Ni-Mn-Ga is a ferromagnetic shape memory alloy that exhibits large, magnetic-fieldand 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- * Corresponding author. 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 asspun Ni-Mn-Ga wire-epoxy matrix composites and of pure epoxy.
Magneto-mechanical actuation of ferromagnetic shape memory alloy/epoxy composites $ http://dx.
AbstractFerromagnetic shape memory alloys (MSMA) exhibit magnetic field-and stressinduced twinning when processed into single crystals, but are brittle and difficult to shape. Embedding slender single crystalline MSMA elements into a polymer matrix can thus provide composites with adjustable magnetic strain actuation behavior. Ni-Mn-Ga single crystalline rods were characterized for their magneto-mechanical behavior and embedded in two different types of epoxy matrices with different volumetric fractions. The magnetic actuation of the composites was measured and shown to depend on the Ni-Mn-Ga volumetric fraction and the matrix stiffness. This behavior was well predicted by finite element simulations of the composite using a simple material model for the strain of the MSMA as a function of the magnetic field and applied stresses. Guidelines for composite behavior prediction could thus be proposed.
This is an author-created, un-copyedited version of an article accepted for publication/published in Smart Materials and Structures, 24 (2015) 065025 (10pp)
a b s t r a c tSingle crystals of ferromagnetic Ni-Mn-Ga shape memory alloys show magnetic-field and stress induced twinning, leading to shape memory. Adaptive composites can thus be produced by embedding single crystalline particles or bamboo-structured Ni-Mn-Ga fibres into a polymer matrix. To guarantee a durable performance of these composites, adhesion between reinforcement phase and matrix should be quantified and optimised. The influence of annealing and surface treatment with an aminosilane of meltspun Ni-Mn-Ga fibres on their strength and adhesion to an epoxy matrix was investigated using single fibre tension and fragmentation tests. Annealing of the melt-spun Ni-Mn-Ga fibres changed the surface from a "pimpled" to a smooth microstructure. This resulted in a reduced adhesion of the annealed fibres in comparison to the as-spun fibres embedded in an epoxy matrix. As-spun fibres exhibited an interfacial shear strength (IFSS) comparable to the shear strength of the epoxy matrix so that the silylation did not change the adhesion significantly. For the annealed fibres, the silane treatment improved the IFSS by 67%. Furthermore, the silylation increased the fracture strength of the Ni-Mn-Ga fibres due to surface flaw healing or forming of a protective surface coating.
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