Effect Of Heat Treatment And Deformation On Low Frequency Damping Capacities Of Magnesium Alloys And Their Mechanisms

On the purpose to research and develop high damping Mg alloys, we designed and fabricated pure Mg, Mg-Al alloys, Mg-Ni alloys and Mg-Si alloys which containing elements which possess different solubilities in Mg. The microstructures characteristics of these Mg alloys were observed by OM and TEM. The mechanical properties were tested by tensile test and hardness test. The strain dependent and temperature dependent low frequency damping capacities of these alloys were studied by DMA. We studied the stabilities of the damping capacities in these Mg alloys by heat treatment, hot extrusion and small tensile deformation at room temperature. The point defects types which influence the damping capacities of Mg alloys were analyized by positron annihilation method. This paper revealed the low frequency damping behaviors of Mg alloys and their influence factors, established a good foundation to develop high damping Mg based materials with high performances.The research results shown that, the damping mechanism of pure Mg and Mg alloys is dislocation damping. The damping capacities of Mg alloys were strongly influenced by the type and amount of alloying elements. When Al which has high solubility was added into Mg, the damping capacity will heavily decrease. But small amount of Al atoms (<1%) could changed the distribution condition of foreign atoms in Mg and predominantly increased the damping value at high strain level. When Ni or Si which has very low solubility was added in Mg, and there are still largeα-Mg phase in these alloys, the damping capacities of these alloys remain high value, especially at small strain level. There are damping peaks P1 and P2 in pure Mg, Mg-Ni and Mg-Si alloys, which located at 80oC and 230oC, respectively. P1 is considered to be induced by the interaction between dislocations and the point defects in the crystal lattice of Mg, and P2 is the grain boundaries damping peak which is caused by the grain boundaries sliding at high temperature.Heat treatment has remarkable influence on the damping capacities in Mg which possessα-Mg phase with certain size. This is caused by the diffusion and redistribution of the nonequilibrium point defects groups inα-Mg phase by heat treatment. The point defects of as-cast Mg are existed as nonequilibrium point defects groups inα-Mg phase, and the amount of weak pinning points on dislocations are very small; when the heat treatment temperature is relatively low, the point defects diffussed along dislocations, and this made the amount of weak pinning points on dislocations increased, and the amount of them in the crystal lattice decreased; when the heat treatment temperature is higher than 350oC, the diffusion rate of point defects is strongly increased, and the point defects groups decomposed rapidly and reach balance very fast. Some of the point defects are located in the grain boundaries, and parts of them are evenly distributed in the crystal lattice. Though the amount of these point defects is not large, they have strong influence on the damping capacities and P1 and P2 damping peaks in Mg alloys when the distribution condition of them is changed. Compared with Mg alloys withα-Mg phase, the influence of heat treatment on damping capacities of Mg-Al alloys is very small. This is because that the Al content is high enough, which made the length LC between weak pinning points on dislocations decreased to a certain value, therefore the Mg-Al alloys got the saturated damping values. Though heat treatment could remarkably changed the microstructures of these Mg-Al alloys, it will not strongly change the content of Al in it, and then, the damping capacities of these alloys will not strongly influenced by heat treatment.The room temperature damping capacities of Mg-1%Al and Mg-1%Si alloys were decreased after deformation, and the influence of deformation on the damping capacity of Mg-1%Si alloys is more remarkable than that of Mg-1%Al. When the room temperature tensile deformation is higher than 3%, the P1 peak will be inhibited; subsequent annealing will not recover the inhibitition of P1 peak which was caused by extrusion. Mg-1%Al and Mg-1%Si alloys could abtain high damping values at high temperature range after deformation, but these high damping values are not stable. The high damping values at high temperature region is related to the movement of dislocations and grain boundaries during recovery recrystalization. The damping capacity could reback to the damping level of as-cast alloys when the recrystalization process is finished.