Pure magnesium exhibits the highest damping capacity among all the commercial metals [1], but its mechanical properties are so poor that it cannot be directly used as the structural material. Accordingly, several magnesium alloys have been developed to increase the mechanical properties and maintain the high damping capacity, e.g. Mg-0.6%Zr. Recently with the development of metal matrix composites (MMCs) technology, it becomes possible to introduce the appropriate reinforcements into pure magnesium to prepare magnesium matrix composites both with high damping capacity and high mechanical properties. Rawal et al. [2] studied Grf/Mg-0.6%Zr and Grf/Mg-1%Mn composites and they found that damping capacities of these composites were improved compared with their matrix alloys. Zhang [3] fabricated the (SiCw + B4Cp)/ZK60A Mg alloy matrix composite and the experimental results revealed that damping capacity of the composite was much better than the matrix alloy when temperature was higher than 50 ◦C due to the activation of dislocations near the interfaces. Actually, most of MMCs have better damping capacities than the matrix alloys [4–10]. The improvement of damping in MMCs can be contributed to the mechanisms of increasing crystal defects (e.g. dislocations), better damping capacity of the secondary phase (e.g. graphite powders), and absorption of vibration energy at the interfaces between the reinforcement and the matrix. The present study attempted to prepare pure magnesium matrix composite materials and to understand their damping behaviors. For the fabrication of magnesium matrix composites, pure magnesium (>99.9 wt%) was used as the matrix and the hybrid SiC particulates and mullite (Al2O3·SiO2) shot fibers were the reinforcements because the hybrid technology of particulates and shot fibers was developed to produce magnesium matrix composites with the advantages of better mechanical properties and lower cost [11]. The diameter of SiC is about 10 μm, while the diameter of mullite fibers is about 10–30 μm and the length is about 200–400 μm. In these composites. the total volume fraction of reinforcements was 8 vol% and 18 vol%, respectively. The composites were fabricated by the liquid pressure infiltration process of fiber-particulate preform. During processing, the preform was first prepared by the mixture of short fibers and particulates. Then it was preheated and infiltrated by the magnesium melt under pressure of argon gas in the infiltration furnace. Finally, the composites were extruded into rods at 350 ◦C and the specimens for tensile and damping tests were cut from the rods. Tensile tests were carried out on the MTS-810 testing machine at the strain rate of 2 × 10−3 s−1. Damping behaviors were studied on the MFIFA (multi-functional internal friction apparatus) with the specimens of 60 × 5 × 1 mm3. MFIMA is an inverted torsion setup in a vacuum chamber, which was produced by the Chinese Academy of Sciences. The forced vibration mode was used under the testing conditions of strain amplitude (e) 10−6–10−4, vibration frequency ( f ) 1 Hz, and temperature (T ) is about 12 ◦C. The results of tensile strength of pure magnesium and two composites are compared in Fig. 1. It is shown that the tensile strength of the two composites with 8 vol% and 18 vol% reinforcements are increased tremendously, namely about 110% and 170% improvement compared with pure magnesium, respectively. The increase of tensile strength of magnesium matrix composites are expected and the higher modulus and strength of reinforcements are the main reasons for the enhancement in the tensile strength. Fig. 2 shows the curves of damping capacity-strain amplitude of pure magnesium. It can be seen that damping capacity of pure magnesium is strongly dependent on strain amplitude and it reaches a high value Q−1 = 0.11 when e is 10−4. The damping behaviors of magnesium matrix composites are different from pure magnesium, as shown in Fig. 3. For the 8 vol% composite, the damping shows a weak dependence on strain amplitude. However, for the 18 vol% case, the damping hardly improves with increase of strain amplitude and it has the lowest damping capacity of Q−1 = 0.0022, which is far below that of pure magnesium. From Fig. 3, it also can be seen that damping of 18 vol% composite is higher than the one of 8 vol% when e at 1 × 10−6–2 × 10−5. But when e is larger than 2 × 10−5, the damping of 8 vol% composite becomes higher due to the rapid increase with strain amplitude. Therefore, it can be concluded that the strong dependence of damping on strain amplitude in pure magnesium resulted in the high damping capacity. On the other hand, damping capacity of the composite is decreased owing to the reduction of its damping dependence on strain amplitude and the reduction becomes greater when the reinforcements are increased. Damping mechanism of pure magnesium was explained by Granato-Lucke’s dislocation damping theory [12, 13]. In the G-L model, the dislocation line
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Mar 1, 2005
Dacia Jackovich + 3
Open Access