[112] Oriented Terfenol-D composites

Magnetostrictive composites containing Terfenol-D particulate promise improved durability and elevated frequency response when compared to monolithic Terfenol-D. The higher frequency response of magnetostrictive composites increases power density and widens the design space for devices employing magnetostrictive materials such as SONAR transducers or ultrasonic actuators. However, performance of magnetostrictive composites in terms of saturation magnetostrictivn and piezomagnetic coefficient significantly trails commercially available monolithic Terfenol-D. This dissertation attributes the decrease in composite magnetostrictive properties to a lack of particulate crystal orientation. To test this hypothesis, composites with particles preferentially oriented along [112] were produced using magnetic shape anisotropy. While particulate orientation does influence the behavior of magnetostrictioe composites, there are several other factors that require consideration. These factors include the magnetoelastic behavior of monolithic Terfenol-D, the elastic behavior of the matrix material, and the geometric arrangement of Terfenol-D particulate and matrix. All of these effects were studied in both oriented composites and non-oriented composites using a specially designed magnetoelastic testing apparatus capable of determining the magnetic and elastic material properties as a function of applied magnetic field and applied mechanical stress. Magnetoelastic testing results reveal that the preferentially oriented composites achieve significantly increased saturation magnetostriction as compared to non-preferentially oriented composites and nearly equal to commercially available monolithic material. Small field measurements indicate that the oriented particle composites also show increased peak piezomagnetic coefficients and peak magnetomechanical coupling as compared to non-oriented composites. The increases in saturation magnetostriction and other properties are attributed to crystal orientation. This conclusion is based on isolating the effect of orientation from the effects of stress induced anisotropy, 1–3 architecture, and demagnetization fields. Observations of composite stiffness as a function of applied magnetic field indicate that translation of non-180° ferromagnetic domains strongly influences elastic properties. Furthermore, the movement of domains in response to applied stress results in energy absorption during cyclic mechanical loading. The amount of energy absorption peaked at low stress amplitudes in absence of an applied magnetic field, and peaked at increasing stress amplitude with increasing applied magnetic field.