Damping behavior and mechanism of foamed Zn-Al eutectoid alloy

The present work investigates the damping behavior and mechanisms in foamed Zn-Al eutectoid alloy prepared by air pressure infiltration process. Pores in the foamed Zn-Al eutectoid alloy have macroscopic size of the order of a millimeter (0.5-1.0 mm) and in large proportions, typically up to 65 vol%. The damping behavior of the foamed Zn-Al eutectoid alloy is characterized by internal friction (IF). IF and relative dynamic modulus measurements at maximum surface shear strain of 20 x 10(-6) have been made using a multifunction internal friction apparatus (MFIFA) at three discrete frequencies of 0.5, 1.0, and 3.0 Hz over a temperature range from room temperature to 400 degreesC while continuously changing temperature. Two IF peaks were found in the IF-temperature curves. The first is a grain boundary IF peak, which is associated with the diffusive flux on a crystalline boundary of Al. Its activation energy has been calculated to be (1.12 +/- 0.04) eV and the pre-exponential factor tau(0) is 10(-14) s in IF measurements. The second is a phase transition peak, which results from the transformation of Zn-Al eutectoid. Experimental results show that this phase transition IF peak is a function of T/f (T rate of temperature change, f vibration frequency), but is not proportional to T/f. Finally, the damping capacity of the foamed Zn-Al eutectoid alloy, with different macroscopic pore volume fractions and two different macroscopic pore sizes, were compared with that of bulk Zn-Al eutectoid alloy specimens. The damping capacity of the materials is shown to increase with increasing volume fraction of macroscopic pores and increase with decreasing macroscopic pore size. The operative possible damping mechanisms in the foamed Zn-Al eutectoid alloy were discussed in light of IF measurements.