A Synchrotron X-Ray Study of Texture Induced by Application of Magnetic Fields During Phase-Transformations in Shape-Mem
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A Synchrotron X-Ray Study of Texture Induced by Application of Magnetic Fields During Phase-Transformations in Shape-Memory Ni-Mn-Ga R. Vaidyanathan, S.Yilmaz1*, R.C. O’Handley2 and D.C. Dunand1 Advanced Materials Processing and Analysis Center and Mechanical, Materials and Aerospace Engineering Department, University of Central Florida, Orlando FL 32826, USA 1 Department of Materials Science and Engineering, Northwestern University, Evanston IL 60208, USA 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, USA * presently at Istanbul Technical University, Department of Mechanical Engineering, TR-80191 Istanbul, Turkey ABSTRACT Ni-Mn-Ga alloys can exhibit a thermoelastic phase transformation near room temperature, which is associated with the shape-memory effect (i.e., temperature-induced strain recovery after twinning) or superelasticity (i.e., strain recovery after a stress-induced phase transformation). This work uses a synchrotron X-ray transmission technique to investigate texture induced by applying magnetic fields during the phase transformation in polycrystalline Ni2MnGa. Synchrotron X-ray radiation is ideally suited for such investigations since the measurements are representative of twinning in the bulk, in contrast with measurements from conventional X-ray sources that represent surface measurements affected by surface relaxation. Magnetic texturing of polycrystalline Ni2MnGa, by cooling through the phase-transformation in the presence of a magnetic field, has potential to lead to polycrystalline materials with more compatible fieldinduced strains and hence increased twin boundary mobility upon application of a magnetic and/or stress field. INTRODUCTION The intermetallic compound Ni2MnGa exhibits a thermoelastic phase transformation near room-temperature from a high-temperature cubic austenite phase (BCC, a=0.5825 nm) to a tetragonal martensite phase (BCT, a=0.590 nm, c=0.554 nm) [1, 2]. The shape-memory effect has been observed upon thermal transformation of single crystals previously deformed by twinning in the martensitic phase [3-9]. Similarly, the superelastic effect was reported in a single-crystal deformed in the austenitic state, with reversible stress-induced formation of martensite [2, 10]. Since Ni2MnGa is ferromagnetic (TCurie= 103 °C [1]], there has been considerable interest in inducing reversible twinning (and thus large, reversible macroscopic strains) by the application of a magnetic field in martensitic Ni2MnGa. A magnetically-actuated Ni2MnGa shape-memory alloy would have response times much faster than current, thermallyactuated two-way-shape-memory NiTi alloys [6, 7, 11]. Magnetically-induced twinning has been demonstrated in monocrystalline Ni2MnGa samples with reversible strains as high as 6.1% at room temperature [12]. However, in polycrystalline form, Ni2MnGa shows much smaller field-induced twinning strains [8]. Due to the difficulty in fabricating monocrystalline Ni2MnGa, there is an interest in understanding the mechanis
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