Compressive Fracture Behavior of Bi-added Ni₅₀Mn₂₈Ga₂₂ Ferromagnetic Shape Memory Alloys

Abstract

In order to develop NiMnGa/polymer composite materials, a production of single-crystal-like NiMnGa particles is important and should be developed for better quality. Although mechanical pulverization is a promising method by utilizing intrinsic intergranular brittleness of NiMnGa polycrystalline ingots, the amount of lattice defects introduced during mechanical crushing needs to be minimized. This must be achieved by enhancement of intergranular brittleness of NiMnGa particles. In this study, the effect of Bi addition on the compressive fracture behavior of polycrystalline Ni50Mn28Ga22 was investigated where Bi was expected to be segregated to the grain boundaries in NiMnGa, similar to Bi segregation to the grain boundaries in Ni. It was found that only intergranular fracture was observed in Ni50Mn28Ga22 polycrystals with 0.3 at.% Bi addition, although a mixture of intergranular and transgranular fracture was observed in Bi-free Ni50Mn28Ga22 polycrystal. Microalloying of Bi into NiMnGa enhances intergranular embrittlement. A number of spherical particles of Bi were confirmed on the fractured surface of Bi-doped NiMnGa polycrystals. The formation of Bi particles is a proof of the grain boundary segregation of Bi in NiMnGa.

Magnetic field-induced rubber-like behavior in Ni-Mn-Ga particles/polymer composite

“…Particles were fabricated by the careful disintegration of ingot at 300 °C (above melting point of bismuth, 271 °C) into the individual grains. As bismuth segregated at the grain boundaries, the decreasing of the adhesion force between grain boundaries, due to a liquid state of bismuth, resulted in the intergranular easy-fracture, whereby the single crystalline grains with the clean and shiny faces were obtained after mechanical cleaving 17,20 . The powder was divided by the sieves into three fractions having different size of particles: below 75 μm, 100 μm–250 μm and over 250 μm, which were used for the measurement of the transformation temperatures, fabrication of composite and optical observation of the twin rearrangements under magnetic field, respectively.…”

“…In order to prepare magnetostrain-active Ni-Mn-Ga particles (condition (i)) with a reduced amount of the lattice defects and low surface roughness, which act as the obstacles to twin boundaries motion 19 , we have recently suggested an innovative method of a drastic intergranular embrittlement enabling easy disintegration of the polycrystalline Ni-Mn-Ga alloy into the individual grains 20,21 . The embrittlement was achieved by adding of 0.03 at.% of Bi which entirely segregates to the grain interfaces facilitating an easy cleaving of the Ni-Mn-Ga ingot into single crystalline grains above melting point of Bi 20 . In the present work, this technology was used to synthesize a Ni-Mn-Ga powder.…”

Compression response of Ni–Mn–Ga/silicone composite and study of three-dimensional deformation of particles

“…Particles were obtained by a careful mechanical crush of the ingot. Added bismuth segregates at the grain boundaries leading to a significant reduction of an adhesion force between the grain boundaries whereby facilitating the intergranular fracture and formation undeformed particlesgrains [20]. A fraction of particles having cross-section range of 100-250 μm was separated.…”

“…Recently, this method was upgraded by using a newly designed Ni-Mn-Ga-(Bi) material with a much higher brittleness than standard Ni-Mn-Ga alloys. This embrittlement, stemming from the weakened intergranular forces, due to segregation of entire Bi at the grain interfaces, facilitates an easy ingot cleaving [20]. As a result, one can obtain a single crystalline powder containing the minimally deformed grains.…”