Elasticity Study in Ferromagnetic Shape Memory Alloys
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Elasticity Study in Ferromagnetic Shape Memory Alloys Liyang Dai, James Cullen, Jun Cui and Manfred Wuttig Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, USA ABSTRACT The temperature dependences of the elastic constants of Ni0.50Mn0.284Ga0.216 and Fe3Pd were studied. Measurements were conducted by the ultrasonic continuous wave method. Anomalous behavior of the elastic constants temperature dependence in austenitic NiMnGa was observed, especially an abrupt 15% softening of C11 at the Curie temperature. The latter anomaly was found to be strongly influenced by the presence and orientation of applied magnetic fields. In martensitic NiMnGa, the temperature dependences of the velocities of all eleven elastic wave modes had abrupt changes at 220K, which indicate a structural phase change from the tetragonal to a second phase at lower temperature. The temperature and magnetic field effects in Fe3Pd were also studied by the elastic constants measurements. INTRODUCTION Ferromagnetic shape memory alloys (FSMA) are a new class of active materials, which combine the properties of ferromagnetism with those of a diffusionless, reversible martensitic transformation. These materials have been scientifically interesting due to the possibility of inducing the shape memory effect with an external applied magnetic field. In these FSMA either inducing the austenite/ martensite transformation or rearranging the martensitic variant structure with an applied field will induce a reversible shape change. This effect has been named "ferromagnetic shape memory effect". While the former effect has been studied since the 1960's in martensitic steel and is relatively well understood, the latter has only recently received attention. The variant rearrangement phenomenon is a complicated one that depends on twin boundary mobility, the magnetic properties of the materials, proper biasing of the initial microstructure, proper specimen orientation, and specimen shape, among others. To date, Ni2MnGa and Fe3Pd are the only two alloys reported exhibiting giant (>0.5%) field-induced strain1-4. A great deal of work has accumulated that has detailed the characteristics of the martensitic transformation in these alloys2,5-7. The dependence of a solid's elastic properties on temperature in the vicinity of a structural transformation provides insight into the nature of the transition and has been the focus of many studies8-13. It is often found that one of the shear moduli, most often (C11-C12)/2, tends to decrease as the martensitic start temperature, MS, is approached from above, in some cases becoming as small as one or two G Pa at the transformation temperature. This softening of the modulus is the result of the tendency of the crystal lattice to transform to a new structure that is obtainable from the austenite by a continuous deformation of the lattice. Ni-Mn-Ga alloys with specific compositions develop a modulated structure10,11,14-17 at temperatures above the martensitic transition. This premart
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