Effect Of Ho Addition On The Microstructure And Mechanical Properties Of ZMT613 Magnesium Alloy

The unique advantages for Magnesium(Mg)alloys,such as low density,excellent damping capability and favorable recyclability,render them as ideal materials for the applications in microelectronics,aerospace and automobile industries for the purpose of weight reduction and high fuel efficiency.However,the widespread utilization of Mg alloy is significantly hindered due to its low strength and high cost either in alloying element addition or special processing technology.Although Mg alloys with a large amount of rare earth elements added,such as Mg-Gd-Y-Zn-Zr magnesium alloys,exhibit super-high strength,their high alloying element costs make them unsuitable for industrial mass production.Therefore,the further application of magnesium alloys urgently requires the development of a low-cost,high-strength Mg alloys.The microstructure and mechanical properties of the as-cast,as-extruded and as-aged ZMT613-xHo alloys has been systematically investigated by optical microscope(OM),scanning electron microscope(SEM)equipped with energy dispersive X-ray spectrometer(EDS),different scanning calorimetry(DSC),EBSD,transmission electron microscopy(TEM)and ambient tensile mechanical property testing,and there are some conclusions in this paper are obtained and summaried as followed:(1)The as-cast ZMT613 alloy is mainly composed of?-Mg matrix and compounds segregated at interdendrites including?-Mn,Mg-Zn,Mg₂Sn and the new ternary phase of Mg_xSn Ho only appearing in in Ho-containing alloys,the new phase resulting from Ho addition will consume the Sn elemen,thereby suspressing the formation of Mg₂Sn phase.Moreover,the addition of Ho element can significantly refine the?-Mg dendrites,and the space of secondary dendrites of ZMT613-0.8Ho alloy is calculated as 22.7?m,which is much smaller than the 32.8?m of the ZMT613.Furthermore,it can be seen that the addition of Ho element will also weaken the effect of homogenization treatment.The compounds with large size increase with the content of Ho element increasing,attributed to their numerous high melting point,and further weakening the homogenization effect.(2)Under the hot extrusion condition,the number of second phases increases with the content of Ho increasing gradually,accompanied by the varying grain size showing1.75?m,2.88?m,2.49?m and 2.22?m for the alloys with the Ho contents increasing gradually.The basal texture intensity shows a fluctuating trend,and the concerning values are 3.60,17.22,5.50 and 7.43 for the ZMT613-xHo alloys,respectively.According to the microstructure and mechanical properties of the extruded alloys,it can be found that the0.1 wt.%Ho addition to the ZMT613 alloy produces significant texture intensifying,ascribed to the many Ho atoms dissolution behavior into Mg lattice,and thereby strengthening axial ratio c/a.For the alloy with 0.4 wt.%Ho addition,massive Ho atoms combine with Sn and Mg atoms to form firstly Mg_xSn Ho compounds and increase the density of precipitates,which promotes recrystallization,refines grains and decreases texture intensity.With the further addition of Ho being 0.8 wt.%,numerous Ho atoms are mostly precipitated at the grian boundaries leading to foming the bulky precipitates with continuous distribution along extrusion direction,which noy only prohibits the dynamic recrystallization(DRX)and increases the basal texture intensity,but also causes the stress concentration to deteriate the ductility from 17.1 for 0.4 wt.%Ho-containing alloy to 10.9%for the ZMT613-0.8Ho alloy under external loading.Therefore,the ZMT613-0.8Ho alloy exhibits the most excellent mechanical properties in the as-extruded ZMT613-xHo alloys,with the yield tensile strength(YTS),ultimate tensile strength(UTS)and elongation to failure being 235 MPa,346 MPa and 17.1%,respectively.After study,the proper solid solution prameter for ZMT613-xHo alloy is 420?/2 h.(3)After aging treatment,a large number of fine and dispersed second phases are precipitated in the alloy,including Mg-Zn and Mg₂Sn compounds,as well as the granular Mgx Sn Ho phase only appears in the Ho-added alloy.Furtheremore,it can be found that adding Ho to the ZMT613 alloy leads to the coarsening precipitates as the Mg-Zn intermetallics accompanied by the particle-deplted zones(PD zones),resulting in the strong dislocation pining effect and grain refining,promoting the strength inprovement.For the ZMT613-xHo alloy under single-aged treatment concerning the 180?/12 h,ZMT613-0.4Ho has the optimal comprehensive mechanical properties with the YTS,UTS and elongation are 320 MPa,400 MPa and 5.19%,respectively.For the alloys under two-step aged process being the 90 ^oC/24 h+180?/8 h,the ZMT613-0.8Ho alloy has the best comprehensive mechanical properties,its YTS,UTS and elongation are 345 MPa,405 MPa and 6.33%respectively.