Evolution Of Microstructure And Mechanical Properties Of Mg-Li Alloy Sheet Processed By Accumulative Roll Bonding

Magnesium-lithium(Mg-Li)alloys have potential applications in the fields of aerospace,military,automobile and electronic industries because of their exceptionally light-weight and excellent electromagnetic shielding performance.However,due to the drawback of low strength,wide application of Mg-Li alloys is restricted.Accumulative roll bonding(ARB)technique is the only promising severe plastic deformation(SPD)techniques in industrial scale-up for continuous manufacturing bulk ultrafine-grained(UFG)metallic sheets or plates,and the obtained UFG materials possess high strength.In this dissertation,the Mg-5Li-1Al sheets were prepared by conventional accumulative roll bonding(ARB)technique and cross accumulative roll bonding(CARB),respectively.The rolling repeated for six passes.The interface number is increased with the increase of rolling passes,the subsequent rolling sufficiently improves the bonding performance of interfaces introduced in the previous cycle.When the total reduction is higher than 90.89%,the interface between two sheets is disappeared,the sheets are bonded completely.Static recrystallization is the main reason of improving bonding performance of the interface of the Mg-5Li-1Al sheets.The ARB technique could effectively refine grains of the Mg-5Li-1Al alloy.After six rolling passes,the mean grain size of the alloy reaches 100nm,the homogeneity of structure is improved at the same time.The CARB technique could increase the homogeneity of structure and refine grains of the Mg-5Li-1Al alloy,but the refining rate is slower than the ARB technique.During ARB processes,the deformation mechanism of the Mg-5Li-1Al alloy is changing from twinning deformation to shear deformation to form macro shear zone,then DRX occurs.The grain refining mechanism is from twin DRX(Dynamic Recrystallization)to rotate DRX.While during CARB processes,the deformation mechanism of the Mg-5Li-1Al alloy is from twinning deformation to slip deformation,then becomes shear deformation.The grain refining mechanism is twin DRX firstly,continuous DRX in the middle passes,then becomes rotate DRX.With the increase of rolling passes,the hardness and strength of the Mg-5Li-1Al sheets are enhanced,and the elongation is changed in different degrees.Increasing rolling passes and changing rolling directions between two passes could weaken the mechanical anisotropy of the alloy.Meanwhile,the CARB process could improve the ductility of the Mg-5Li-1A1 alloy.The mechanical behavior of the Mg-5Li-1Al alloy during deformation processes are governed by two main strengthening mechanisms:grain refinement and work hardening.The interface bonding quality and the oxide numbers at the interface also influence the mechanical properties of the ARB-processed sheets.At the same time,the texture of the Mg-5Li-1Al alloy is rolling texture.With the increase of ARB passes,the basal texture density increases and the deflection of the c-axis of grains increases firstly and then decreases.Meanwhile,the non-basal slip especially the cone slip system is activated.During CARB processes,with the change of rolling directions,the pole density distribution rotate 90° and the deflection of the c-axis of grains becomes weaken.The prismatic systems and cone slip systems also activated.Then,the Mg-8Li-3Al-1Zn sheets were prepared using ARB process for five passes.The interface bonding strength of the Mg-8Li-3A1-1Zn sheets is higher than that of the Mg-5Li-l Al sheets.The sheets are bonded with same phases,most of which are identified asα phase.The ARB technique refines the grains of Mg-8Li-3Al-1Zn alloy,the refining process begins at the tips and boundaries of P phase.With the increase of ARB passes,the a phase andβ phase are gradually elongated,broken into small pieces and scattered distribution,the AlLi and MgLi2Al particle phases become more and more dispersive.The hardness and strength are enhanced with the increase of ARB passes,whereas the elongation value first decreases,then it slightly increases.The elongation value of the Mg-8Li-3Al-1Zn ARB-processed sheets maintains 11%-15%.The strengthening mechanisms of the Mg-8Li-3Al-1Zn alloy are grain refinement,work hardening and the second phase particles strengthening.Finally,the six passes α/β alternative Mg-Li composite sheets were fabricated with ARB process.The composite sheets not only possess high strength of the Mg-5Li-lAl alloy,but also possess better ductility of the Mg-12Li-lAl alloy.Because the cross shear zone formed near the interface,the interface bonding quality becomes better.With the increase of ARB passes,the hardness value of the Mg-5Li-lAl alloy and the Mg-12Li-l Al alloy is enhanced.Meanwhile,the strength of the composite sheets first increases,then maintains in a stable range.The elongation decreases during initial several passes.After five passes,the elongation increases.In the α/β alternative Mg-Li composite sheet,the texture of the Mg-5Li-l Al alloy is rolling texture.With the increase of ARB passes,the texture density first increases,and then decreases.The main slip of the Mg-12Li-lAl alloy is the(110)middle temperature slip plane,the(321)high temperature slip plane is also activated with the increase of ARB passes.In order to further increase bonding performance of the interface and comprehensive mechanical properties,the α/β alternative Mg-Li composite sheets were annealed at different temperature(250℃,300℃,350℃,400℃,450℃)for different time(15min、30min、60min、120min).After annealing treatment,certain thickness diffusion layer forms near the interface on the side of Mg-5Li-l Al alloy.With the increase of annealing temperature and annealing time,the thickness of the diffusion layer becomes large.In the diffusion layer 1,the structure forms single row grains,and the grain size grows with the thickness of the diffusion zone increases.Through synthetic analysis with ToF-SIMS,XRD and TEM,the diffusion layer 1 is Li3Mg7 and the diffusion layer 2 is Li0.92Mg4.08.Under different annealing treatment,the microstructure,bonding properties and strength of the Mg-Li composite sheets change.The sheet annealed at 350℃ for 30min or at 400℃ for 15min possesses the best comprehensive properties.Because of larger strains in the ARB6 composite sheet,the recrystallization needs more time,the ARB6 sheet annealed at 400℃ for 30min possesses the best comprehensive mechanical properties.