Phase Transition and Texture Evolution in the Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys Studied by a Neutron Diffractio
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THE Heusler Ni-Mn-Ga ferromagnetic shape-memory alloys (FSMAs) with compositions close to the stoichiometric Ni2MnGa recently attracted great attention for their interesting physical properties, such as the magnetic shape-memory effect (MSME), magnetocaloric effect (MCE), and magnetoplasticity (magnetic-fieldinduced strain (MFIS)).[1–10] The MSME and MCE can be achieved through a magnetic-field-induced first-order martensitic phase transformation, associated with a second-order magnetic transformation. The MFIS in the Ni-Mn-Ga FSMAs is generated by the rearrangements of the martensitic twin variants through the twinboundary motion under the application of a magnetic field. Actually, the study of the martensitic-phase transformation from the high-temperature parent phase to low-temperature martensite is very important for Z.H. NIE, Graduate Student, Y.D. WANG, Y.D. LIU, and L. ZUO, Professors, and G. WANG, Researcher, are with the Key Laboratory for Anisotropy and Texture of Materials (MOE), Northeastern University, Shenyang 110004, P.R. China. Contact e-mail: [email protected] G.Y. WANG and P.K. LIAW are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. J.W. RICHARDSON, formerly Senior Research Scientist, Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, is deceased. This article is based on a presentation given in the symposium entitled ‘‘Neutron and X-Ray Studies for Probing Materials Behavior,’’ which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee. Article published online July 22, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
understanding the MFIS in this alloy system.[8] It is well known that the transformation strain and magneticfield-induced strain are strongly dependent on both of the applied prestresses and the orientation of parent austenite and martensitic variants.[2,3] So far, a fundamental scientific issue has not been well studied concerning the roles of stress or deformation on the selections of martensitic variants during the phase transformation in this alloy system. The explorations on some novel fabrication processes for enhancing the preferred grain orientation (the crystallographic texture) in the polycrystalline FSMAs do benefit the improvement of their performance properties. The neutron diffraction technique is advantageous over the traditional diffraction techniques, such as the X-ray diffraction and electron diffraction, in the study of the phase transition in many structural and functional materials. With increasing the atomic number of the chemical elements, an irregular change in the neutronscattering length provides an accurate tool for structural characterizations, especially in the Ni-Mn-Ga FSMAs that consist of the elements with the similar scattering factors for X-ray beams. Moreover, the penetrabilit
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