Compressive Fracture Behavior of Bi-added Ni 50 Mn 28 Ga 22 Ferromagnetic Shape Memory Alloys
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Compressive Fracture Behavior of Bi-added Ni50Mn28Ga22 Ferromagnetic Shape Memory Alloys Hirotaka Tanimura1*, Masaki Tahara1, Tomonari Inamura1 and Hideki Hosoda1 1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan *Graduate Student, Tokyo Institute of Technology ABSTRACT In order to develop NiMnGa/polymer composite materials, a production of single-crystallike 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. INTRODUCTION The ferromagnetic shape memory alloy NiMnGa is a promising candidate for a new actuator material exhibiting a large magnetic field induced strain (MFIS) due to the reorientation of martensite variants. MFIS is induced not by heating/cooling but by magnetic field that enables one to control the field at a higher frequency than usual for shape memory alloys such as TiNi. That is why NiMnGa has attracted a great deal of interest during past decades. Ullako and coworkers reported 0.2% MFIS in a single crystal of Ni2MnGa [1]. Recently, up to 6% MFIS has been reported in single crystal NiMnGa with 10M martensite [2]. The material does not, however, recover its original shape spontaneously upon removing the magnetic field. On the other hand, polycrystalline NiMnGa cannot exhibit such large MFIS. This is explained by the suppression of the twin boundary motion stemming from the constraints among grains of the martensite phase. In addition, the intrinsic intergranular brittleness of polycrystalline NiMnGa is also a drawback for practical applications. We have, therefore, proposed and developed NiMnGa/polymer composite materials which are composed of single-crystal-like NiMnGa particles and polymer matrix [3]. This composite is expected to exhibit large MFIS due to the motion of single-crystal-like NiMnGa and spontaneous shape recovery upon removing the magnetic field by elastic back stress from the polymer matrix. Based on the theory of elasticity, a calculated MFIS of around 1% in a 50vol%Ni
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