High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification
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TRODUCTION
IT is well established that an Al-8.5Fe-1.3V-1.7Si (8009 Al) type alloy processed by a rapid solidification and powder metallurgy route exhibits remarkable creep resistance up to temperatures around 700 K.[1,2,3] This creep resistance is due to the high volume fraction (,0.27) of fine incoherent particles (less than 50 nm in diameter) of the intermetallic Al12(Fe,V)3Si phase and the low coarsening rate of these particles at high temperatures. The high-temperature creep behavior of the Al-8.5Fe1.3V-1.7Si alloy was investigated by a number of authors, specifically by Carren˜o et al.[1] and Peng et al.[2,3] Carren˜o et al. applied the strain-rate-change technique in tension at temperatures ranging from 523 to 823 K and at strain rates ranging from 1026 s21 to 1022 s21. Every tensile test was carried out in less than 9 ks. Peng et al. applied the constantload compression-creep test technique in the temperature range from 573 to 723 K. Most of the creep strain rates were obtained from the stress incremental tests: at any applied stress, the specimen was crept to about 0.02 strain, at which point the authors assumed an attainment of the steady state, and then the applied stress was increased and the procedure repeated. The measured creep strain rates did not fully cover three orders of magnitude. Neither Carren˜o et al. nor Peng et al. have presented any evidence on the attainment of steady-state deformation. Both of the aforementioned authors found linear relations between their measured strain rates and applied stress in double logarithmic coordinates; the slopes of the straight lines (values of the apparent stress exponent mc) were found S.J. ZHU, Associate Professor, is with the Department of Mechanical Engineering and Intelligent Systems, The University of Electro-Communi´ , Senior Sciencations, Chofu, Tokyo, 182-8585 Japan. K. KUCHARˇOVA tist, and J. CˇADEK, Professor, are with the Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zˇizˇkova 22, 616 62 Brno, Czech Republic. Manuscript submitted November 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
to be much higher than ,5 (the value of mc for minimum creep strain rate in aluminum, e.g., Reference 4). The apparent activation energy of creep (Qc), estimated from the temperature dependence of the measured strain rates, was found to be much higher than the value of the activation enthalpy of the lattice self-diffusion in aluminum, i.e., 142 kJ mol21.[5] Such high values of mc and Qc are typical for dispersionstrengthened aluminum alloys (e.g., References 6 and 7). An interpretation of the creep behavior of the alloy was attempted,[2,3] alternatively, in terms of the true-thresholdstress concept and in terms of the concept of thermally activated detachment of dislocations from incoherent Al12(Fe,V)3Si particles. The analyses of creep data have not been found to be conclusive as to one of the previous concepts. This is because the authors[2,3] did not account for the temperature dependence of the true threshold stress being m
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