Magnetostriction and Ordering in Fe-16.4 AT.% Al Single Crystals

Fe-Al alloys are of interest as high strength, ductile and low cost magnetostrictive alloys. This work examines how the nature of short range ordering and possible presence of B2 and DO3 ordered domains change with thermal history in Fe-16.4 at.% Al single crystal and how this influences magnetostriction. Fe-16.4 at.% Al was chosen as to remain in the α-phase region but have as high an Al content as possible. Fe-16.4 at.% Al single crystal was prepared using high-vacuum arc-melting and were grown using a vertical Bridgman growth process. Single crystal samples with (200), (220), and (111) orientations were prepared. X-ray diffraction (XRD) was used to characterize crystal orientation. The three different thermal histories examined were (a) as-grown condition, (b) annealed at 1373 K (c) high temperature anneal followed by low temperature anneal at 573 K. The composition analyses were made using an electron microprobe. Magnetic measurements were made using a vibrating sample magnetometer, and the saturation magnetization values obtained were nearly the same for samples with three different thermal histories. XRD examination was carried out to characterize the phases present and short range ordering (SRO). Samples from all of the three different thermal treatments indicated a presence of SRO and a very small of amount of long range ordering (LRO) domains. XRD patterns of [200]- and [111]- oriented samples of the as-grown crystal sample and low temperature annealed samples indicate the presence of SRO that tends predominantly towards B2 like local arrangements. A very small contribution to SRO from DO3-like arrangement is also suggested. However, in the high temperature annealed sample there is no indication of the presence of DO3-like arrangements. The (3/2 λ100) value of 170 ppm observed for the as-grown Fe-16.4 at.% Al single crystal sample is higher than the (3/2 λ100) value of 141 ppm observed in the high temperature annealed Fe-Al alloy single crystals. The low-temperature annealed sample showed a magnetostriction value of about 160 ppm. The lower value of magnetostriction compared to as-grown and low temperature annealed samples is likely related to the observed differences in the nature of SRO.