Xanes and Exafs Analysis of Ball-Milled Fe-Ni
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XANES AND EXAFS ANALYSIS OF BALL-MILLED Fe-Ni P.J. SCHILLINGa , R.C. TITTSWORTHb, V. PALSHINb, J.XUb, E. MAc, and J.-H. HEc Mechanical Engineering Department, University of New Orleans (UNO), New Orleans, LA 70148, USA, [email protected] b Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, LA 70806, USA c Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218, USA a
ABSTRACT EXAFS and XANES analysis have been applied in a study of ball-milled nano-crystalline iron-nickel alloys prepared with overall compositions spanning the two-phase region. EXAFS analysis was used to determine bond distances for like and unlike atom pairs in single phase face-centered cubic Fe-Ni alloys, demonstrating the presence of bond dilation for unlike neighbors. Using XANES analysis, the compositions of the two coexisting solid solutions were determined to characterize the ball-milled two-phase region. The compositions of the coexisting face-centered and body-centered cubic phases were found to be consistent with the overall composition of the mixture. The results obtained for the negative heat-of-mixing Fe-Ni system were used for comparison to the positive heat-of-mixing Fe-Cu system.
INTRODUCTION High energy ball milling has attracted a great deal of attention as a means of synthesizing alloy phases with extended solubilities, including amorphous and nano-crystalline alloys [1-5]. In particular, the Fe-Cu and Fe-Ni systems have been extensively studied [1, 6-13]. These two systems are interesting to compare since both form nano-crystalline alloys (Fe100-xCux and Fe100-xNix). In both systems, a bcc alloy is formed in Fe-rich mixtures and an f.c.c. alloy is formed in Fe-poor mixtures, with a two-phase region at intermediate compositions – x ≈ 20–50 for Fe100-xCux and x ≈ 10-40 for Fe100-xNix. The results of ball-milling are quite similar, despite the fact Fe100-xCux represents a positive heat of mixing (+∆H) system and Fe100-xNix represents a negative heat of mixing system (-∆H) system. Due to its ability to provide element-specific structural information is systems lacking long range order, x-ray absorption spectroscopy has been employed in the study of these nano-crystalline alloys. Extended x-ray absorption fine structure (EXAFS) spectroscopy has been used to establish the presence of atomic-level alloying in single phase alloys in the immiscible Fe-Cu system [10,14]. Harris et al. [14] used EXAFS to establish near-neighbor mixing and a dilation of bonds between unlike neighbors which accounts for the lattice expansion in f.c.c. Fe100-xCux (x = 50 – 70) ball-milled mixtures. In the two-phase region of Fe100-xCux (x = 20 – 50), x-ray absorption near edge structure (XANES) spectroscopy has been used to determine the phase fractions and compositions of each phase [15]. In the current study, the same approach – EXAFS analysis of single phase alloys and XANES analysis of the two-phase system – is applied to ball-milled Fe100-xNix mixtures.
EE8.4.1
EXPERIMENTAL
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