Research On The Superplasticity Of Mg-9.3Li-1.79Al-1.61Zn Alloy

As the lightest structural alloy that can be obtained now in industry, magnesium-lithium alloy has many advantages, such as high specific strength and stiffness, superior damping capacity, high dimensional stability, good electromagnetic shielding characteristics, and good machinability. Mg-Li alloy has become one of the hot research fields of the light alloy. The ultra-light feature makes it have a good application in aerospace, weapons,3C industry, shipbuilding industry and nuclear energy field. Nevertheless, the application of the Mg-Li alloys is restricted, due to the lower absolute strength, and the deficiency of research on their hot deformation behavior. In order to overcome those disadvantages existed in Mg-Li alloys, the alloying and deforming process are applied.Starting from melting, the mechanical properties of Mg-9.3Li-1.79Al-1.61Zn alloy at room temperature and high temperatures were investigated by tensile test under different deformation conditions.The analysis focused on the characteristics of high-temperature superplastic deformation. The evolutions of grain structures and cavities, fracture modes, microstructures such as precipitated phases, dislocations, and subgrains were studieded by means of OM, SEM and TEM. Based on the true stress-true strain curves and DRX microstructures, constitutive equation of thermal deformation and DRX model were obtained and the constitutive model describing the superplastic deformation mechanism was established to know the superplastic deformation mechanism of the present alloy. Besides, the electro-superplastic effect of the present alloy was investigated by inputting pulse current into the specimens during tensile test to study on the influences of pulse current on their mechanical properties, grain structures and other microstructures, including precipitated phases, dislocations and subgrains.The main reseraches and conclusions are summarized as following:(1) The new Mg-9.3Li-1.79Al-1.61Zn alloy sheets composed of α-Mg phase and β-Li phase were prepared by the process of melting, rolling and annealing. The tesile test was conducted at room temperature and in the range of deformation temperature from 473 to 623K, strain rate from 1.67 X 10-4 to 1.67 X 10-2s-1 and the maximum elongation to failure of 617.6% was demonstrated at 573K at the initial strain rate of 1.67 ×10-4s-1。(2) Based on the true stress-true strain curves, the influences of temperatures and strain rates on flow stress were studied and the strain rate sensitivity index values at different temperatures were calculated. The evolutions of grain structures and cavities, and microstructures during deformation process were studieded.(3) Constitutive equation of thermal deformation was built and the values of deformation activation energy were calculated.(4) Based on the curves of true stress-true strain and dynamic recrystallization microstructures, dynamic recrystallization model were obtained to describe the influences of deformations on dynamic recrystallization.(5) The constitutive model describing the superplastic deformation mechanism was built by calculating the values of stress exponent, grain size exponent, deformation activation energy. Combined with analysis of dislocations and subgrains, superplastic deformation mechanism of Mg-9.3Li-1.79Al-1.61Zn alloy was investigated.(6) The electro-superplastic effect of the alloy was investigated to know the influences of pulse current on the mechanical properties, dynamic microstructures and other microstructures. It is concluded that pulse current can improve the plasticity and refine the grains of alloy at proper pulse current parameters.