Dynamic Mechanical Property Of Several Magnesium Alloy Under High Strain Rate

Dynamic mechnical properties of magnesium alloys are required for the design of part in addition to static mechnical properties in many applications. Thus, it is important to study the mechnical behavior of magneisum alloy under dynamic loading. In order to investigate the dynamic mechnical properties, deformation mechnism and failure behavior of several magnesium alloys at high strain rates, Split Hopkinson Pressure Bar(SHPB) tests are conducted respectively on rolled AZ31 B sheet at the strain rates of 700s-1-3000s-1, on extruded extruded AM30 magnesium alloy at the strain rates of 1000s-1-3000s-1, and on ordinary die casting and vacuum die casting(-75 kPa,-94kPa) AT72 Magneisum alloy at the strain rates of 3000s-1-7000s-1. Moreover, spacing ring limit are uesd in dynamic compression of extruded AM30 magnesium alloy in order to control the strain from 2% to18%.The microstructure and mechnical properties of as-cast(ordinary die casting and vacuum die casting) and deformed(along RD, TD and ND direction) magnesium alloy are analyzed in details. The microstructures are observed by optical microscope(OM). The fractures are observed by scanning electron microscope(SEM). Transmission electron microscopy(TEM) is uesed to identify the fine microstructure. The dynaimc failure and deformation mechnisms of magneisum alloys are discussed based on the evolution of microstructure. The results show that the basal texture in the rolled AZ31 B sheet causes the anisotropy of dynamic mechnical properties. The tensile twinning { }2101 happens in RD and TD direction. The results also show that the stress of rolled AZ31 B alloy increases with the increase of strain gradually and demonstrates the strain strengthening effect. The compress twining { }1101 is hard to activate and continuous yielding happens in ND direction. The dynamic fracture strength is 364 MPa, 365 MPa and 308 MPa in RD, TD and ND direction, respectively. The tongue patterns are observed on the fracture surfaces in RD and ND direction. The mechanism of dynamic fracture is cleavage fracture in TD direction. The deformation localization causes the temperature increasing and results in partial melting and dynamic recrystallization forming. Meanwhile, the fractures are propogated along the partial melting, grain boundary of dynamic recrystallization and twinning.Compared with rolled AZ31 alloys, the frature strength of extruded AM30 alloy is 390 MPa and 345 MPa in ED and TD directions during dynamic compressive tests. It means that the dynamic mechanical properties are similar and shows weak anisotropy. The twinnings are observed in ED and TD directions. Meanwhile, the twinnings are more in ED direction than those of TD direction. The microstructure evolution is analysed by spacting ring limit technology for extruded AM30 alloys:(1) The tensile twinning { }2101 happens during plastic deformation when the c-axes is perpendicular to the loading direction. The amount of twinning is more in ED direction than that of TD direciton;(2) The tensile twinning { 2101} happens again in original crystal orientation by subgrain rotation. Due to the rotation with no simultaneity, the fine equiaxed grains are formed;(3) The untwinning behavior occurs when the equiaxed rotates the same directino(c-axes paralle to the loading direction);(4) The new slip system is activated due to deformation localization with the increase of temperature;(5) The slip causes the dislocation, and then stress is increased due to tangled dislocation. The compressive twinning occurs when the stress reaches the critical resolved shear stress of compressive twinning;(6) Twinning changes lattice orientation and creatived new slip system. The mechanism of twinning is lattice rotation and results in fine subgrains. The subgrains are divided into fine equiaxed and formed deformation bands;(7) The cracks happens when the slip and twinning can not support the deformation. The tip of cracks have high stress and form a lot of compressive twinning;(8) The alloys are broken when the slip can not move and compressive twinning can not happen. The fractures are propagated along the deformation bands with fine grain, and finally break.Compared with wrought Mg alloys, there is no texture in die casting Mg alloys. The dynamic mechanical properties is decreased due to casting defects. The growth of porosity is one of the main factors resulting in fracture. The threshold of porosity is the fracture threshold. The dynamic tensile strength of normal die casting and vacuum die casting of AT72 alloys(-75 MPa,-94MPa) is 76 MPa, 69 MPa å'Œ 70 Mpa at strain rate of 500s-1. The dynamic tensile strength is 88 MPa, 84 MPa å'Œ 85 MPa when the strain rate increased. The true stress-strain curve of dynamic tensile is moved up with increasing strain rate and presented strain rate strengthening. The mechanism of dynamic fracture is intergranular fracture. The dynamic compress strength of normal die casting and vacuum die casting of AT72 alloys(vacuum degree:-75 MPa,-94MPa) is 107 MPa, 107 MPa and 113 MPa at strain rate of 4300s-1. The mechanism of dynamic compress fracture is quasi-cleavage crack. The amount of casting defects is not the main factor influenced the dynamic mechanical properties, but the defects or not is critical factors.The different dynamic mechanical properties of magnesium alloys are caused by different micro-deformation mechanism. When compression deformation along the rolling direction of RD, TD and the extrusion direction of ED, the deformation mechanism is tensile twinning. When the twinning is almost exhausted and the untwining ends, the deformation mechanism turns to slipping. When compression deformation is along the ND direction, the deformation mechanism is slipping. When compression deformation is along the TD direction and die casting magnesium alloys deform, the deformation mechanisms are both twinning and slipping. However, the defect in die casting magnesium alloys has a strong effect on its dynamic mechanical properties.