Effects Of Alloying Elements And Processing On Deformation Mechanisms And Properties Of Mg-Li Alloys

The demand for weight reduction in automotive,aerospace and electronic industries makes magnesium a material of high interest due to the combined properties of the low density,high specific strength,excellent electromagnetic shielding et al..Unfortunately,the widespread application of Mg alloys is still challenging due to their poor room temperature formability.Addition Li in Mg to form Mg-Li alloy can significantly increase the room temperature ductility,and the plastic deformation mechanisms of binary Mg-Li alloy were systemically studied in this dissertation.However,the disadvantage of alloying Li in Mg alloys makes the material strength too low to meet the practical industrial application.Alloying is one of the most effective methods to overcome the drawback.Besides,the poor corrosion resistance property of Mg-Li alloy is another factor to restrict their widespread application.As we known,surface treatment is the most effective way to improve the corrosion resistance properties.To solve the issues mentioned above,studies on microstructure,deformation mechanisms and properties of binary,ternary and quaternary Mg-Li based alloys as well as electroless Ni-P plating on Mg-Li based alloys were mainly carried out in this dissertation from the perspective of improving the strength and ductility of the Mg-Li based alloys and the adaptability of the coating.The following conclusions are drawn:(1)For the a phase of the Mg-3Li based alloys and Mg-9Li based alloys,both alloys mainly form basal texture during the thermomechanical treatments,while the basal texture of Mg-9Li based alloys is much weaker than that of Mg-3Li based alloys,accompanying with non-basal slip activation.The deformation modes in Mg-9Li-(6Al)alloys are basal slip first,and then pyramidal slip.The relative activity of pyramidal slip decreases after a large extent of deformation;the activation of non-basal slip in Er contained Mg-9Li based alloys is main prismatic slip.For the ? phase of the Mg-9Li based alloys and Mg-15Li based alloys,with the rolling reduction of 40%,binary Mg-9Li alloy mainly shows strong ? fiber and binary Mg-15Li alloy shows ? and ? fibers,while the Al contained Mg-9,15Li-6Al alloys mainly exhibit ?,?,? and ? fibers.With the rolling reduction to 75%,all the Mg-9Li based alloys and Mg-15Li based alloys mainly form ?,? and ? fibers.With respect to single phase Mg-3Li based and Mg-15Li based alloys,duplex Mg-9Li based alloys have the most excellent mechanical properties,which can be attributed to the stress propagation between the a phase and ? phase through collaborative deformation,and the dislocation pinning and crack propagation effectively hindered by ?/? interfaces.(2)A1 was added into binary Mg-Li alloys to form ternary Mg-Li-Al alloys,which can significantly improve the strength,accompanying with the decrease in ductility.This can be mainly attributed to the formation of MgLiAl2 and AlLi intermetallic compounds with Al addition.The cast Mg-Li-Al alloys show a positive strength differential effect,resulting from the {1012} twinning which can be more easily activated under tensile loading than compressive loading;On the other hand,the tension/compression yield stress increase with increasing the Li addition,which can be ascribed to the increased critical resolved shear stress of {1012} twinning;Er was employed in ternary Mg-Li-Al alloys to form Mg-Li-xAl-xEr to obtain higher ductility without damaging the strength.This trend is mainly attributed to the Er addition weakening the basal texture and activation of the prismatic slip as well as the evenly distributed of uniform and fine AlLi,Al3Er formation.Moreover,the Er addition in LA96 alloy also greatly enhances the corrosion resistance properties.(3)Compared to the unidirectional rolling,cross-rolling makes the alloys have a significant deformation strengthening effect,which can be mainly attributed to the change in strain path,making the a phase distribute disorderly in the ? phase and interact with each other into a network,at the same time,the number of second phase particles increases during cross-rolling,which can greatly improve the strength.On the other hand,due to the disordered distribution of matrix phases,uniform plastic deformation is blocked,and the ductility is greatly lowered.(4)The basal {0002}<1100>texture of single a phase Mg-3Li and duplex(a + p)phase Mg-9Li-6Al-6Er alloys was weakened after tensile deformation,accompanying with the emergence of weak {1120}<1100>component.A clear hysteresis loop at plastic strain of about 1%is observed during loading/unloading stress-strain curves,which is ascribed to the anelasticity resulting from {1012} twins upon unloading.For the Mg-3Li alloy,lattice strain in the {10.0} grain families was saturated around 84.5 MPa,and decreased abruptly around the 132.6 MPa(fully developed plasticity),indicating that prismatic slip is operated around the yield point;The orientations can be changed between soft and hard families due to the deformation mode transition,and load redistribution continuously occurs between soft and hard grain orientations.For the Mg-9Li-6Al-6Er alloy,due to the collaborative deformation of a phase and ? phase,the deformation of a phase is restricted by the ? phase,when the lattice stress in ? phase reaches the limit,instability will not occur immediately,but transmit the stress to the adjacent well-deformed soft ? phase,resulting in a greater deformation coordination behaviour.(5)Uniform and compact electroless Ni-P coatings with ultrasonic-assisted have been successfully deposited on duplex Mg-Li based alloy.Environmentally-friendly Ce(NO3)3-KMnO4 solution as pre-treatment also proved to be feasible.The Ni-P coatings are mixed with amorphous-crystalline features and well adhered to the substrates.Ultrasonic processing significantly improves the corrosion resistance of Ni-P coatings due to the formation of uniform,dense and fine structures,which strengthen the passive film.