Field and temperature induced giant strain in single crystal Ni-Mn-Ga

Abstract

We analyze the effect of temperature coupled to dc or ac magnetic fields on the strain properties of an oriented single crystal of Ni 2 MnGa. The sample has an austenite-martensite phase transformation temperature T M = 160 K and reverse transformation at T A = 180 K. No external stress was applied before or during the tests. It is shown that the shape memory features may be deeply influenced by the presence of external magnetic fields: during heating or cooling through the transformation an ac field applied along the [100] direction is able to trigger an irreversible strain of 2.5% [100] at T A . This value can be compared with a 1.2% [100] strain with a dc field and about 0.17% in absence of field (both at T A ). The peak cyclic strain at T A is 0.45% when a cyclic field is applied at constant temperature.It is shown that, in this single crystal, twin rearrangements leading to large values of strain only occur in limited temperature ranges, in correspondence with the phase transformations at T M and T A .Index Terms-Magnetic shape memory alloys, magneto-elastic materials and magnetostriction, martensite twins, NiMnGa.

Shape memory ferromagnets

“…The typical values of magnetically induced strains observed in experiments with unstressed samples are 0.2-2% [68,70,94,102,104,[122][123][124][125][126][127][128]. The highest magnetically induced strains have been observed within temperature ranges from M s to M f and from A s to A f ; outside these ranges magnetically induced strains are negligible [104,123].…”

Analysis of Rheological Properties of MR Fluid Based on Variation in Concentration of Iron Particles

“…It indicates that the surfactant of MR fluid plays an important role. When Science Publications AJNT external magnetic field was applied, all three samples exhibit Bingham fluid behaviour with yield stress, representing increase in the shear stress along with the shear rate (Pasquale et al, 2001). It can be seen that the shear stress and shear viscosity increased by magnetic field force.…”

Magnetoelastic Nature of Ferromagnetic Shape Memory Effect

“…For instance, at the parent-tointermediate phase transition in Fig. 2(b) it is about 0.06% (the measurements were carried out in a cyclic magnetic field [70,71]). According to the expression ' / 2 1 ) (…”

“…2(a) illustrates a typical temperature evolution of the lowfrequency elastic modulus for the single crystalline Ni 2 MnGa sample on cooling [65]. The resonance ultrasonic measurements of the polycrystalline samples show similar temperature [65,70,71]. T M and T A are characteristic temperatures of the forward and reverse martensitic transformations, respectively; T I is the temperature of premartensitic transformation into the intermediate phase.…”

“…For instance, at the parent-tointermediate phase transition in Fig. 2(b) it is about 0.06% (the measurements were carried out in a cyclic magnetic field [70,71] δ is the magnetoelastic parameter, the occurrence of the right-hand-side minimum in the curve of Fig.2(b) is described by the concurrent minimum of elastic modulus in Fig.2(a).…”