| Magnesium alloys are characterized with low density,high specific strength and toughness,superior properties for absorbing vibration or shock energy,excellent electric and thermal conductive properties,etc,which have broad application prospects in the automobile,electronic and aircraft industries.Especially,the Mg-Zn-Sn alloys,which have been considered as the novel low-cost high-strength heat-resistant magnesium alloys,have been payed more and more attentions due to their excellent heat-treatable strengthening and heat resistance.Therefore,it is of great significance in theory and engineering to develop high strength magnesium alloy through heat treatment optimization and minor Sr addition.Effects of solution treatment&aging treatment and minor Sr on microstructure and properties have been explored on the Mg-4.5 Zn-4.5Sn-2Al alloy and the Mg-4.5Zn-4.5Sn-2Al-0.6Sr alloy in the present study,and the results are as followes.(1)During the solution treatment of the Mg-4.5Zn-4.5Sn-2Al-0.6Sr alloy,the higher solution temperature plays a more important role in dissolving the high melting point phases,when compared with prolonged solution time.The residual intermetalic phases after the optimal solution treatment(310?×4h+340? ×28h+460? ×2h)are confirmed to be the mixture of the Mg2Sn and MgSnSr.The volume fraction of the residual second phases,ultimate tensile strength and the elongation to rupture of the as-solutionized alloy are 1.2%,238 MPa and 12%,respectively.(2)By comparing the solutioned microstructure,it is revealed that minor Sr addition promotes the dissolution of Mg2Sn in the matrix through the refinement of Mg2Sn and the modification of its morphology.Besides,it is also beneficial to improve the thermal stability of grains by forming the fine dot-like MgSnSr phase,within the grains and grain boundaries.(3)During the single aging at 150 ? and 200 ?,two hardness peaks are discernible in all the age hardening curves,corresponding with the precipitation of ?1'followed by the precipitation of Mg2Sn.Small amount of Mg2Sn particles nucleate on the tip of the ?1' long rod,forming the two-phase T-shaped particles.(4)During the double aging(70?×10h+200?),a large amount of GP zones are uniformly dispersed within grains at the pre-aging stage.This accelerates the aging process and brings about an obvious refinement and a larger amount of the precipitates,promoting the formation of the two-phase T-shaped particles,which is beneficial to the thermal stability during aging treatment.The curved-like phase,which is about 30nm and formed by the dot-like Mg2Sn and the tiny needle-like ?1',is also found in the Sr-containing alloy.The peak aged Sr-free alloy exhibits the UTS of 273 MPa,the Er of 11.3%and the HV0.1 of 91HV,while the Sr-containing alloy exhibits the UTS of 284 MPa,the Er of 8.3%and the HV0.1 of 96HV.(5)Minor Sr addition slows down the single aging speed and brings about an obvious refinement and a larger amount of the precipitates in the Mg-4.5Zn-4.5Sn-2Al alloy,which improves the mechanical properties in return.Moreover,minor Sr also refines the ?1' size and increases its number density during the double aging process,which stimulates the nucleation of the Mg2Sn on the tip of thec ?1',forming more T-shaped phases,and thus improves the thermal stability of the aged alloy. |
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