Magnetic-field-induced recovery strain in polycrystalline Ni–Mn–Ga foam

Abstract

Recently, we have shown that a polycrystalline Ni-Mn-Ga magnetic shape-memory alloy, when containing two populations of pore sizes, shows very high magnetic-field-induced strain of up to 8.7%. Here, this double-porosity sample is imaged by x-ray microtomography, showing a homogenous distribution of both pore populations. The orientation of six large grains-four with 10M and two with 14M structure-is identified with neutron diffraction. In situ magnetomechanical experiments with a rotating magnetic field demonstrate that strain incompatibilities between misoriented grains are effectively screened by the pores which also stop the propagation of microcracks. During uniaxial compression performed with an orthogonal magnetic bias field, a strain as high as 1% is recovered on unloading by twinning, which is much larger than the elastic value of Ͻ0.1% measured without field. At the same time, repeated loading and unloading results in a reduction in the yield stress, which is a training effect similar to that in single crystals.

Enhancing the Elastocaloric Cooling Stability of NiFeGa Alloys via Introducing Pores

“…It has been proved that the porosity in polycrystalline materials effectively overcome the constraints due to grain boundaries as only six grains are found in (2.3 × 3 × 6.2) mm 3 NiMnGa open pore foam . Actually, during the compression in the foam, the struts (near the nodes) are those critical points which are mostly affected by plastic hinging or plastic bending .…”

“…It has been proved that the porosity in polycrystalline materials effectively overcome the constraints due to grain boundaries as only six grains are found in (2.3 Â 3 Â 6.2) mm 3 Ni─Mn─Ga open pore foam. [67] Actually, during the compression in the foam, the struts (near the nodes) are those critical points which are mostly affected by plastic hinging or plastic bending. [68] Moreover, the research explains that the sample deforms at various regions in several orientations under different strengths [67] which produce the strain incompatibilities result in the crack initiation and finally fracture at grain boundaries in polycrystalline bulk alloys.…”

“…[67] Actually, during the compression in the foam, the struts (near the nodes) are those critical points which are mostly affected by plastic hinging or plastic bending. [68] Moreover, the research explains that the sample deforms at various regions in several orientations under different strengths [67] which produce the strain incompatibilities result in the crack initiation and finally fracture at grain boundaries in polycrystalline bulk alloys. But the pores may act as a buffer against the stresses of strain mismatch, even if the grains span over more than one strut or node, by reducing the internal constraints.…”

Review and perspectives: shape memory alloy composite systems

“…oading-unloading cycles at 37°C with constant deformation rate of 0.5 mm/min on (a) pseudoelastic NiTi with 51% porosity a 1% porosity Calculation of YoungÕs modulus by the Gibson-Ashby Eq n = 2, E(Ti) = 110 GPa, q(Ti) = 4.50 g/cm 3 specimen with 0.115% MFIS (more than an order of magnitude higher than a fine-grained, dense specimen). Chmielus and coworkers [95] used neutron diffraction to experimentally demonstrate that the MFIS was a result of pores reducing internal constraints. Subsequent efforts sought to build upon this idea by creating porous specimens with bimodal pore architectures [477,96].…”

Next Generation Autofocus and Optical Image Stabilization System for Camera Modules Using Magnetic Shape Memory Actuators

“…Hence, this parameter determines the maximum output force for an MSM element with a fixed size. The magnetic stress is primarily determined by the MSM alloy microstructure and is commonly considered to not exceed 3 MPa [25], [26]. However, alloys with magnetic stress as high as 3.5 MPa have also been reported [27].…”