Relaxation mechanisms in Fe-Al-C alloys
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IRON-RICH alloys of the Fe-Al system are widely used because of their favorable combination of mechanical, magnetic, and dissipative properties. Besides their application as a functional material, Fe-Al alloys are attractive for structural applications because they show higher strength than iron, high corrosion resistance, and are relatively inexpensive. Binary Fe-Al alloys containing sufficient Al content may produce long-range ordering of two types: D03 (Fe3Al) and B2 (FeAl). The Fe3Al phase is stable at lower temperatures, while the FeAl phase is stable at higher temperatures for Al ⬍35 pct. At high temperatures, Fe-Al is a disordered bcc solid solution (designated A2). In contrast to the many studies of long-range ordering in binary Fe-Al alloys (e.g., References 1 through 4), a limited number of articles have examined carbon-atom short-range ordering accompanying D03 or B2 ordering[5,6,7] and the corresponding dissipative properties. The use of an internal-friction method allows a study of the parameters of carbon-atom diffusion and shortrange order caused by the Fe-Al order-disorder transition.[8–11] The loss maximum (Q⫺1) for the relaxation peak and, in particular, for stress-induced jumps of interstitial atoms in bcc metals (known as Snoek relaxation[12]), is described by the Debye equation:[13] Q⫺1 ⫽ ⌬ ⭈
1 ⫹ ( )2
[1]
where ⌬ is the relaxation strength, ⫽ 2f, and f is the
I.S. GOLOVIN, Professor, is with the Materials Science Department, Russian State Technology University MATI, Moscow, 121552 Russia, now with the Institute for Materials, Technical University of Braunschweig. Contact e-mail: [email protected] D. MUKHERJI, Scientist, is with the Institute for Materials, Technical University of Braunschweig, 38106 Germany. T.V. POZDOVA, Scientist, is with the Physics of Metals Department, Tula State University, Tula, 300600 Russia. N.Y. ROKHMANOV, Senior Researcher, is with the School of Physics, Kharkiv National University, Kharkiv, 61077 Ukraine. Manuscript submitted November 1, 2001. Dedicated to the memory of our colleague and friend Alexandr B. Lebedev. METALLURGICAL AND MATERIALS TRANSACTIONS A
measuring frequency. In practice, it is usual to measure Q⫺1 vs T (T being the temperature). The jump of carbon (C) atoms under stress in Fe-based alloys is the elementary step of carbon diffusion, and its temperature dependence is described by the well-known Arrhenius equation: ⫽ 0 ⫹ exp (H/kBT ), where H is the activation energy and, kB is the Boltzman’s constant. Some differences in the carbon Snoekpeak parameters in ␣ -Fe are reported in the literature: the peak temperature is 314 K[14] or 308 K[13,15] for 1 Hz, and the corresponding activation energy for C in ␣ -Fe is about 0.87 ⫾ 0.01 or 0.835/0.83 eV, respectively. A value of H ⫽ 0.84 ⫾ 0.04 eV was determined by magnetic after-effect spectroscopy.[16] A Zener peak caused by stress-induced reorientation of solute metallic atom pairs in solid solution is observed in Fe-Al alloys, with an activation energy of about 2.5 eV (H depends
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