Finite-temperature Anisotropic Elastic Properties of Ni-Mn-In Magnetic Shape Memory Alloy
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1200-G06-02
Finite-temperature Anisotropic Elastic Properties of Ni-Mn-In Magnetic Shape Memory Alloy Kristen S. Williams1,2 and Tahir Cagin2,1 1 Materials Science & Engineering Graduate Program, Texas A&M University, College Station, TX, U.S.A. 2 Department of Chemical Engineering, Texas A&M University, College Station, TX, U.S.A. ABSTRACT Designing magnetic shape memory materials with practicable engineering applications requires a thorough understanding of their electronic, magnetic, and mechanical properties. Experimental and computational studies on such materials provide differing perspectives on the same problems, with theoretical approaches offering fundamental insight into complex experimental phenomena. Many recent computational approaches have focused on firstprinciples calculations, all of which have been successful in reproducing ground-state structures and properties such as lattice parameters, magnetic moments, electronic density of states, and phonon dispersion curves. With all of these successes, however, such methods fail to include the effects of finite temperatures, effects which are critical in understanding how these properties couple to the experimentally-observed martensitic transformation. To this end, we apply the quasi-harmonic theory of lattice dynamics to predict the finite-temperature mechanical properties of Ni-Mn-In magnetic shape memory alloy. We employ first-principles calculations in which we include vibrational contributions to the free energy. By constructing a free energy surface in volume/temperature space, we are able to evaluate key thermodynamic properties such as entropy, enthalpy, and specific heat. We further report the elastic constants for the austenite and martensite phases and evaluate their role as a driving force for martensitic transformation. INTRODUCTION Magnetic shape memory alloys (MSMAs) differ from traditional SMAs in that they are characterized by strong magnetoelastic coupling. The Heusler-type metals, such as Ni-Mn-X (X:Al,Ga,In,Sn,Sb), undergo martensitic transformations that are sensitive to alloy composition, external pressure, and applied magnetic field. Certain compositions display unique structural responses to external magnetic fields, undergoing either magnetic twin reorientation or fieldinduced phase transformations that lead to a macroscopic shape memory effect. In Ni-Mn-Ga, this effect generates recoverable strains that are an order-of-magnitude larger than those associated with the most common commercial magnetostrictors [1]. In the Ni-Mn-In system, martensitic transformation has only been observed for compositions with increased Mn content [2-4]. This magnetic-field-induced transition is driven by the relative size of magnetization between the parent and martensite phases, with stabilization of the austenite at high fields [2]. Alloys of Ni-Mn-In undergoing these transitions exhibit large magnetic-field-induced strains [2], giant (sometimes inverse) magnetocaloric effect [2,5-7], and high magnetoresistance [7-9], making them attractive materi
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