Ni–Mn–Ga ferromagnetic shape memory wires

Abstract

Ni–Mn–Ga ferromagnetic shape memory wires (Ni2.10Mn0.98Ga0.92, mean diameter 170 μm) are obtained by the rotating water bath melt spinning technique. The compositional heterogeneity linked to its dendritelike structure gives rise to a complex and broad martensitic transformation (MT). The reduced value of magnetization in the as-spun sample is ascribed to Mn–Mn antiferromagnetic interactions at structural defects as atomic disorder, vacancies, and antiphase boundaries structures. Moreover, the observed low temperature magnetic relaxation process is characterized by a splitting in the zero-field-cooled/field-cooled magnetization curves and the frequency shift in the ac magnetic susceptibility. The results are interpreted in terms of the coexistence of a reduced magnetization state and nearly noninteracting magnetic clusters. A high temperature treatment optimizes both the MT and the magnetic characteristics (i.e., decrease in the hysteresis of the MT and magnetization recovery, respectively).

Effect of chemical ordering annealing on martensitic transformation and superelasticity in polycrystalline Ni–Mn–Ga microwires

“…An enhancement of magnetization and a reduction of MT hysteresis were observed in Ni-Mn-Ga microwires after annealing at 800°C. The improved MT and magnetic characteristics are related to the improvement of antiferromagnetic Mn-Mn exchange interactions associated with reduction in the density of defects (such as vacancies, atomic disorder and antiphase boundaries) [24].…”

Shape Memory and Huge Superelasticity in Ni–Mn–Ga Glass-Coated Fibers

“…The fiber shows a perfect superelasticity behavior in a comparatively broader temperature region (from 30 to 60 • C). Differently to the stress-strain curves in Ni-Mn-Ga single crystals, many serration flows occurred during the transformation [18][19][20]25,26]. The critical stress for the stress-induced martensitic (SIM) transformation, σ, which corresponds to the applied stress when the martensitic transformation starts, is plotted against temperature, T, in Figure 6.…”

“…The fiber shows a perfect superelasticity behavior in a comparatively broader temperature region (from 30 to 60 °C). Differently to the stress-strain curves in Ni-Mn-Ga single crystals, many serration flows occurred during the transformation [18][19][20]25,26]. The shape memory experiments of the fiber were carried out by a dynamic mechanical analyzer (DMA), where the fiber was loaded to 0.05 N (~58 MPa) at room temperature before it was heated to 60 • C. While maintaining this constant load, the fiber was subjected to a thermal cycle across the transformation temperature range from 60 to −70 • C. The stress-assisted SME of the Ni54.9-Mn23.5-Ga21.6 fiber was observed by DMA.…”

“…However, as the grain size is still as large as 10 to 50 μm, the recovery ratio of the rods is less than 70%, and the maximum shape memory strain is only 4.2%. In-rotating-water-quenching technique was previously used to fabricate a Ni-Mn-Ga fiber about 170 μm in diameter with reduced magnetization and broad martensitic transformation, but the mechanical properties were not greatly improved [19]. Our previous work reported a Ni53.96-Mn24.12-Ga21.92 fiber exhibiting large superelasticity at room temperature [20].…”

Superelasticity by reversible variants reorientation in a Ni–Mn–Ga microwire with bamboo grains

“…Gómez-Polo et al [8] prepared wires by rotating water bath melt spinning and observed coexistence of austenite and the 7M and 5M martensites. Zhukova et al [9][10][11][12][13][14][15][16] prepared glass-coated wires by the Taylor method for Ni-Mn-Ga and Ni-Mn-In/Ni-CoMn-In alloys and investigated the effect of the internal stress between the metallic core and the glass sheath on the microstructure, phases and magnetic properties.…”