Study On The Microstructure And Mechanical Properties Of The Rapidly Solidified And Sintering Mg-Gd-Zn Series Alloys

Magnesium alloys have a high specific strength,good castability and excellent machinability.They have widely used in the aerospace,automotive and other fields.Our study adopts a new manufacturing process:rapid solidification ribbon+spark plasma sintering process to prepare high performance magnesium alloys.Additionally,we are also discussed the influence of process parameters on the microstructure and its evolution,mechanical properties of magnesium alloys,during the manufacturing process of magnesium alloys.The main findings of this paper are as follows:(1)Based on the idea that Mg-Gd-Zn magnesium alloys which could form LPSO phase,icosahedral quasicrystal(IQC)phase and W phase with strengthening effect,three alloy are determined:Mg96.9Gd2.7Zn0.4,Mg61.6Gd4.4Zn34 and Mg97Gd1Zn2.They will form the LPSO phase,the quasicrystal phase and the W phase,respectively.The results show that,in the conventional solidified as-cast Mg96.9Gd2.7Zn0.4 alloy,the secondary phase is Mg3(Gd,Zn)which distributes along the grain boundary with a continuous net-shape,and the matrix contains up to 1.4 at.%Gd.This alloy shows a higher compressive yield strength and compressive strength of 170 MPa and 292 MPa,and possesses a good plasticity:the strain at fracture is 0.15.The as-cast Mg61.6Gd4.4Zn34 alloy is composed of abundant intermetallic compounds and a small amount of Mg matrix.The compounds contain ? phase(Mg59Gd6Zn35),H1 phase(Mg26Gd14Zn60),(Mg,Gd)7Zn3 phase,and MgZn phase.The strength is high but the plasticity is slitghtly poor in the as-cast piece,the compressive strength is 539 MPa(no obvious yield stage),and the strain at fracture is 0.04.For the as-cast Mg97Gd1Zn2 alloy,a plentiful of W phase distribute along the grain boundary,which size is coarser and the matrix contains a little Gd.The strength of the as-cast alloy is lower,and the compressive yield strength and compressive strength are 132 MPa and 234 MPa,respectively.The strain at fracture is 0.13.(2)The processes have a certain effect on the rapid solidified(RS)ribbons,which includes the heating power,heating time,injection pressure,the pressure in the device,and the speed of the copper roller.For the RS Mg96.9Gd2.7Zn0.4 ribbons,the optimum paremeters are:the heating power of 12.5kW,heating time of about 30s,injection pressure of 20Pa,the pressure of 1.5×10-3Pa Ar atmosphere,and the speed of the copper roller 30m/s(90r/s The size of the ribbon 3600 mm × 3 mm×60 ?m).The grain size of the RS ribbon is less than 1 ?m,the content Mg3(Gd,Zn)phase is significantly reduced.For the RS Mg61.6Gd4.4Zn34 ribbons,the optimum paremeters are:the heating power of 12-20kW,heating time about 10s,injection pressure of 20Pa,the pressure of 1.5?10-3Pa Ar atmosphere,and the speed of the copper roller 35m/s(95r/s),the surface quality of this RS ribbon is fine.The size of the ribbon 800 mm x(3-5)mm×29.1?m.The RS ribbons mainly contain a large number of quasicrystals with a size of less than 100nm,and a small part of amorphous phase,or a petal-like shape quasicrystals with a size about 20?m,and H1 crystal phase.The microstructure is related to the preparation process of the ribbon.For the RS Mg97GdiZn2 ribbons,the optimum paremeters are:the heating power of 12.5kW,heating time of 31s,injection pressure of 20Pa,the pressure of 1.5×10-3Pa Ar atmosphere,and the speed of the copper roller 30m/s(90r/s).The size of the ribbon 2000 mm x 3 mm×50 vm.The RS ribbons possess both an average grain size of about 2 ?m and a W phase size of only several tens of nanometers.(3)The sintering processes have a certain effect on the sintering bulk alloys.The main process parameters have:the sintering temperature,sintering time and sintering pressure.For the sintering Mg96.9Gd2.7Zn0.4 bulk,its optimum paremeters are:the sintering temperature of 410-450?,the sintering time of 5min,and a sintering pressure of 45MPa.The microstructure is uniform in the bulk,and the grain size is a range of 5-25?m.Plentiful lamellar LPSO phase and the Mg3(Gd,Zn)phase with an average size of 106 nm uniformly distribute on its matrix.The yield strength,ultimate compressive strength and strain at fracture of the sintering bulk are 316 MPa,425 MPa and 0.16,respectively.For the sintering Mg61.6Gd4.4Zn34 bulk,its optimum paremeters are:360?/5 min/45 MPa.The quasicrystal phase existed in the RS ribbons has disappeared after the sintering process,and the microstructure is mainly composed by the block H1 phase and plentiful of short rod-like ? phases with a size of 1 ?m.The strength of the sintering alloy is still high,the plasicity is yet poor:the ultimate compressive strength and the strain at fracture are 563 MPa and 0.044.For the sintering Mg97GdiZn2 bulk,the optimum sintering temperature is 410?/10min/45MPa(or 450?/50min/45MPa).A large number of spherical W-phase particles with a size of 10-810 nm uniformly distribute on the matrix.The alloy shows a lower yield strength than the former two bulks.The yield strength,ultimate compressive strength and the strain to fracture of the alloy are:200 MPa,390 MPa and 0.24,respectively.(4)The analysis of strengthening-mechanism of the sintering alloys show that,for the sintering Mg96.9Gd2.7Zn0.4 bulk,the strengthening of grain-boundaries has contributed about 35%of the yield strength,additionally,the strengthening effect from solid solution and fine lamellar LPSO phase are close to 27%and 25%,respectively.The nano-sized Mg3(Gd,Zn)phase contributes about 13%.For the sintering Mg61.6Gd4.4Zn34 bulk,the high strength is mainly due to existing plentiful of intermetallic compounds.For the sintering Mg97Gd1Zn2 bulk,the nanoparticle W phases contributes at least 50%of the yield strength,meanwhile,it also remarkably improves the plasticity of the alloy(the strain of fracture is greater than 0.2 at the room temperature).(5)The magnesium alloys containing LPSO phase or W phase are suitable for the preparation of rapid solidification ribbons+low temperature sintering.The alloys has uniform microstructure,a certain content of secondary phases with small size,the sintering bulks show the excellent mechanical properties,which have closed to the high performance of Mg alloys prepared by the traditional process.However,the alloy containing the quasicrystalline phase is not suitable for the preparation of the process.