Magnetic properties and thermal ordering of mechanically alloyed Fe–40 at% Al Q. Zeng, I. Baker * Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, USA Received 8 May 2005; accepted 28 July 2005 Available online 28 September 2005 Abstract A metastable, nanostructured, disordered bcc Fe–40 at% Al solid solution was produced from elemental powders using a high-energy ball mill. The effects of milling and subsequent annealing on (1) the formation of disordered nanocrystals, (2) changes in the lattice parameter and grain size, and (3) the disorder-to-order, ferromagnetic-to-paramagnetic transformations were studied by examination of both the microstructure and magnetic measurements. Oxidation and contamination of the powder during processing may play a significant role in the thermal ordering. q 2005 Published by Elsevier Ltd. Keywords: A. Iron aluminides (based on FeAl); B. Magnetic properties; B. Order/disorder transformations; C. Mechanical alloying and milling 1. Introduction Fe–Al alloys have long been of interest for a broad range of applications. They form protective oxide scales in oxidizing environments [1], have reasonably low densities, and exhibit good intermediate-temperature mechanical properties compared with most iron or nickel-based alloys [2,3]. However, commercial development of these materials has been limited by their low ductility at room temperature and low mechanical strength above 600 8C. One approach for improving their ductility is by reducing the crystallite size to the nanometer range [4,5], another is by particle- induced slip homogenization [6,7]. ‘Mechanical alloying’ (MA) [8,9] via ball milling is capable of producing nano-structured materials on an industrial scale. MA is a solid state, dry milling process that leads, through a micro-sandwich morphology, to the mixing of elemental powders and eventual alloy formation [10]. This technique has become popular due to its comparatively low cost, great flexibility in the selection of processing parameters, and variety of attainable products. However, a serious problem with the ball milling of fine powders is the susceptibility to contamination from the milling media (balls and vial) or from the gas environment, both of which can affect materials properties [11]. According to the Fe–Al phase diagram, the ordered bcc or B2 phase exists over the composition range 36–50 at% Al at room temperature [12,13]. The alloy of interest here, Fe- 40 at% Al is paramagnetic at room temperature when fully ordered, but becomes ferromagnetic when strained [12–22]. For light deformation, e.g. by cold-rolling, the paramagnetic to ferromagnetic transition has been linked to an increase in the number of Fe–Fe nearest neighbors both in the anti- phase boundary (APB) coupled dislocation partials and in APB tubes [14–17,19–22]. The ferromagnetism arises mainly from the APB tubes and the magnetic moment can be calculated using the local environment model applied to these tubes [12,15,54]. No lattice expansion or general disorder is observed after this light deformation. For heavy deformation, e.g. by mechanical milling (MM), the paramagnetic to ferromagnetic transition is due to disorder throughout the material and this disordering is associated with lattice expansion in FeAl [12,18]. Therefore, related effects such as volume expansion could affect the magnetic properties. Hernando et al. [13] suggested that the lattice expansion of Fe–Al led to the narrowing of its electronic band structure and changed the density of states at the Fermi level, thus, increasing the magnetic moment. Very recent ab initio [23] and TB-LMTO [24] theoretical calculations Intermetallics 14 (2006) 396–405 www.elsevier.com/locate/intermet 0966-9795/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.intermet.2005.07.005 * Corresponding author. Fax: C1 603 646 9856. E-mail address: ian.baker@dartmouth.edu (I. Baker).