Structural and Magnetic Transitions in Fe-, Co-, and Al-Substituted Ni-Mn-Ga Ferromagnetic Shape Memory Alloys
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I.
INTRODUCTION
SMART materials, viz., shape memory alloys based on Fe, Ni, Cu, and Ti systems, are finding an everincreasing role in todayÕs technological devices as sensors, actuators, and transducers. Their shape memory effect is attributed to a large recoverable strain generated due to thermoelastic martensitic transformations. While conventional shape memory alloys require a thermal energy impetus, Ferromagnetic shape memory (FMSM) alloys undergo conversion/reorientation of the martensite variants under a magnetic field. As no temperature change usually occurs during application of the FMSM alloys, they respond more quickly, which opens the possibility for higher-frequency applications. The functional requirement for the alloys is the stability of the martensite phase at higher temperatures, with easy reorientation of the martensite variants under a magnetic field. The driving force for reorienting the martensite variants by detwinning i.e., moving the twin boundaries, is provided by Zeeman energy. A high magnetocrystalline anisotropy energy coupled with high saturation magnetization is, therefore, required for these materials.[1,2] The FMSM alloys, thus, offer an opportunity for materials development in terms of compositional and structural modifications, in order to optimize the requirements for shape memory applications. R.P. MATHUR, Scientist ÔFÕ, R.K. SINGH, Scientist ÔBÕ, V. CHANDRASEKARAN, Scientist ÔGÕ, and P. GHOSAL, Scientist ÔDÕ, are with the Defence Metallurgical Research Laboratory, Hyderabad 500 058, India. Contact e-mail: raghumathur@rediffmail.com S. RAY, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee 247 667, India. Manuscript submitted January 12, 2007. Article published online August 4, 2007. 2076—VOLUME 38A, SEPTEMBER 2007
Though FMSM phenomena are displayed by many alloys, single crystals of off-stoichiometric Ni2MnGa Heusler alloys have shown large magnetic-field-induced strains at magnitudes of from 6 to 10 pct at room temperatures.[3–6] The stoichiometric Ni2MnGa alloy shows a magnetic transition at 376 K and structural martensitic transformation at 202 K. Both Ni- and Mn-rich off-stoichiometric compositions increase the martensite transformation temperatures.[7,8] The Ni-rich martensite does not show FMSM phenomena and is being studied more for its magnetocaloric properties. The Mn-rich compositions are employed for FMSM applications, because they exhibit a five- or sevenlayered modulated structure of martensites and low detwining stresses.[5,9] The Mn substitution for Ga in these alloys increases the martensite transformation temperature; however, the Curie temperatures are lowered.[8] Ternary Ni50Mn30Ga20 (at. pct) shows the maximum operating temperatures possible as the martensite transformation approaches the Curie temperature.[10] The alloys with a martensite transformation temperature higher than the Curie temperature normally show a nonmodulated martensite structure undesirable for FMSM applications.[11] Additional imp
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