Effect Of B₄C And Sn Compound Addition On Microstructure And Mechanical Properties Of Mg-Y-Zn-Mn Alloy With Long-Period Stacking Ordered Structures

The magnesium alloys with long-period stacking ordered?LPSO?structure as a new type of high-strength alloy have attracted extensive attention of researchers due to their excellent performance.The Mg-Y-Zn alloys with LPSO structure have become a hotspot because of their unique microstructure and superior properties.Such alloys based on Mg-Y-Zn have been manufactured through a series of methods including alloying,modification and hot deformation.Owing to the high price of rare earth Y,it becomes an important research problem on how to obtain more LPSO phase under the same Y element content.This paper selected Mg₉₄Y_2.5Zn_2.5Mn₁ alloy as the based alloy and prepared Sn-modified alloy and ally with B₄C and Sn addition by conventional casting method.Firstly,the effect of micro addition of Sn on the microstructure and mechanical properties of Mg₉₄Y_2.5Zn_2.5Mn₁ alloy was studied.Then,B₄C was doped into the alloy with optimal amount of Sn.The influence of B₄C on Mg₉₄Y_2.5Zn_2.5Mn₁ alloy containing Sn was explored.The effect of B₄C on LPSO phase and magnesium matrix formation was analyzed.Subsequently,the two alloys were carried out solution treatment to further investigate the changes on the microstructure and mechanical properties.The addition of B₄C and Sn to the W-phase spheroidization and the conversion between the 18R-LPSO and the14H-LPSO were studied.Finally,the effects of two important parameters?temperature and speed?on the dynamic recrystallization?DRX?,microstructure and properties of the extruded alloys were investigated.The influence of B₄C on dynamic recrystallization behavior was discussed.The results are summarized as follows:?1?Sn addition significantly promoted the formation of LPSO phase in the as-cast Mg₉₄Y_2.5Zn_2.5Mn₁ alloy.When the addition amount of Sn was 0.35 wt.%,the volume fraction of the LPSO phase reached 31%.The grains were refined and there were still large dendrites.The growth of the 18R-LPSO phase was obviously directional and paralleled with the W phase.The tensile strength and elongation of the alloy reached 209.8MPa and 6.7%,which was 22.2%and 17.5%higher than that of Mg₉₄Y_2.5Zn_2.5Mn₁ alloy.?2?With 0.02 wt.%B₄C and 0.35 wt.%Sn addition,the microstructure of Mg₉₄Y_2.5Zn_2.5Mn₁ alloy were obviously refined,and the grain was changed from irregular dendrite to equiaxed dendrite.B₄C hindered the growth of LPSO phase in a certain direction,thus changed LPSO phase growth pattern.The compound addition of B₄C and Sn not only refines the grains but also ensured the volume fraction of the 18R-LPSO phase.The tensile strength and elongation of the based alloy with Sn and B₄C compound addition were 255MPa and 8.8%.?3?The secondary phase mainly consisted of block 18R-LPSO phase,lamellar 14H-LPSO phase and granular W phase in the heat-treated Mg₉₄Y_2.5Zn_2.5Mn₁ alloy,Mg₉₄Y_2.5Zn_2.5Mn₁ alloy with 0.35 wt.%Sn addition and Mg₉₄Y_2.5Zn_2.5Mn₁ alloy with 0.35 wt.%Sn and 0.02 wt.%B₄C addition.?4?The microstructure of as-heated alloy containing 0.35 wt.%Sn was uneven.After solid solution treatment,the microstructure of alloy containing B₄C and Sn became uniform,and the spheroidization effect of W phase was better.The tensile strength and elongation of heat-treated alloy with Sn and B₄C compound addition were 252MPa and 18%,respectively.?5?The Mg₉₄Y_2.5Zn_2.5Mn₁ alloy with 0.35 wt.%Sn and 0.02 wt.%B₄C addition had obvious DRX behavior through direct extrusion at different temperature and speed.The mechanical properties of the alloys were improved.The extrusion speed was constant,the higher the extrusion temperature,the more completeness the DRX was.The extrusion temperature was constant,the faster the extrusion speed,the more complete the DRX was.The DRX behavior was delayed by B₄C addition.The volume fraction of DRXed grains decreased from 75.8%in the Sn-doped alloy to 68.95%in the alloy with the addition of B₄C and Sn.?6?When the extrusion temperature was 360°C and the extrusion speed was30 mm/min,the mechanical properties of the alloy were the best,the tensile strength reached 347.5 MPa,yield strength was 320MPa and the elongation was21.2%,which were greatly higher than that of Sn-modified alloy.