Study Of Dynamic Mechanical Property And Deformation Mechanism Of Fine Crystal Magnesium Alloy

Magnesium alloys hold promising future in many structural applications because of their ultra-low density and high specific strength and stiffness.They also exhibit excellent conductivity,damping capacity and electromagnetic shielding properties which may entail wide applications in communication,wiring,aviation and spaceflight.However,poor corrosion resistance and low fracture toughness are still the obstacle for their applications.Also,hexagonal close-packed(HCP)structured Mg alloys have low symmetry of slip systems that contribute to low ductility at room temperature.Many researches have been conducted to improve the ductility and strength of magnesium alloys through various approaches.It has been found that grain refinement,among which equal channel angular pressing or extrusion(ECAP/ECAE)has stood out to be the most popular technical approach to concurrently improve the strength and ductility of Mg-alloys.Moreover,numerous efforts have been undertaken on magnesium and Mg-alloys toward the microstructure and mechanical property relationship.However,there have only been sporadic investigations on the effect of strain rate on the mechanical behavior of this important type of ultralight structural material.Therefore,understanding the mechanical mechanisms and the influence factors of the material response in a large range of strain rates has great significance in producing high-quality magnesium alloy.The primary objective of the present study is to explore the effect of microstructure and strain rate on the mechanical behavior and deformation mechanisms of Mg alloys.The main mechanical properties considered in this study consist of the uniaxial compression and tension property,three point bending fracture property and impact property.The main work and conclusions are briefly summarized as follow:(1)A commercial magnesium alloy,AZ31 B in hot-rolled condition,is subjected to severe plastic deformation via several passes of equal channel angular pressing(ECAP)to modify its microstructure.Electron backscatter diffraction(EBSD)is used to characterize the microstructure of the as-received and ECAPed material.(2)Mechanical properties of the AZ31 B specimens are evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates.The yield strength,ultimate strength and failure strain/elongation under compression and tension are compared in detail to sort out the effects of factors in terms of microstructure and loading conditions.The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression,and the stress-strain curves exhibit clear sigmoidal shape,pointing to dominance of mechanical twinning responsible for the plastic deformation under compression.All compressive samples fail prematurely via adiabatic shear banding followed by cracking.Significant grain size refinement is identified in the vicinity of the shear crack.Under tension,the yield strength is much higher,with strong rate dependence and much improved tensile ductility in the ECAPed specimens.Tensile ductility is even much larger than the malleability under compression.(3)Mode I quasi-static and dynamic fracture experiments are conducted on pre-cracked three point bending specimens by using electric motor-driven universal tensile-compressive load frame and modified split-Hopkinson pressure bar,respectively.Two sets of specimens with different initial textures are considered here: one set of the specimens are machined from the hot rolled AZ31 B Mg alloy plate.The others are treated by four passes of ECAP after they are cut from the initial material.They are with the finer grain size.Digital image correlation(DIC)technique is used to determine the strain contours around the crack tip and EBSD is employed to analyze the texture evolution after tests.It is found that the dynamic fracture toughness of finer grain specimen is higher than that of coarse grain specimen.The fracture toughness of both sets of specimens is enhanced by increasing the loading rates.The strain contours around the crack tip generated by DIC technique agree well with the numerical simulation results.Texture analysis shows the formation of tensile twinning in the ligament ahead of the crack tip in the coarse grain specimen but no sign in fine grain specimen.The brittle features e.g.cleavage planes and twinning lamellas are observed on the fracture surface of coarse grain specimen by scanning electron microscope(SEM).However,the relative ductile features such as micro-voids surrounding by tear ridges present on the fracture surface of fine grain specimen.(4)Plate impact experiments are conducted on both sets of AZ31 B Mg alloy as mentioned above.The Hugoniot elastic limit(HEL)and spall strength are calculated by analyzing the free surface velocity profiles of the specimen.SEM is used to observe the microvoids produced along the spall plane and EBSD is employed to examine the texture distribution in the middle cross section of the recovered specimen.It is found that the spall strength increases with increase of impact velocity.Texture and grain size have a slight effect on the spall strength.Elastic precursor decay appears in the as-received specimen but it is absent in ECAPed specimen.The HEL of as-received specimen is higher than that of ECAPed specimen over all the impact velocities while the spall strength is just opposite.The spallation is dominated by coalescence of microvoids emerged along the tensile twinning boundaries and grain boundaries in brittle process for as-received specimen while with inner dimples forming by the pull out of grains in ductile process for ECAPed specimen.The slightly high spall strength and relatively ductile spall process may have important significance in the armor protection field.