The Dynamic Mechanical Response And Failure Behaviour Of AM80 Magnesium Alloy At Various Temperatures

With the rapid development of automobile,weaponry and aerospace industries,higher lightweight and energy efficient are required.Magnesium alloys being the lightest metal structure material,are more and more utilized in industrial manufacture to replace the original steel or aluminum alloy parts.AM80 magnesium alloy with low price and excellent comprehensive mechanical performance is one of the most promising magnesium alloys for large-scale industry applications.Except for static loading,components prepared by magnesium alloys are inevitably subjected to dynamic loads at various temperatures during their services,such as impact ing and explosion.At present,systematic investigations have been conducted on the mechanical response behavior and the corresponding microstructure evolution of magnesium alloys under quasi-static load.Additionally,there has been an increase in the number of researches on dynamic load.However,the research on dynamic load of magnesium alloy is still scarce and far below adequacy.Therefore,system atic research on the dynamic mechanical response behavior,microstructure evolution and fracture failure mechanism of magnesium alloys with various forming processes and heat treatment states is extraordinarily necessary to carry out for improving the application and operation reliability in the industrial fields of automotive,weaponry,aerospace and aircraft etc.AM80 magnesium alloys with different forming and heat-treated conditions were used in the present work.In order to simulate the microstructure evolution at variou s temperatures and strain rates,compression tests with special strians were carried out by a Spli-Hopkinson pressure bar（SHPB）using a series of pressure rings with different designed thickness.The microstructure evolution,microscopic deformation mechansim and fracture failure mechanism of the studied alloy a t different conditions were discussed with the imaging techniques of optical microscopy（OM）,electron backscatter diffraction（EBSD）and transmission electron microscopy（TEM）.The following conclusions were drawn.（1）A completely different stress response behavior at room temperatur e was exhibited by the solution-treated AM80 cast alloy under quasi-static and dynamic impact loads.At quasi-static load,the flow stress decreased with the increase of the applied strain rate,showing a negative strain rate sensitivity（SRS）.It was only shown at medium to high strain.Under dynamic load,the flow stress increased with the increase of loading strain rate,illustrating a strong positive SRS.（2）A Johnson-Cook（J-C）equation was built by modifying the parameter of strain rate coefficient C,which can characterize the mechanical behavior of the solution-treated AM80 cast alloy.The simulations based on the modified J-C equation were in good agreement with the experimental and constitutive fitting re sults,which can be used to predict the dynamic mechanical behavior of the studied alloy in a wide range of strain rates.（3）When dynamic compressed at room temperature,the deformation twins generated in the solution-treated AM80 cast alloy increased with the increase of strain rate.Then it decreased as the applied strain rate increased to 5000 s^-1,and some adiabatic shear bands were generated in the area with high-density of twin-twin intersections.At temperatures of 150°C and 250°C,the deformation twins decreased and the volume fraction of dynamic recrystallization increased with increasing the strain rate.As the applied strain rate increasing to 5000 s^-1,a fully dynamic recrystallization occurred and the initial deformed structures were replace d with recrystallization grains.The dynamic recrystallization at high temperature nucleated preferentially at grain boundaries,twin boundaries and the intersections of both,and then propagated along the twin boundaries.（4）At room temperature,the dynamic deformation mechanism of AM80 alloy was dominated by tension twin and basal slip,and supplemented with non-basal slip.At high temperature,intercoordination among the slip,mechanical twin and dynamic recrystallization under the guidance of non-basal slip was the deformation mechanism of the studied alloy.The relatively lower density of mechanical twins and the occurrence of dynamic recrystallization due to the various deformation mechanisms were the main reasons for the lower flow stress as compared with that at room temperature.The mutual promotion of adiabatic temperature rise and deformation localization led to the negative SRS of the flow stress in the later period of deformation.（5）The dynamic mechanical properties of the extruded AM80 alloy at room temperature were strongly dependent on the applied strain rate and heat treatment state.The deformation twins increas ed with increasing the strain rate and strain.In the later period of deformation with applied strain rate increasing to 6000 s^-1,the twin density in the local high-strain region decreaseed,and a lot of cell-like dislocation structures and some adiabatic shear bands along t he cracks tip were detected.In addition,a number of nan-scale dynamic recrystallization grains were generated in the as-extruded AM80 alloy,which was mainly attributed to the initial strain energy as well as the relatively strong basal texture.（6）The microcracks in AM80 alloy nucleated preferentially at the intersection of twin and grain boundaries,and then extend ed along the twin direction until passing through the entire grain.By contrast,the main cracks nucleated at the edge of the compressive surface with the highest stress concentration,and then propagated along the maximum shear stress direction.When the adiabatic shear band occurred,the main cracks initiated on the shear band and then extend along it until failure.Deformation twins and microcracks were the two important factors that affect ed the crack propagation,and the interaction of both resulted in the deflection of the main crack.