Study On A Novel Mg-Zn-Y-Zr Cast Damping Alloy

A series of requirements of having high strength, high damping and easy to forming for structural materials have been raised as the development of modern aerospace industry. Magnesium and its alloys are getting an increasing number of attention for the low density, high specific strength and stiffness and good processing performance. Besides, magnesium alloys have such advantages as excellent damping capacities which other metallic materials do not have. It is a trend to use magnesium alloys with both high strength and high damping as structural material in aerospace industry. Magnesium alloys reinforced with icosahedral quasicrystal phase (I-phase) have become a hot spot and have the potential to meet the requirements above. However, most quasicrystal reinforced magnesium alloys have a large amount of rare earth and other elements, which is unfavorable for lowering density and reducing cost of alloys. Hence, in this subject, a kind of quasicrystal reinforced Mg-Zn-Y-Zr alloys were successfully fabricated through adding Zn and Y elements into a Mg-Zr high damping binary alloy under "multi element micro alloying" principle. The composition range of quasicrystal phase in the Mg-Zn-Y-Zr alloys with low content of alloying elements was systematically studied as well as the microstructure, mechanical properties and damping capacities of Mg-Zn-Y-Zr alloys with different content of quasicrystal phase. Further modifications, including Nd alloying, heat treatment and hot extrusion, were carried out on the Mg-Zn-Y-Zr alloys with good comprehensive performance. The aim is to strengthen the alloys under the premise of maintaining their damping capacities as possible. The phase transformation, strengthening and damping mechanism of Mg-3.0%Zn-0.6%Y-0.6%Zr alloy (ZWK3) and Mg-3.0%Zn-0.6%Y-1.2%Nd-0.6%Zr alloy (ZWEK3) under different heat treatment process were mainly investigated through metallographic analysis, scanning electron microscope, X-ray diffraction and transmission electron microscope. The connection between microstructure evolution and property change has been revealed to provide theoretical and experimental basis for further development of high strength and high damping magnesium alloys.The matrix alloy used in this study is Mg-0.6%Zr alloy. First of all, the effect of Zn/Y ratio (in weight) on phase composition, mechanical properties and damping capacities was studied. The results show that when the ratio is2and3there is W-Mg3Y2Zn3phase forming in the alloys and when the ratio is up to5the I-Mg3Y-Zn6comes up. The W-phase and I-phase are both beneficial for mechanical properties and the I-phase have better strengthening effect. The I-phase/matrix interface is stable and coherent so there is no resident stress and dislocation tanglement generation, and is complete without cracks during loading which makes the Mg-Zn-Y-Zr alloy containing the I-phase have the best strength and damping combination. Mg-Zn-Y-Zr alloys with different content of I-phase were obtained by changing the amount of Zn and Y elements with a changeless Zn/Y ration of5. The room temperature tensile strengths of the Mg-Zn-Y-Zr alloys are obviously improved as the increasing content of I-phase up to a maximum of216MPa while the damping capacities decline in different degrees. The Mg-3.0%Zn-0.6%Y-0.6%Zr alloy (ZWK3) exhibits the best comprehensive properties. The room temperature tensile strength is206MPa, the elongation is18.6and the damping value is0.03at a strain of1x10-3, which is far higher than that of AZ91D alloy and commercial pure Al. After the addition of Nd element, the damping capacities are obviously higher than that of ZWK3alloy. The Mg-3.0%Zn-0.6%Y-1.2%Nd-0.6%Zr alloy (ZWEK3) have the best comprehensive performance.The ZWK3and ZWEK3alloys are chosen to be further heat treated. The best heat treatment processes are:high temperature annealing:350℃x8h, T4:425℃x8h, T6:425℉x8h+175℃x24h. There are nano scaled sphere I-phase and short-rod like Mg7Zn3precipitates in the annealed ZWK3alloy and those precipitates strengthen the alloy but lower the elongation. Most phases are dissolved and only the I-phase precipitates exist in the T4treated ZWK3alloy. The tensile strength gets a maximum of224MPa among all the other heat treated ZWK3alloys after T4treatment and the elongation comes back to higher than16%. A large amount of nano MgZn2phase precipitates in the T6treated ZWK3alloy as well as a little long period stack structure. The phase transformation and tensile behavior of the heat treated ZWEK3alloys are similar to those of heat treated ZWK3alloys with a difference that the improvement of tensile strength of the T6treated ZWEK3alloy is more obvious and the strength is up to238MPa. It is the quasicrystal interface strengthening mechanism that mainly effects in the as-cast and annealed alloys and the solution strengthening mechanism in the T4treated alloys. The large amount nano scaled precipitates are responsible for the improvement of mechanical properties of the T6treated alloys. A lot of twins are observed in the T4treated ZWK3alloy after tensile test which are considered to be the main reason why the strength increases with the maintenance of high elongation. The strengths of the heat treated ZWK3alloys decline obviously at200℃while the elongations are significantly increased up to a maximum of25%.Dynamic recrystallization happens in ZWK3alloys during hot extrusion and lots of twins come up in non-recrystallization region. The recrystallization regions as well as the grains get larger as the extrusion temperature increases. The average grain size decreases sharply after extrusion compared with that of the as-cast alloy, from169μm to less than lOμm. Grain refinement brings a great increase in yield strength up to250MPa and the tensile strength improved to285MPa maximum. The elongations of extruded ZWK3alloys are all above11%. The tensile strengths of the extruded alloys change little after T5treatment while the yield strengths and elongations are enhanced further. Twinning is the main deformation mechanism of ZWK3alloys at room temperature while twinning combined with slip at elevated temperatures. The casting performance of the ZWK3and ZWEK3alloys is also studied. The two alloys exhibit excellent liquid formability so that they can be used as novel casting alloys.The damping behaviors of ZWK3and ZWEK3alloys can be most explained with the G-L dislocation model. The precipitates have strong pinning effect on dislocations so as the amount of precipitates increases the damping capacities of the alloys decline. There is no dislocation generated at the I-phase/matrix interface so the interface has little influence on damping capacities. The mobility of dislocations in the T6treated alloys is the worst while the damping capacities better than those of annealed and T4treated alloys, suggesting that there is new damping mechanism besides dislocation damping in T6treated alloys. The slender twins observed in T6treated ZWK3and ZWEK3alloys are considered to be the extra damping source.