Gallium-enhanced phase contrast in atom probe tomography of nanocrystalline and amorphous Al–Mn alloys Shiyun Ruan a , Karen L. Torres b , Gregory B. Thompson b , Christopher A. Schuh a,n a Department of Materials Science & Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA b Department of Metallurgical & Materials Engineering, The University of Alabama, Box 870202, Tuscaloosa, AL 35487-0202, USA article info Article history: Received 7 October 2010 Received in revised form 16 January 2011 Accepted 17 January 2011 Available online 28 January 2011 Keywords: Atom probe tomography Solute segregation Nanocrystalline alloys Amorphous alloys Nanocomposites abstract Over a narrow range of composition, electrodeposited Al–Mn alloys transition from a nanocrystalline structure to an amorphous one, passing through an intermediate dual-phase nanocrystal/amorphous structure. Although the structural change is significant, the chemical difference between the phases is subtle. In this study, the solute distribution in these alloys is revealed by developing a method to enhance phase contrast in atom probe tomography (APT). Standard APT data analysis techniques show that Mn distributes uniformly in single phase (nanocrystalline or amorphous) specimens, and despite some slight deviations from randomness, standard methods reveal no convincing evidence of Mn segregation in dual-phase samples either. However, implanted Ga ions deposited during sample preparation by focused ion-beam milling are found to act as chemical markers that preferentially occupy the amorphous phase. This additional information permits more robust identification of the phases and measurement of their compositions. As a result, a weak partitioning tendency of Mn into the amorphous phase (about 2 at%) is discerned in these alloys. & 2011 Elsevier B.V. All rights reserved. 1. Introduction The properties of nanostructured and amorphous alloys rely on details of the solute content and its distribution at the finest scales. For example, in nanocrystalline alloys, chemical ordering and solute enrichment at grain boundaries affect such properties as strength and thermal stability [1–7]. In amorphous alloys, subtle changes in chemical composition influence crystallization behavior and glass forming ability [8–25]. In dual-phase nano- crystal/amorphous composites, the phase composition, phase fraction and phase distribution impact, for instance, their mag- netic [2,10,11,26–29] and mechanical properties [30–32]. Thus, a sound understanding of solute distribution at scales from the nanometer down to the sub-nanometer regime is central to tailoring the properties of nanostructured and amorphous alloys. Unfortunately, many traditional chemical mapping methods like the Auger microscopy and energy dispersive X-ray spectroscopy in the transmission electron microscope (TEM) lack the resolution necessary for these advanced problems. On the other hand, three- dimensional atom probe tomography (APT) has very high spatial and chemical resolution, as well as equal sensitivity for all elements [33–35]; as a result, APT has been increasingly used to probe the spatial distribution of atoms in nanostructured and amorphous alloys. There have been several APT studies on nanocrystalline alloys. Among these, the most common issue addressed is local chem- istry at grain boundaries [1–5,10,36–43]; nanostructure forma- tion in many such alloy systems is attributed either to the thermodynamic effect of solutes in decreasing grain boundary energy [3–5], or to the kinetic effect of solutes in inhibiting grain growth [39,43]. Additionally, APT studies on some Ni–P [1,2] and Co–P [5] alloys help account for their high thermal stabilities; in such systems, as the alloys are heated to higher temperatures, the extent of solute segregation to grain boundaries increases, thus decreasing the driving force for grain growth. APT has also been employed to elucidate the phase transformation sequence and mechanisms in nanocrystalline alloys [1,2,5,37,42,44]: as tem- perature increases, solute enrichment occurs at the grain bound- aries until a new solute-rich phase precipitates there. Valuable knowledge about amorphous alloys, such as their crystallization mechanism during heat treatment, has also been acquired from APT studies [8–25]. In these studies, clusters of atoms are identified by considering the local density around each atom [9–15,17,20,22–25,28,45,46]. The chemical identity of atoms in these pre-nuclei, in turn, helps rationalize the effects of subtle chemical composition changes on the nucleation beha- vior of the alloys. Dual-phase nanocrystalline/amorphous compo- sites formed by such partial devitrification processes have also Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ultramic Ultramicroscopy 0304-3991/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2011.01.026 n Corresponding author. E-mail address: schuh@mit.edu (C.A. Schuh). Ultramicroscopy 111 (2011) 1062–1072