Domain Structure Produced by Confined Displacive Transformation and Its Response to the Applied Field
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OBTAINING a large reversible field-induced strain is one of the fundamental issues in ‘‘quick and slim’’ actuator and sensor design. In conventional materials, such strain is attainable due to intrinsic piezoelectricity or magnetostriction effect, but the magnitude of this strain is small (normally less than 0.1 pct). As has been demonstrated in recent experimental studies, a promising approach is to use a reorientation of structural domains caused by motion of their boundaries driven by applied fields.[1–4] A rebalancing of volumes of domains with different orientations that is produced by their field-induced rearrangement generates a significant macroscopic strain comparable with the Bain strain of the displacive transformation (DT). If the domain rearrangement is generated by an applied stress, the corresponding macroscopic strain caused by this rearrangement is of an order of magnitude higher than the Hookean strain in most of the conventional solids. The domain reconfiguration process caused by applied stress or magnetic (electric) field has recently received much consideration as it pertains to exploring the efficiency of field-induced strain.[5–8] However, most studies focus on compositionally homogeneous systems, wherein the field-induced strain is found to be irrecoverable without a biased field. Moreover, the recoverable strain response of the compositionally homogeneous domain structures to the biased field often shows a large hysteresis, which is undesirable for most applications.[3] Y. NI, Research Associate, and A.G. KHACHATURYAN, Professor, are with the Department of Materials Science and Engineering, Rutgers University, Piscataway, NJ 08854. Contact e-mail: [email protected] Y.M. JIN, Assistant Professor, is with the Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843. Manuscript submitted December 9, 2007. Article published online April 30, 2008 1658—VOLUME 39A, JULY 2008
A situation is expected to be different if a DT is confined within precipitates whose composition differs from that of the surrounding matrix phase. Examples of such transformations are bcc fi fcc DT in the bcc Curich precipitates[9] and presumably the cubic fi tetragonal DT in DO3 precipitates in Fe-Ga alloys.[10,11] The latter system shows a giant extrinsic magnetostriction.[12–14] This kind of transformation may be expected in many systems, in which the cubic fi tetragonal (or any other symmetry-reducing crystal lattice rearrangement) transformation develops in one of the phases of a two-phase mixture. If the size of precipitates is small and the domain wall energy density is low, the stressaccommodating domain structure within precipitates is miniaturized to nanosize scale.[15] The confinement of the DT should strongly influence the scale and morphology of the domain structure produced without an applied field. In particular, the confinement may lift or significantly reduce the energy degeneration of the domain structure, and thus make the initial domain structure energetically preferable. A deviation of the
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