Isothermal Martensitic Transformation Kinetics in Ni-Mn-Sn Ferromagnetic Shape Memory Alloys
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1200-G02-04
Isothermal Martensitic Transformation Kinetics in Ni-Mn-Sn Ferromagnetic Shape Memory Alloys Patrick J. Shamberger1, Alexandre B. Pakhomov1, and Fumio S. Ohuchi1 1 Materials Science & Eng. Dept., University of Washington, Box 352120, Seattle, WA. ABSTRACT This study examines the kinetics of the martensitic phase transformation in a representative Ni-Mn-Sn Heusler alloy. Here, we present data on isothermal and continuous cooling/warming transformations in both bulk polycrystalline and individual small particle samples. We demonstrate that while the martensite to austenite transformation proceeds very rapidly (faster than the time-scale of our observations), the austenite to martensite transformation has a significant isothermal component. A similar asymmetry is also noted in transformation behavior of individual martensite plates. We conclude that the observed time dependence is due primarily to nucleation-limited kinetics. INTRODUCTION Ni-based Heusler alloys have recently received intense interest due to a magnetic fieldinduced martensitic transformation near room temperature [1, 2]. This transformation is associated with a change in unit cell parameters and a latent heat, allowing for large magnetic strain recovery at constant temperatures (metamagnetic shape memory effect) [3], spontaneous reversible magnetic field-induced strain (two-way shape memory effect) [4], and magnetic heat transfer (magnetocaloric effect) [5]. These effects may lead to high-performance magnetic field controlled actuators and sensors, as well as high efficiency magnetic refrigerators. However, the performance of such functional devices is limited by the rate of the phase transformation. Fast rates of transformation allow for rapid actuation and sensing and lead to efficient thermal cycling, as there is no kinetic contribution to the hysteresis loss, while slow rates of transformation have the opposite effect. Recently, isothermal kinetics have been demonstrated in the Ni-Co-Mn-Sn and Ni-Co-Mn-In systems [6, 7]. Here, we investigate the rate-limiting mechanism of the martensitic transformation in the Ni-Mn-Sn Heusler alloy system by observing isothermal and continuous cooling transformation on the scale of individual martensite plates. EXPERIMENTAL Bulk polycrystalline Ni-Mn-Sn alloy samples were prepared with an arc-melting furnace using standard metallurgical techniques. After melting, alloys were annealed at 1200 K for 24 h in a quartz tube under a continuous vacuum, and were cooled slowly (~10 K/min) in the furnace to promote atomic ordering. Sample preparation methods are described in more detail elsewhere [8]. The composition of the alloy described in this study was determined by wavelength dispersive spectrometry with a JEOL 733 electron microprobe to be Ni = 50.5 (0.1), Mn = 34.0 (0.3), Sn = 15.6 (0.2). X-ray diffraction analysis of powdered samples indicated that only the
Heusler (L21) phase was present at room temperature, and that the sample transformed to the modulated martensite (4O) phase upon cooling. Overall
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