Orientation Relationship Between Magnetic Domains and Twins in Ni 52 Fe 17 Ga 27 Co 4 Magnetic Shape Memory Alloy
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r the last decade, NiFeGa (Co) ferromagnetic shape memory alloys (FSMAs) have been proposed as a new magnetic shape memory alloy, owing to their significantly improved ductility (compared to Ni-Mnbased system[1,2]) by introducing the second phase c (disordered fcc structure) to the matrix,[3–5] low driving magnetic field,[6] high magneto-crystalline anisotropy energy[7] and nonvolatile elements. Interestingly, their martensitic transformation temperature and the Curie temperature can be tailored over a wide range by changing composition and heat treatment.[5,8] More importantly, a relatively large magnetic-field-induced strain (MFIS) can be obtained in NiFeGa (Co) alloys at room temperature.[6] Morito[9] reported that a large MFIS of 8.5 pct could be achieved in Ni49Fe18Ga27Co6 alloy at room temperature under a static compressive stress of about 8 MPa. Therefore, the NiFeGa (Co) system is regarded as a promising candidate for ferromagnetic actuating and sensing materials.
QIAODAN HU, LIANG YANG, ZHENNI ZHOU, YUJIN HUANG, JUN LI, and JIANGUO LI are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. contact e.mail: [email protected] Manuscript submitted October 6, 2016. Article published online March 24, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
The mechanism for MFIS is related to the field induced reorientation of the crystallographic domains, which appears in the process of thermoelastic martensitic transformations (MT) from the high-temperature B2 (L21) phase to different martensite variants, such as 10M, 12M, 14M layered modulated structures as well as non-modulated L10.[10–12] There are two key conditions to obtain a large MFIS. The first is a low critical stress for twin boundary movement. In other words, the magneto-crystalline anisotropy energy in the vicinity of MT should be large enough to move the twin boundaries. The second is the angle between magnetization and the direction of magnetic field applied. When the easy magnetization axis of one martensitic twin variant is parallel to the applied magnetic field, it would grow at the expense of other variants with different orientations.[13,14] In the course of magnetic-field-induced MT, there is a coupled interaction of the magnetic domains and twins. Therefore, the interaction of magnetic domains and the twinned martensitic crystal structure play an important role during the rearrangement of the twin variants and subsequently the magnetic field induced strain.[15] Recently, this coupled structure and their evolutions have been studied by optical microscopy,[13] scanning electron microscopy (SEM),[15,16] magnetic force microscopy (MFM),[14,17] Lorentz transmission electron microscopy (TEM),[18–20] high-resolution interference contrast colloid (ICC) method[21,22] and magneto-optical indicator film (MOIF) technique.[23] In general, these methods are difficult to provide simultaneously some information on the orientation relationship of these structures in the FSMAs, and a good contrast from two magnetic d
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