High-temperature relaxations in aluminum studied by isothermal mechanical spectrometry
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I.
INTRODUCTION
IN strain hardening aluminum, the origin of the hightemperature relaxation peaks is not yet understood. Compilations[1–4] of the published results about the relaxation peaks observed above room temperature in aluminum show very large differences between the various authors. This situation can arise from the fact that, in aluminum, relaxation effects are very sensitive to a large number of experimental parameters[5] such as the sample purity, the strain amplitude, the rate of temperature change, and previous thermomechanical treatments. Consequently, internal friction measurements using the classical anisothermal method, at fixed frequency, are not convenient for a rigorous study of the high-temperature relaxation in this metal. Because it is now possible to describe internal friction spectra vs frequency at fixed temperatures with great accuracy, we carried out experiments with different purity aluminum samples to obtain precise relaxation peak parameters and, hence, solved the controversy about their origin (grain boundary relaxation or network dislocation motion). Results using a 20 pct cold-rolled and high-temperature annealed 5N polycrystal specimen were previously published.[5,6] This article presents the results obtained during successive annealings in cold-rolled 3N polycrystal and concerns the peak observed between 550 and 620 K. II.
The measurement frequencies ranged between 160 and 1024 Hz, and ten discrete frequencies per decade were used. Internal friction was also measured at the pendulum-free frequency (160 Hz) by the free decay method. For each measurement temperature, the internal friction coefficient (Q21) was determined for three maximal vibration amplitudes: 5 3 1026, 1025, and 2 3 1025. III.
EXPERIMENTAL RESULTS
Figure 1 shows internal friction spectra obtained after annealing at increasing temperatures ranging between 327
EXPERIMENTAL PROCEDURE
The 3N aluminum polycrystal was annealed 6 hours at 870 K under vacuum, 67 pct cold-rolled, and then cut into flat bar samples (60 3 5 3 1 mm). Internal friction measurements were carried out with a forced torsional pendulum in a vacuum of 1025 torr. The pendulum and experimental technique were previously published.[7] Before the beginning of an isothermal experiment, the temperature was stabilized 24 hours and held constant during the measurement time (12 hours). ANDRE´ RIVIE`RE, Professor, and PASCAL GADAUD, Researcher, are with the Laboratoire de Me´canique et Physique des Mate´riaux, F86960 Futuroscope, Cedex, France. Manuscript submitted November 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Internal friction spectra obtained after annealings at different temperatures. The measurement temperatures are the same as the annealing ones: (A) 327 K, (B) 363 K, (C) 400 K, and (D) 474 K. εM 5 1025. VOLUME 28A, AUGUST 1997—1661
Fig. 2—Napierian logarithm of Q21 vs logarithm of frequency. Annealing and measurement temperature: 454 K. εM 5 5 3 1026.
Fig. 3—Napierian logarithm of Q21 vs logarithm of frequency. Annealing an
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