Decomposition and thermodynamic property of metastable Fe-Zn solid solutions produced by mechanical alloying
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E.J. Lavernia Department of Chemical Engineering and Materials Science, University of California at Davis, California 95616-5294, and Department of Chemical Engineering and Materials Science, University of California at Irvine, Irvine, California 92697-2575 (Received 27 June 2002; accepted 30 September 2002)
Thermal decomposition of supersaturated single-phase body-centered cubic (bcc) Fe100−xZnx (5 艋 x 艋 65 at.%) solid solutions, processed via mechanical alloying of high-purity metal powders, was investigated using x-ray diffraction and differential scanning calorimetry (DSC). At elevated temperatures the metastable solid solution decomposed into a stable equilibrium aggregate consisting of the pure bcc Fe phase and an intermetallic compound Fe4Zn9. The decomposition temperature decreased with increasing Zn concentration. The enthalpy of decomposition for various Fe–Zn solid solutions measured by the DSC was in the range of 1.2–3.5 kJ/mol. The enthalpy of mixing of the as-milled solid solutions from elemental Fe and Zn powders was estimated to be 0.5–1.7 kJ/mol. In addition, the activation energies of decomposition for these solid solutions were determined on the basis of the Kissinger analysis, and their values appeared to be independent of the Zn concentration in the alloy, with an average of 147 ± 17 kJ/mol. I. INTRODUCTION
The formation and thermodynamic properties of metastable phases by mechanical alloying (MA) have been investigated for a variety of alloy systems in recent years.1,2 Among the alloy systems studied, those with components that are mutually immiscible under equilibrium conditions3–13 are of particular interest. In many cases, an extended solubility was observed even in systems with a large positive heat of mixing (e.g., Cu– Fe,3,4,11,12 Cu–Co,5 and Ag–Cu13). These supersaturated solid solutions produced by the MA process are not in the equilibrium state and therefore at elevated temperatures tend to undergo phase separation or decomposition to equilibrium products. For example, the mechanically alloyed Fe-rich or Cu-rich Fe–Cu solid solutions transformed back to the terminal body-centered cubic (bcc) Fe and face-centered cubic (fcc) Cu phases via a phase separation process.11,12 While mechanically enhanced alloying has been investigated in many systems, there was little attention drawn to the Fe–Zn alloy. At the equilibrium state, Fe and Zn have no mutual solid solubility at room temperature.14 According to Miedema et al.15 the system exhibits a
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 14 Mar 2015
positive heat of mixing (+4 kJ/mol) for the liquid alloy with equiatomic composition in reference to pure liquid states and a negative heat of formation (−5 kJ/mol) for the solid equiatomic compounds. Such a system is different from the binary systems with a large positive heat of mixing for both liquid and solid solutions, e.g., Fe–Cu and Co–Cu. Another distinct feature of this s
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