| Structural materials for aerospace and space are always required to have good damping capacity in addition to excellent lightweight characteristics and mechanical properties.Based on the dislocation mechanisms,the mechanical properties and damping capacity of metals are a pair of contradictions.It is difficult to improve the mechanical properties and damping capacity of metals simultaneously by dislocation strengthening.Therefore,it has become an urgent problem to explore new ways to synergistically improve mechanical properties and damping capacity,and to prepare lightweight alloy materials with excellent vibration and noise reduction,as well as good mechanical properties.In this dissertation,the microstructures of Mg-4Y-2Er-2Zn-0.6Zr and Mg-8Li-4Y-2Er-2Zn-0.6Zr alloys were regulated by means of heat treatment,rolling,extrusion,ultra-high pressure,etc.And ultra-light magnesium-based alloys with high damping and good mechanical properties were prepared.The morphology and type of long-period stacking ordered（LPSO）structure inαsingle-phase magnesium alloys were changed by heat treatment,and LPSO was introduced intoɑ+βdual-phase magnesium-lithium alloys.The effect of LPSO structure on the mechanical and damping capacity of the alloy was comparatively studied.Then,the alloy was rolled,extruded and treated with ultra-high pressure to regulate the microstructure（nano-twins,grain size,dislocation,substructure,phase interface,etc.）,LPSO phase（morphology and type）,special microstructure by ultra-high pressure,etc.,so as to improve the mechanical properties and damping capacity of the alloy.The following results are obtained:Lamellar 14H type LPSO exists in as-cast Mg-4Y-2Er-2Zn-0.6Zr alloy.After heat treatment,LPSO transforms into bulk 18R type.However,there is no LPSO in the as-cast Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy.After heat treatment,the LPSO structure of 18R type was successfully introduced.In the two alloys,the bulk 18R type LPSO can synergistically improve the mechanical properties and damping capacity.During hot rolling,the mechanical properties of Mg-4Y-2Er-2Zn-0.6Zr alloy gradually increase with the increase of rolling reduction.When the reduction is 60%,the mechanical properties of Mg-4Y-2Er-2Zn-0.6Zr alloy are the best.The tensile strength at room temperature is 285 MPa and the elongation is 14.6%.The strengthening is attributed to the combined effect of dispersion strengthening of broken LPSO phase,multiple high-angle kink strengthening of LPSO structural phase,twin and SFs strengthening.At this time,the damping capacity of the alloy was also improved.SFs and twin are new damping sources to improve the damping capacity of rolled alloy.The plastic deformation of extrusion plus rolling increases the tensile strength of Mg-4Y-2Er-2Zn-0.6Zr to 362 MPa,and the elongation remained at 7.8%.Compared to the rolled alloy（reduction of 60%）,the tensile strength is increased by 27%.The main reasons for the significant increase in tensile strength are LPSO phase kink,twin formation in both matrix and LPSO,and cross distribution of SFs and twins.After double plastic deformation of extrusion plus rolling,the damping capacity of Mg-4Y-2Er-2Zn-0.6Zr was also greatly improved.The generation of twins in both matrix and LPSO improves its damping capacity and counteracts the negative effect of increasing the number of strong pinning points.The heat-treated Mg-8Li-4Y-2Er-2Zn-0.6Zr alloys were hot-rolled,cold-rolled,and hot-rolled plus cold-rolled.After comparative research,the Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy cold rolled at room temperature has the highest mechanical properties,the best damping capacity and elastic modulus.The strengthening mechanisms of cold rolled alloys include uniform distribution of broken LPSO particles,DRX,strengthening of twins in both matrix and LPSO structural phase,and kink of LPSO phase.The damping increased from 0.01 as cast to 0.02 as cold rolled,which increased by 100%.The 18R LPSO structural phase and the nano twins in both matrix and 18R LPSO structural phase are new damping sources to improve the damping capacity of the alloy.The elastic modulus of the cold rolled alloy is as high as 48.9 GPa,which is mainly attributed to the existence of high elastic modulus LPSO phase,kink of LPSO phase,and formation of twins.After extrusion plus rolling,the tensile strength of Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy is also greatly increased to 233 MPa,and the elongation is also improved to 24%.The main strengthening mechanisms are the appearance of twins and the kink of both LPSO and matrix phase.Although the damping capacity of the Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy decreases after extrusion plus rolling,the damping value of the low-strain region is still higher than 0.01,which belongs to the high-damping magnesium alloy.The main reason is that,even though the generation of twins can improve the damping of the alloy,it cannot offset the adverse effects of the increase in the number of strong pinning points and the kink of theα-Mg andβ-Li matrix on the damping of the alloy.During the ultra-high pressure（UHP）heat treatment of 6 GPa,the microhardness of the Mg-4Y-2Er-2Zn-0.6Zr alloy is the highest,when the heat treatment temperature reaches1000℃.The tensile strength of the alloy is up to 326 MPa,which is 95%higher than that of ordinary heat treatment alloy,and the elongation can still reach 11.9%.The significant increase in the tensile strength of UHP alloys is mainly attributed to the obvious grain size refinement,solid solution strengthening and the generation of twins.The damping value Q-1of UHP treated Mg-4Y-2Er-2Zn-0.6Zr alloy in the low-strain stage is 0.03,which is three times higher than that of the ordinary heat-treated alloy.The significant improvement in damping is attributed to the introduction of a large number of mobile dislocations and the formation of twins.For UHP treated Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy,the microhardness at1000℃is 80 HV,which is about 86.8%higher than that of the as-cast alloy.The tensile strength of the alloy increased to 277 MPa,which was significantly higher than that of the plastically deformed alloys,while the elongation can still reach 24%.The main strengthening mechanisms for the significant improvement of mechanical properties include:solid solution strengthening,dispersion strengthening,precipitation strengthening of acicular phase,fine grain strengthening and nano-twin strengthening.The damping value Q-1 of Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy after UHP treatment is 0.05,which is higher than that of the Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy in any state.The main reasons for the significant improvement of damping are high-density movable dislocations,the precipitation of a large number of nano-scale particle phases and the formation of twins.For theɑ+βdual-phase Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy,the UHP alloy has the highest mechanical properties and damping capacity. |
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