Structure-Composition-Magnetostriction Correlations in Strong and Ductile Fe-Based Alloys with large Low-Field Magnetostriction

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TECHNICAL: This work examines the large enhancements in the magnetostriction of Fe with W and Mo additions, and how local atomic environment, short range ordering and structural defects can dramatically influence the magnetostriction in Fe-W, Fe-Mo, FeGa, and other alpha-Fe based magnetostrictive alloy single crystals. PI and co-workers have earlier shown that the addition of Ga to Fe results in a large increase in magnetostriction at low applied magnetic fields and that these alloys are strong and ductile. In a recent work, PI and coworkers have discovered a large increase in magnetostriction with W and Mo additions to Fe. The magnitude of magnetostrictive strain achieved in ductile Fe-4.4% W alloys, on a per atom basis, is comparable to that obtained in Fe-20 at.% Ga and Fe-27.5 at.% Ga alloy single crystals. Ongoing investigations also suggest that internal inhomogeneous strains introduced by the structural changes and defects play a much greater role than has been appreciated in determining the magnetostriction in these alloys. The long-term objectives are to gain an improved understanding of magnetostriction in Fe and Fe alloys and formulate the guidelines for the design of alloys with attractive magnetostrictive and mechanical properties. As a part of this effort, the work will focus on detailed examinations of (a) magnetostriction in bcc phase binary Fe-W and Fe-Mo alloy single crystals and the influence of ternary additions (b) the characterization of local atomic environments that shed light on near-neighbor interatomic distances and how they influence the magnetostriction in Fe alloys (c) how short range order is changed with thermal treatments and how it influences magnetostriction in Fe-W, Fe-Mo and Fe-Ga alloys, and (d) the effect of well-defined crystal defects and their distribution on magnetostriction. The defects to be examined include dislocation structures introduced with controlled single crystal deformation, and coherent second phases introduced through precipitation hardening treatments. The work envisaged involves alloy preparation by vacuum arc-melting, single crystal growth using Bridgman technique, structural evaluation, magnetic and magnetostriction measurements, and structure-composition-property correlations. Transmission electron microscopy will be used to characterize the defects. Extended X-ray absorption Fine Spectrum (EXAFS) measurements using a synchrotron radiation source will be used to probe the local atomic environments and determine the mean inter-atomic distances, coordination number and type of neighboring atom, and mean-square disorder of neighbor distance. Elastic measurements will be made using the resonance ultrasound spectroscopy technique. X-ray diffraction scans, rocking curve scans and X-ray topography measurements will be performed to assess the crystal orientation, short range order and the structural defects. NON-TECHNICAL: The work will further the knowledge that will enable the design and development of a new generation of inexpensive and high performance low-field magnetostrictive alloys for use in sensor and actuator applications. The work will (i) lead to Ph.D. dissertations of two graduate students, (ii) provide research opportunities for undergraduate and high school students, (iii) be used as an educational element to attract undergraduate students to metallurgical engineering, in particular women and underrepresented minority students, and (iv) enhance the undergraduate- and graduate-courses in Magnetic Materials, Metals Processing and Physical Metallurgy. Besides providing a basis for a more scientific understanding of magnetostriction in Fe alloys, this work will lead to the development of inexpensive and high performance alloys for use in acoustic sensors and actuators and magnetomechanical devices that are very rugged and of interest in a wide range of defense and commercial applications that would include nano-positioning devices, MEMS devices, sonar devices, active vibration damping devices, load and torque sensors, and electrical delay lines.