Effect of Aging Treatment on the Damping Capacity and Mechanical Properties of Mg-6Al-1Zn Alloy

The damping capacity and mechanical properties of Mg-6Al-1Zn alloy after heat treatment were investigated. The damping characteristics of un-heat-treated, solution treated, and aged Mg-6Al-1Zn specimens were determined by measuring the damping ratio and the logarithmic decrement of free vibrations of a bending beam clamped at one side. The microstructural evaluations confirmed that the β-Mg17Al12 phase was reprecipitated after aging and increased with an increase in aging time. The peak level of damping ratio and logarithmic decrement was obtained after 34 hr of aging time, over which the damping capacity declined according to increasing amount of strong pining points.


Introduction
Magnesium alloys have great potential for the applications in suppressing mechanical vibration and attenuating wave propagation for the control of noise and the stabilization of structure components because of their low density and good damping capacity [1][2][3][4]. However, the use of magnesium alloys has been limited to narrow range because of the limited mechanical properties such as low hardness and low tensile strength [5]. Recently, a large number of research studies have focused on improving the mechanical properties of magnesium alloys by alloying [6,7], plastic deformation [8][9][10][11][12], or heat treatment [9,13]. However, the improvement of mechanical properties often results in a decline of damping capacity [14,15]. For further engineering applications of magnesium, balancing the damping behavior and the mechanical properties has been a critical challenge [14,16].
The damping capacity of magnesium alloys is considered to be associated with different characteristics of precipitation, such as the presence and interaction of precipitation, and crystal lattice defects, such as dislocations and impurity atoms [17][18][19]. A good combination of mechanical properties and damping capacity could be achieved if the dislocations are firmly pinned by the impurity atoms and the phase precipitates, but on the other hand the dislocations are allowed to vibrate between these pinning points [20][21][22].
In the present work, the effect of heat treatment (solid solution and different aging durations) on the internal damping capacity and mechanical properties of Mg-6Al-1Zn (commercially known as AZ61) alloy and their relations was investigated. The damping tests were conducted using impulse-frequency response method with single cantilever vibration mode at room temperature. The intrinsic damping behavior was discussed in light of the microstructure characteristic and damping data.

Material and Experimental Procedure
The experiments were conducted using Mg-6Al-1Zn alloy (AZ61). The chemical composition of the used material is   Table 1. Figure 1 shows the SEM micrograph of unheat-treated (as-received) AZ61 alloy specimen. The un-heattreated specimen shows a lamellar mixture of depleted Mg solid solution ( -Mg) and intermetallic Mg 17 Al 12 compound. The intermetallic compound is revealed to be a mixture of discontinuous and continuous Mg 17 Al 12 -phase at grain boundaries [23]. The EDS of the un-heat-treated specimen shown in Figure 1(b) reveal that the elemental compositions of both gray and bright phases include Mg, Zn, O and Al. The un-heat-treated specimens were subjected to solid solution treatment at 410 ∘ C for 24 hr followed by water quenching. The -phase and eutectic phase (formed at the grain boundaries during the casting into the matrix) disappeared and the matrix transformed to single phase, supersaturated solid solution. The specimens were aged at 200 ∘ C afterwards, for different intervals of time. The Xray diffraction pattern of the solution treated and aged for 34 hr is shown in Figure 2. The heat treatment of AZ61 alloy is fundamentally a microstructure transformation process in which Mg 17 Al 12 phase dissolves during solid solution and then reprecipitates during aging and increases with the increase in aging time [24].
Damping test was performed using impulse-frequency response method with single cantilever vibration mode, which can simply and rapidly measure damping property of the material. The testing schematic diagram is shown in Figure 3. The dimensions of the damping test specimens were 50 mm × 20 mm × 2 mm. The damping test was conducted at room temperature. The measurement sequence consists of recording the free vibrations of the specimens excited by knocking it with an adequate hammer. After the exciting hammer hits the specimen, the amplitude of vibration gradually decreases with time as the vibration energy is dissipated. These decayed amplitudes and frequency of vibration were transferred to oscilloscope and computer by an accelerometer attached on the specimen. The amplitude decay as a function of time and the vibration modes was detected by the acquisition data system.
Several techniques are used to quantify the level of damping in a vibration structure [25][26][27]. Frequency domain approach (half-power bandwidth method) was used in this work as it is most commonly used to measure damping ratio in frequency domain [28]. This method is based on the frequency response function curve and consists in obtaining the maximum amplitude related to the natural frequency of the system and the two frequencies that have response amplitude equal to the maximum divided by the square root of two points corresponding to half power point. The loss factor of this method can be defined as where is the frequency at which the peak of the curve is obtained and 1 and 2 are the two frequencies, for which the frequency response is 1/√2 times the one at resonance. The damping ratio in a single degree of freedom is given by For small damping ( ≪ 1), the logarithmic decrement, , is defined as where −1 is the internal friction and can be calculated from the bandwidth Δ by The Scientific World Journal   Figure 4 shows frequency response variation as a function of frequency transformation using calculations carried out in MatLab program. The signals input into fast Fourier transform (FFT) were settled by Fourier transformation using MatLab program to measure the damping capacity from damping test of cantilever. The resonant frequencies of the un-heat-treated, solution treated, and aged AZ61 at 200 ∘ C for different intervals of time were calculated from these curves and listed in Table 2. For a quantitative measure of damping behavior, the half-power bandwidth method was used. The frequency response function is plotted by linear curve fitting as in the Figure 4, and the resonant frequency ( ) and half-power bandwidth frequency (Δ ) are calculated and damping loss factor ( ) can be determined using the data in (1). The Δ was determined from the resonant peak value. The usual convention is to consider points 1 and 2 to be located at frequencies on the response curve where the amplitude of response of these points is 1/√2 times the maximum amplitude. The bandwidth at these points is frequently referred to as half-power bandwidth. Mandal et al. [25] reported that half-power bandwidth method is useful for only small loss factor and the thickness of the specimen must be smaller than the corresponding wavelength. Figure 5 illustrates the corresponding damping ratio, , of the AZ61 as a function of the aging time at 200 ∘ C, compared with the un-heat-treated and solution treated specimens. The un-heat treated specimen has 2.58 × 10 −3 damping ratio. After solid solution, the damping ratio value slightly decreases to 2.24 × 10 −3 , which is resulted due to dissolution of the -Mg 17 Al 12 phase. However, after aging, the damping ratio continuously decreases to approximately 0.5 hr of aging time. An increasing trend is observed in the damping ratio up to 34 hr aging time, after the damping ratio shows a reduction. The peak drop of 1.8 × 10 −3 is observed at 167 hr aging time.

Results and Discussion
The damping ratio of the specimen aged at 34 hr increased because of increasing the interface area of the two phases with different stiffness, which is caused by discontinuous -phase precipitation inside -Mg matrix. Xu et al. [29] confirmed that numerous discontinuous Mg 17 Al 12 were distributed at the newly formed dynamic recrystallized grain boundaries as shown in Figures 6(a) and 6(b). Enhancement of damping properties is recognized to be attained by dislocation, interface of particles with different stiffness, and high damping particle or phase.
The decline in damping ratio after 34 hr aging time is appraised to be caused by the hardening of microstructure coinciding with the transition from -Mg matrix with discontinuous -phase to lamellar structure (Figure 6(b)). The SEM micrograph shown in Figure 6(a) reveals that the alloy exhibits some Mg 17 Al 12 precipitates at the grain boundaries and becomes interconnected to form a network. As it can be seen from Figure 6(b), there is substantive lamellar -phase distributing in the microstructure. The lamellar structure which is the product of the discontinuous precipitation always initiates at grain boundaries (in Figure 6(a)) and grows perpendicular to the boundary. The previous studies observed that the lamellar structures appear mostly under 200 ∘ C and it is hard to see them above 250 ∘ C [30]. The damping ratio of specimen aged at 167 hr aging time decreased by about 30% after formation of lamellar structure. Figure 7 illustrates the logarithmic decrement, , of the un-heat-treated, solution treated, and aged AZ61. The figure describes the effect of the Mg 17 Al 12 precipitates on the decay property. It has already been observed from the SEM investigation that, after solid solution, the -Mg solution becomes in the supersaturated state and thereby there will be a number 4 The Scientific World Journal   of solute atoms distributing along the grain boundaries that play a pinning role, which makes the movement of grain boundaries more difficult and thus the decay is decreased. After aging treatment up to 34 hr aging time, most of the solute Al atoms precipitate in the -phase and decrease the concentration of solute Al atoms in the -Mg matrix and the spacing between weak pining points becomes large. Thus, the diffusion of Mg atoms along grain boundaries becomes smooth and consequently the logarithmic decrement after aging process increases, as shown in Figure 7. At longer aging time (>34 hr) the precipitation hardening which is based on the principle of restricting dislocation mobility generally leads to reduction in the decay values. Figure 8 illustrates the corresponding hardness measurements (HV) of the AZ61 alloy. The un-heat-treated specimen has hardness value of 64 HV. After 24 hr solid solution treatment, the hardness value decreases to 57 HV. The small deviations exhibited by the un-treated and the solution treated specimens are assumed to be arisen from the fact that the un-treated specimen has already somephase. However, the Figure shows a slight decrease in the hardness value in the initial stages of aging treatment up to approximately 2 hr, which may be associated with a decrease in the dislocation density due to recovery and grain boundary relaxation processes.
After prolonged aging time up to 34 hr, it is clearly noticed that the HV values are slightly increased. Thereafter, with further increase in the aging time up to approximately 167 hr, the dependence of hardness values on the aging time became more obvious. The increase in hardness values is likely to be due to the precipitation of -phase in the matrix. By extending aging time (184 hr), the effect of grain growth becomes more dominant than the strengthening effect due to -precipitation [13]. Figure 9 shows the tensile properties curves at ambient temperature for the AZ61 alloy aged at 200 ∘ C. At the beginning of aging (up to 2 hr), the tensile strength and the yield stress slightly decreased with aging time. When being aged up to 34 hr, all samples have shown slightly increased strength value. When aging time exceeds 34 hr, the tensile strength value increases due to the development of Mg 17 Al 12 precipitates

Conclusions
The damping behavior of heat treated AZ61 alloy has been investigated. The following have been found.   properties values. When being aged for 167 hr, the dependence of mechanical properties on the aging time becomes more obvious.