| The as-extruded magnesium alloys,with the advantages of fine grains,high strength and superior plasticity,have wide application prospects in the fields of automobile,aviation,aerospace,national defense and military industries.As a new type of wrought magnesium alloys with high strength and good heat resistance,the Mg-Zn-Sn-Al alloy system has draw much attention for low cost and good heat-treatable strengthening effects.The Mg-Zn-Sn-Al-based wrought alloys can been further strengthened by means of minor Ca/Sr additions and aging.In this paper,effects of minor Ca/Sr additions and aging on microstructure,mechanical properties,creep resistance and corrosion resistance of the as-extruded Mg-4.5Zn-4.5Sn-2Al alloy have been investigated.The research conclusions are as follows:?1?The as-extruded Mg-4.5Zn-4.5Sn-2Al alloy is characteristic of complete dynamic recrystallization,with the grain size of 5.8?m,and the Mg2Sn phase is distributed along the extrusion direction as strips.The tensile strength,the yield strength and the elongation are 354 MPa,216MPa and 19.6%respectively.?2?Minor Ca/Sr additions can improve mechanical properties of the as-extruded alloy by refining grains and modifying the morphology and the distribution of the second phases.Especially,with the 0.2Ca-0.6Sr addition,grains are refined to about3.8?m and the number density of the smaller residual Mg2Sn phase is increased.The tensile strength,the yield strength and the elongation are up to 368 Map,242MPa and22.9%respectively.Moreover,the Ca/Sr additions have great impacts on high temperature hardness and creep resistance of the as-extruded alloy.The 0.6Sr addition results in the highest hardness above 150?.With the 0.2Ca and the 0.6Sr addition,The steady creep rates of the as-extruded alloys decrease to 5.293×10-7/s and4.73×10-7/s respectively,and the creep resistance is remarkably improved.However,the 0.2%Ca-0.6%Sr addition brings about the inferior creep resistance.?3?Extrusion brings about the higher corrosion current density of the Mg-4.5Zn-4.5Zn-2Al alloy in 3.5%Na Cl solution,changes the corrosion behavior from the localized pitting corrosion to the severely comprehensive corrosion,andreduce corrosion resistance.Minor Ca/Sr additions have negative effects on corrosion resistance of the as-extruded alloys.?4?As compared with single aging,The double aging brings about stronger precipitation strengthening response for the as-extruded alloys.Both strength and creep resistance can been notably improved.Under the Peak I of double aging state,the tensile strengths of the aged Mg-4.5Zn-4.5Sn-2Al-X?0,0.2Ca,0.6Sr and0.2Ca-0.6Sr?alloys are 12 MPa,25MPa,30 MPa and 27 MPa higher than the corresponding as-extruded alloys,the steady creep are 59.7%,73.7%,53.6%and71.0%lower than the corresponding as-extruded alloys.?5?With minor Ca/Sr additions,the aging precipitation process is retarded,and the appearances of hardness peaks are delayed,but the precipitation strengthening effects are reinforced.With the additions of 0.2Ca,0.6Sr and 0.2Ca-0.6Sr,the tensile strengths of double aged alloys under Peak?are 22 MPa,11MPa,and 29 MPa higer than the as-aged Mg-4.5Zn-4.5Sn-2Al alloy,the steady creep are 55.9%,30.6%and23.0%lower than the as-aged Mg-4.5Zn-4.5Sn-2Al alloy. |
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