Electronic structure of Fe-based amorphous alloys studied using electron-energy-loss spectroscopy H. J. Wang, 1 X. J. Gu, 2 S. J. Poon, 2 and G. J. Shiflet 1 1 Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA 2 Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA Received 3 January 2007; revised manuscript received 25 April 2007; published 16 January 2008 The local atomic electronic structures of Fe-Mo-C-B metallic glasses are investigated using electron energy- loss spectroscopy EELS. The fracture behavior of this Fe-based amorphous alloy system undergoes the transition from being ductile to exhibiting brittleness when alloyed with Cr or Er atoms. In addition, the glass-forming ability is also enhanced. This plastic-to-brittle transition is suggested to correlate with the change of local atomic short-range order or bonding configurations. Therefore, the bonding configuration of Fe-Mo- C-B-ErCr amorphous alloys is investigated by studying the electronic structure of Fe and C atoms using electron energy-loss spectroscopy. It is shown that the normalized EELS white line intensities of Fe-L 2,3 edges decrease slightly with an increasing amount of Er additions, while no noticeable difference is obtained with Cr additions. As for the C K edge, a prominent change of edge shape is observed for both alloy systems, where the first peak corresponding to a 1s →1 * transition increases with increasing Er and Cr additions. Accordingly, it is concluded that changes in the local atomic and electronic structure occur around Fe and C atoms when Er and Cr are introduced into the alloys. Furthermore, it is pointed out that the formation of Er-C and Cr-C carbide like local order inferred from the observed C K edge spectra can provide a plausible explanation for the plastic-to-brittle transition observed in these Fe-based amorphous alloys. In spite of the complexity of elec- tronic and atomic structure in this multicomponent Fe-based metallic glass system, this study could serve as a starting point for providing a qualitative interpretation between electronic structure and plasticity in the Fe- Mo-C-B amorphous alloy system. Complimentary techniques, such as x-ray diffraction and high-resolution transmission electron microscope are also employed, providing a more complete structural characterization. DOI: 10.1103/PhysRevB.77.014204 PACS numbers: 61.43.Dq, 79.20.Uv I. INTRODUCTION Since the development of Fe-based bulk metallic glasses, there is an increasing interest in these alloys as structural materials because of several attractive physical properties. These properties include high strength and hardness, corro- sion and wear resistance, coupled with relatively low mate- rials cost. However, the lack of plasticity has limited the development of Fe-based metallic glasses as structural mate- rials. Recently, an Fe-Mo-C-B bulk metallic glass with the composition of Fe 65 Mo 14 C 15 B 6 has been found to exhibit some plasticity ca. 0.8% in compression. 1 A ductile-to- brittle transition occurs when lanthanide Ln elements or Cr are added with the purpose of enhancing the glass formability. 2 Most studies on Fe-based metallic glass sys- tems primarily focus on understanding the excellent soft magnetic properties of the alloys, as well as the methods of enhancing glass formability through empirical alloying strat- egies and atomic packing processes. 3,4 With rising interest and importance regarding Fe-based metallic glasses, a more broad understanding in the factors determining plastic performance is required. Generally, the differing behaviors in hardness and soft- ness, or brittleness and ductility in a solid material are related to a difference in resistance to plastic flow. 5 Since all the metallic glasses exhibit a dislocation-free homogeneous amorphous structure, the factor which could possibly deter- mine the level of plastic resistance is considered directly re- lated to chemical bonding. At a most fundamental level, the interatomic forces, i.e., bonding, are determined by local electronic structure. Investigating the changes and differ- ences in bonding configurations among these alloys with various Er or Cr additions that cross the ductile-to-brittle chemical regime is a first step towards constructing the cor- relation between plasticity and electronic structure. The base Fe-Mo-C-B bulk metallic glass selected for the present investigation contains two transition metals and two metalloid elements. Carbon is the main metalloid element because of its important role in forming bonds with all tran- sition metals and lanthanides. As a result, a better under- standing concerning the electronic structure and bonding configuration associated with carbon atoms should lead to more detailed information about how bonding configuration affects plasticity. In addition, Fe is the dominant element 50 atomic fraction. The change of electronic structure around Fe through alloying will offer information about bonding configurations and bonding formation. Therefore, a practical first step is to begin an investigation of the Fe elec- tronic structure with various Er or Cr additions. The exis- tence of metal-metal, metal-metalloid, and metalloid- metalloid bonding makes the analysis of the electronic structure within the alloys extremely difficult because of the complexity of bonding configurations. Consequently, the cur- rent electronic structure study will be focused only on C, Cr, and Fe atoms. Electron energy-loss spectroscopy EELS is employed to carry out the electronic structure study. This tool provides detailed information on the chemical environment of excited atoms. In transition metals, there may be a large density of unoccupied d states above the Fermi level, yielding promi- nent “white lines” in the near-edge structure of an EELS spectrum. The white lines, which generally contain double sharp peaks at threshold, arise through transitions from the PHYSICAL REVIEW B 77, 014204 2008 1098-0121/2008/771/0142048 ©2008 The American Physical Society 014204-1