| Mg-Zn-Y alloy is an important commercial wrought magnesium alloy. The addition of Y in Mg-Zn alloy can effectively refine the grains, improve the structure of precipitation phase and enhance the ageing strengthening effect, which make the Mg-Zn-Y alloy having high strength and good plasticity. So Mg-Zn-Y alloy is widely used in the fields of automotive, electronics, aerospace and defense industry, etc. However, the commercial application of Mg-Zn-Y alloys has been limited because of poor heat resistance and high temperature mechanical properties. As a alloying elements, Alkaline-earth elements Sr can effectively improve the high temperature properties of magnesium alloys. Compared with the expensive rare earth elements, Sr has the advantages of light weight and low price.In this thesis,Sr was added into Mg-3Zn-1Y alloys and the effect of Sr on microstructure and high-temperature properties of Mg-3Zn-1Y alloys was studied. Five kinds of Mg-3Zn-1Y-xSr(x=0, 0.3, 0.6, 0.9, 1.5) alloys are designed and prepared in the present study. The microstructure and high temperature mechanical properties of Mg-Zn-Y-x Sr alloy were researched, and the strengthening mechanism of secondary phases on the high temperature mechanical properties and fracture mechanism of the alloys were analyzed.The results show that:(1) Mg-3Zn-1Y-x Sr alloys are mainly composed of ?-Mg, Mg3Y2Zn3(W phase) and a new phase of Mg(Zn, Sr) which was defined as S phase. By the TEM analysis, the S phase was confirmed to be Mg6Zn2Sr1 and it was proved that S phase has better thermal stability compared with W phase by DSC thermal analysis.(2) Sr has refining effect on Mg-3Zn-1Y-xSr alloys grains, and Mg-3Zn-1Y-1.5Sr alloy has the smallest grain size. Besides, with increasing content of Sr, the eutectic compound along the grain boundary become coarse and Mg-3Zn-1Y-1.5Sr alloy is transformed into a severe continuous mesh structure.(3) The dynamic recrystallization occurs in Mg-3Zn-1Y-xSr alloys during the hot-extruding and the grains are refined, and the eutectic compounds along the grain boundary are broken into small particles and are distributed parallel to the extrusion direction. The volume fraction of the S-phases increases with the increase of Sr content in the as-extruded alloys and the morphology of the S phase is transformed from ellipsoid, stubby to long fiber successively.(4) After high temperature tensile test at 250?, the result indicate: With increment of S-phases volume fraction,the strength of as-extruded alloys increase and the elongation decrease, and when the volume fraction of S-phases reach the level of 8.39%(Sr content is 0.9%wt), the alloy shows the highest UTS of 204 MPa and YS of 171 MPa with the elongation of 61.34% at a temperature of 250 oC. Besides, the creep strain rate of extruded Mg-3Zn-1Y-0.9Sr alloy at 250 C is 5.63×10-9s-1.(5) The main strengthening mechanism of elevated temperature tensile strength of Mg-3Zn-1Y-0.9Sr alloy are pinning strengthen by W-phase and short fiber reinforced by S-phase with spindle-shape. |
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