Processing,Microstructure And Mechanical Properties Of Rapidly Solidified Powder Metallurgy High Strength Magnesium Alloys

Magnesium alloys have broad application prospects in many engineering fields due to their low density,high specific strength and easy recyclability etc.However,magnesium alloys still suffer from low strength and poor corrosion resistance,which nowadays seriously limits their development and application.Thus pursuing improved strength has been remained as an important topic for magnesium alloys in recent years.Rapidly solidified powder metallurgy(RS/PM)technology is a practical and effective technology to prepare high strength of magnesium alloys,which not only brings about grain refinement but also leads to unusual composition and microstructure.This paper aims to develop and prepare high strength RS/PM Mg-Zn-Y,Mg-Al-Zn system alloys and Mg based composites.The effects of the processing and the alloy composition on the microstructures and mechanical behaviors will be systematically investigated.Furthermore,comprehensive studies are conducted on the microstructural evolution and the strengthening mechanisms of the RS/PM Mg alloys and Mg based composites.(1)In order to determine the appropriate processing parameters,the effects of the powder particle size,sintering and extrusion process on the microstructure and mechanical properties of Mg-Zn-Y alloy have been investigated in detail.The cooling rate of Mg alloys during the argon atomization processing in this study is calculated to be 104～107K/s.The RS Mg94Zn2Y4 powders are much finer than the conventional cast alloys.The cold-compacted ability for Mg alloys is bad.It is shown that the round billet produced by warm compaction and sintering under the condition of 300 ℃+600 MPa exhibits highest density and best mechanical properties.With the decrease of powder particle size,the strength of RS/PM Mg alloy increases and the plasticity decreases.The RS/PM Mg alloy prepared by powders with the size lower than 74 μm possesses the best comprehensive mechanical properties.(2)Six Mg100-x-yZnxYy(x= 5.4～7,y= 5.4～7,at.%)alloys with the atomic ratios of Y/Zn = 2,1.5 and 1,respectively,are designed and prepared by RS/PM technology.It is shown that the phase compositions of the RS/PM alloys are mainly influenced by Y/Zn atomic ratio.For the alloys with Y/Zn= 2 or 1.5,the microstructures are composed of α-Mg and Mg12ZnY phase,while for alloys with Y/Zn= 1 mainly contain α-Mg,Mg12ZnY and Mg3Zn3Y2 phase.It is also found that the chemical composition and content of X-phase are influenced by Y/Zn ratio of RS/PM alloys.X-phase is most easily obtained when the Y/Zn ratio is 1.5.The ability to form X-phase depends on Y/Zn ratio,which follows the descending sequence of Y/Zn= 1.5,Y/Zn =2,Y/Zn = 1.It is found that X-phase is composed of 14H,18R,24R LPSO(long period stacking order structure),α-Mg and stacking faults,which means that X phase doesn’t possess the sole chemical composition.The synergistic strengthening effect of X-phase and W-particle is stronger than that of single X-phase.For the alloys containing same Y content,the one with the Y/Zn ratio of 1.5 exhibits best mechanical properties.(3)High strength Mg-Zn-Y composites reinforced by Nano SiC particulates(SiCp)are fabricated by RS/PM technology.The effect of volume fractions(0.6,1.2,1.8 and 2.4 vol.%)of Nano SiCp on the microstructure and mechanical properties of the Mg-Zn-Y composites are investigated.It indicates that the Nano SiCp indeed plays an important role in enhancing the matrix and that 1.2 vol.%addition of Nano SiCp is optimal.The main strengthening mechanisms are attributed to the thermal mismatch strengthening,fine grain strengthening and Orowan strengthening.(4)In order to decrease the cost and expand the application fields of Mg alloys,an AZ80 alloy with super-high tensile strength is fabricated by rapidly solidified powder metallurgy(RS/PM)and low temperature extrusion technology.By reducing extrusion temperature from 340 ℃ to 170 ℃,the α-Mg grains are significant refined to be 0.55 μm.The RS/PM alloy extruded at 170 ℃ exhibited the tensile yield strength of 448 MPa,the ultimate tensile strength of 480 MPa and the microhardness of 137 HV.These excellent mechanical properties are mainly attributed to the ultra-fine grains,the high-density dislocations and subgrains,and dispersive nano-scale β-Mg17Al12 particulate strengthening.