The Effects of Composition and Aging on the Martensite and Magnetic Transformations in Ni-Fe-Ga Ferromagnetic Shape Memo
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The Effects of Composition and Aging on the Martensite and Magnetic Transformations in Ni-Fe-Ga Ferromagnetic Shape Memory Alloys T.M. HEIL, M.A. WILLARD, and W.T. REYNOLDS, Jr. The martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X2YZ) were used to investigate how the martensite and magnetic transitions change with alloy composition and isothermal aging above and below the B2/L21 ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 and 35 K, respectively. Aging a Ni53Fe19Ga28 alloy for 3600 seconds at various temperatures shifts the martensite start temperature and the Curie temperature by almost 70 K. Transmission electron microscopy investigations were conducted on the aged Ni53Fe19Ga28 alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and aging treatments. I. INTRODUCTION
DURING the past ten years, studies of Ni-Mn-Ga and other ferromagnetic shape memory alloys have intensified with the impetus for the development of multifunctional materials. Use of the ferromagnetic shape memory effect is envisioned for sensing and actuating applications in aerospace and other fields. While considerable attention has been paid to alloys that exhibit both ferromagnetic and shape memory behaviors, understanding the wide varieties of structural and magnetic transitions is still a challenge. The intricacies of the phase transformations and magnetic behavior in these alloys need to be clarified to enable efficient tailoring of properties for practical applications. The prominent challenges facing the design and use of these specialized alloys are a limited range of operating temperatures, phase transformations that have high sensitivity to variations in composition, brittleness of the alloys, and changes in transformation temperatures due to aging effects. In 2002, Oikawa et al. published the first studies of ferromagnetic shape memory behavior in Ni-Fe-Ga alloys.[1,2] As in analogous Ni-Mn-Ga alloys, the ferromagnetic shape memory effect in Ni-Fe-Ga alloys is found at compositions near the prototype Heusler stoichiometry (X2YZ). Ni-FeGa alloys have since emerged as promising candidates for sensing and actuating applications because (1) they have the same crystal structures as Ni-Mn-Ga alloys that have shown up to a 9.5 pct recoverable magnetic field induced strain;[3] (2) they have magnetocrystalline anisotropy T.M. HEIL, National Research Council Associate, National Research Council, and M.A. WILLARD, Materials Scientist, are with the United States Naval Research Laboratory, Washington, DC, 20375. Contact e-mai
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