Microstructure And Properties Of High-modulus Mg-RE-Zn Magnesium Alloys

Magnesium（Mg）alloys with low density,excellent damping capacity,high specific stiffness,recyclability,etc.,have great potential applications.However,the low elastic modulus（40-45GPa）of Mg alloys makes them unable to meet the rigidity requirements of components in the aerospace and military industries.In addition,vibration and noise have become one of the three major public hazards.Mg alloys are widely used in vibration and noise reduction materials due to their good damping capacity.However,the currently developed high damping Mg alloys tend to have a low elastic modulus.The interaction between low elastic modulus and high damping capacity greatly limits the further application of Mg alloys in practical engineering,since attempts to increase the elastic modulus tend to reduce the damping capacity of Mg alloys.Therefore,the development of light Mg alloys with high elastic modulus and excellent damping capacity is of great significance to military and civil fields such as automobiles,aircraft,instruments and meters.In this work,Si,Al and Li elements were added to Mg-RE-Zn alloys by alloying method.The effects of alloying elements on the microstructure,mechanical properties and damping capacity of Mg-RE-Zn alloys were investigated.The influence mechanism of elastic modulus,strength and damping capacity of Mg alloys were also analyzed.At last,the synergistic improvement of elastic modulus and damping capacity in the Mg-RE-Zn alloys was obtained,and the design idea of Mg alloys with high elastic modulus and excellent damping capacity with high strength was proposed,which provided a reliable theoretical support for the development and application of high-performance Mg alloys.The main results of this paper were as follows:（1）In the Mg-Y-Zn-xSi（x=0.3,0.6,1.2 wt.%）alloys system,the number of Mg₃Zn₆Y and LPSO phases gradually decreased,while that of Si Y and Mg₂Si phases gradually increased with the increase of Si content.In addition,with increasing Si content,the gain size increased and then decreased,and the texture intensity increased sequentially.The as-extruded Mg-Y-Zn-1.2Si alloy exhibited the optimal comprehensive mechanical properties with the elastic modulus,ultimate tensile strength and tensile yield strength of 49.7GPa,303MPa and 210MPa,respectively.In addition,with increment of Si content,the damping capacity of as-cast and as-extruded Mg-Y-Zn-xSi alloys at room temperature and high temperature were significantly improved,and an obvious P₂ peak appeared in the as-extruded alloys at high temperature.（2）In the Mg-Gd-Y-Zn-Mn-xSi（x=0,0.8 wt.%）alloys system,the addition of 0.8wt.%Si led to the almost disappearance of LPSO phase and the appearance of a large number of RE+Si particles,along with coarsening of the grain size and a significant weakening of the basal texture.Compared with the Mg-Y-Zn-xSi alloys and Mg-Gd-Y-Zn-Mn alloy,the extruded Mg-Gd-Y-Zn-Mn-0.8Si alloy achieved a synergistic improvement in elastic modulus and damping capacity,with an elastic modulus of 49.3 GPa and excellent room-temperature damping and high-temperature damping capacity.Moreover,the room temperature damping value was～0.076 at1×10^-3 strain amplitude.The high elastic modulus was due to the formation of high modulus second phases Gd₅Si₃and Si Y.High damping capacity at room temperature was attributed to the generation of a large number of movable dislocations and weak basal texture.With the increase of temperature,the as-extruded Mg-Gd-Y-Zn-Mn-0.8Si alloy exhibited high damping capacity,which was mainly related to dislocation movement,grain boundary migration and interface slip.（3）In the Mg-Y-Zn-xAl-yLi（low content Al and Li,x=0,0.5,y=0,2 wt.%）alloys system,the as-cast Mg-Y-Zn and Mg-Y-Zn-2Li alloys mainly includedα-Mg+18R LPSO.The addition of Al element led to the formation of new Al₃（Y,Zn）phase and contributed to the generation of 14H LPSO phase during solid solution treatment.After extrusion,the Mg-Y-Zn and Mg-Y-Zn-2Li alloys exhibited uniform microstructure,high recrystallization degree and low texture intensity,while the unrecrystallized structure and texture intensity were significantly enhanced after the addition of Al elements.The as-extruded Mg-Y-Zn-2Li alloy showed the highest elastic modulus of48.8GPa,and the as-extruded Mg-Y-Zn-0.5Al alloy displayed the highest ultimate tensile strength and tensile yield strength of 375MPa and 279MPa,respectively.In addition,the addition of Al and Li was benefical to the improvement of the independent strain damping capacity at room temperature and high temperature damping capacity of as-cast and as-extruded alloys,as well as appearance of P₂ peak,but reduced the damping capacity at high strain amplitude.（4）In the Mg-Y-Zn-xAl-yLi（high content of Al and Li,x=0,7,y=0,5 wt.%）alloys system,after adding Al and Li elements,LPSO phase disappeared and a new Al-Li phase and Al₂Y phase were formed.Compared with the Mg-Y-Zn-xAl-yLi（low content of Al and Li）alloys,the elastic modulus and damping capacity of as-cast Mg-Y-Zn-7Al-5Li alloy were improved synchronously,with high elastic modulus of52.9GPa and excellent room-temperature and high-temperature damping.Moreover,the room temperature damping value was～0.022 at 1×10^-3strain amplitude.The high elastic modulus was attributed to Li in solid solute,the formation of high modulus Al-Li compounds and Al₂Y particles.High damping capacity in the Mg-Y-Zn-7Al-5Li alloy at room temperature was mainly due to the existence of a large number of movable dislocation density.With the increasing temperature,the superior damping capacity was mainly related to dislocations movement,grain boundary gliding and interface slip.After rolling,the grain refinement and the formation of fine Al-Li phase and nano precipitated Al_0.9Li_34.3Mg_64.5Zn_0.3 phase obviously enhanced the strength and duclity.When the rolling reduction increased to 80%,the mechanical anisotropy of the alloy was significantly improved by the weak symmetry texture,the reduction of dislocation density and the activation of non basal<c+a>slip.