. The observed cobalt particle size distributions in different cobalt batches produced with unvarying reaction parameters was reproducible: The mean diameter of primary cobalt particle varied only 5% from the mean value of 76 nm in different batches. Increased carbon precursor concentration decreased mean diameter of cobalt particles to 17 nm. The produced nanoparticles were used as filler material in 0–3 type metal–polymer composites. Composite samples with varying filler loading were fabricated with mixing extrusion and injection moulding techniques. The magnetic properties of the fabricated composites were measured up to 1 GHz. In order to analyse the particle distribution in composite matrix and its effect on magnetic properties the microstructure was studied.     

Bioactivity of ceramic–polymer composites with varied composition and surface topography

HAPEX trade mark (40 vol % hydroxyapatite in a high-density polyethylene matrix) and AWPEX (40 vol % glass-ceramic apatite-wollastonite in a high-density polyethylene matrix) are composites designed to provide bioactivity and to match the mechanical properties of human cortical bone. HAPEX trade mark has had clinical success in middle ear and orbital implants, and there is great potential for further orthopaedic applications of these materials. However, more detailed in vitro investigations must be performed to better understand the biological interactions of the composites. In this study, the bioactivity of each material was assessed. Specifically, the effects of controlled surface topography and ceramic filler composition on apatite layer formation in acellular simulated body fluid (SBF) with ion concentration similar to those of human blood plasma were examined. Samples were prepared as 1 x 10 x 10 mm(3) tiles with polished, roughened or parallel-grooved surface finishes, and were incubated in 20 ml of SBF at 36.5 degrees C for one, three, seven or 14 days. The formation of an apatite layer on the composite surface after immersion was demonstrated by thin-film X-ray diffraction, environmental scanning electron microscopy and energy dispersive X-ray analysis. Variations in sample weight and solution pH over the period of incubation were also recorded. Significant differences were found between the two materials tested, with greater bioactivity in AWPEX than HAPEX trade mark. Results also showed surface topography to be important, with rougher samples correlated to earlier apatite formation. Osteoblast-like cells proliferated favourably on both composite materials, with many filopodia connections, preferential attachment to ceramic particles and contact guidance effects evident.