Effects Of Heat Treatment And Accumulative Roll Bonding On Microstructure And Mechanical Properties Of Mg-14Li-3Al-1Y-0.8Zr-0.4Sc Alloy

Magnesium-lithium alloys have excellent comprehensive properties,especially ultra-low density and high yield-tensile strength ratio,so they can be used in aerospace and other fields.The absolute strength of the magnesium-lithium alloy itself is low.The comprehensive mechanical properties of the alloy can be significantly improved by microalloying,heat treatment and deformation processing.In this paper,the micro structure and mechanical properties of Mg-14Li-3Al-1Y-0.8Zr-0.4Sc alloy at different heat treatment processes and subsequent accumulative roll bonding(ARB)were studied.Moreover,the effect of the composite second phases on the performance improvement of sheets was discussed.The micro structure and mechanical properties of the alloy were characterized and analyzed by OM,XRD,SEM,EDS and room temperature tensile tests.After adding three alloying elements of Y,Sc and Zr,a large number of second phase particles of Al2Y and Al3Sc composites are formed in the LA143(Mg-14Li-3Al)alloy,and the Zr elements are uniformly distributed in the matrix.The microstructure distribution in the as-cast Mg-14Li-3Al-1Y-0.8Zr-0.4Sc alloy is uneven because there is a large amount of aggregated second phase.After homogenization treatment at 200?/10 h,the micro structure of the alloy is relatively uniform,and the second phase particles are dispersed in the grains and grain boundaries of the alloy.In the rolling process,as the amount of deformation increases,the strength of the alloy gradually increases,but the elongation decreases due to work hardening.After the homogenization treatment and ARB process of Mg-14Li-3Al-1Y-0.8Zr-0.4Sc sheet,the microstructure of the sheet changed significantly.The obviously directional band structures appeared in the first,second and third passes,because the uneven deformation of the rolling process results in the occurrence of micro-deformation zones and severe deformation zones in the alloy.However,as the number of rolling passes increases,the distance between these deformation zones gradually decreases,and the microstructure in the alloy gradually becomes uniform.After the fourth pass,the grains in the alloy were remarkably refined,and the uneven band-like structure formed by rolling completely disappeared.After the fifth pass,the yield strength,ultimate tensile strength and yield-tensile strength ratio of the sheet were increased to 211 MPa,250 MPa,and 84.63%,and the elongation was 14.5%.During ARB process,the micro structure of the sheet bonding layer also changed significantly.The inclusions were crushed,refined,embedded in the matrix,and the metals on both sides were gradually welded together to form a metallurgical bond.The optimum solution and aging treatment process(T6)of Mg-14Li-3Al-1Y-0.8Zr-0.4Sc alloy was finally determined.The parameters are 360?/1h and 150?/24 h,respectively.After the optimal T6 heat treatment,a large amount of dispersed second phase particles precipitated in the alloy,and then sheets are subjected to 5 ARB passes.The yield strength,ultimate tensile strength and yield-tensile strength ratio of ARB5 sheet reached 233 MPa,269 MPa and 86.64%,respectively.It can be seen that the strength of the sheet is more obviously increased after the same ARB process,indicating that a large amount of precipitated second phase particles in solid solution and aging heat treatment have a significant strengthening effect.Comparing the mechanical properties of the 1,3,and 5 pass sheets after the homogenization treatment and T6 heat treatment,and analyzing the change of micro structure in Mg-14Li-3Al-1Y-0.8Zr-0.4Sc sheet,the conclusion can be obtained that the second phase particles uniformly distributed in the grains and at the grain boundaries together can refine the grains and pin dislocations in ARB process.The Al2Y and Al3Sc particles in the grain effectively hinder the movement of dislocations,causing that Mg-14Li-3Al-1Y-0.8Zr-0.4Sc alloy can withstand larger external stress,and the composite particles at the grain boundary pin the grain boundaries,inhibiting the growth of the grain boundaries,thereby refining the grains.