Dynamic Mechanical Property And Failure Behavior Of Extruded Mg-Gd-Y Magnesium Alloy At High Strain Rate

Mg-Gd-Y magnesium alloys which are thought as a very potential high strength magnesium alloys, have been received much global attention by researchers. However, at present so much work are focus on alloying and heat treatment of Mg-Gd-Y magnesium alloys and the mechanical tests are almost undet static or quasi-staic loading. The study on mechanical properties and microstructures of Mg-Gd-Y magnesium alloys under high strain rates have not been caried out indetails, which limits the application of Mg-Gd-Y magnesium alloys in practical engineering. Thus, the research on dynamic mechanical behavior of Mg-Gd-Y magnesiuam alloy can not only enrich the theory of dynamic mechanical property of magensium alloy, but also deeply know the high-speed plastic deformation mechanism of magensium alloy. This research will guide the improvement of deformation process when magensium alloy under high-speed and large-deformation rolling or under high-speed extrusion. Meanwhile this research can provide the theoretical foundation of the design and application of Mg-Gd-Y magnesiuam alloy components uesd in automobiles, airplanes and protective armor in which the dynamic mechanical properties must be considered.For the purpose of investigating the dynamic mechanical behavior and failure mechanisms of extruded Mg-Gd-Y magnesium alloy at high strain rates, the Split Hopkinson Pressure Bar(SHPB) was used for investigating the dynamic compressive properties of extruded Mg-Gd-Y magnesium alloy along different directions at ambient temperature and the dynamic compressive properties of extruded Mg-Gd-Y magnesium alloy at high temperatures. The Split Hopkinson Tensile Bar(SHTB) was used for investigating the dynamic tensile properties of extruded Mg-Gd-Y magnesium alloy along at ambient temperature. The strain rates of the dynaimc tests were from 500s-1 to 3500s-1.The microstructures, fracture morphology and adiabatic shear bands of the specimens after dyniamic tests were analyzed by optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM). Dynamic mechanical properties under different loading and temperatures, the dynamic deformation mechanism,the evolution of microstructure and crack performance of extruded Mg-Gd-Y magnesium were discussed. The main conclusions are:Extruded Mg-Gd-Y magnesium alloy at ambient temperature under dynamic compression along ED, TD and ND directions has no apparent yielding point. When strain rates rise, extruded Mg-Gd-Y magnesium alloy has the positive strain rate effect.The dynamic compressive strengthes of extruded Mg-Gd-Y magnesium alloy along ED, TD and ND directions are respectively 535 MPa, 508 MPa and 489 MPa. The dynamic compressive strains of extruded Mg-Gd-Y magnesium alloy along ED, TD and ND directions are respectively 15%, 14% and 15%. The dynamic compressive properties do not exhibit strong anisotropy and along ED direction the mechnical properties are relatively optimal. It is due to the fine grains and the main deformation mechanism of non-basal-plane slips.When temperature increases under similar strain rate, the dyniamic compressive fracture strength of extruded Mg-Gd-Y magnesium alloy decreases while the plasticity increases. At ambient temperature and the strain rate of 2826s-1, extruded Mg-Gd-Y Magnesium alloy has the largest dynamic compressive fracture strength, which is 535 MPa. At 300℃ temperature and the strain rate of 3344s-1, extruded Mg-Gd-Y Magnesium alloy has the largest dynamic compressive strain, which is 20%.Extruded Mg-Gd-Y magnesium alloy has dynamic asymmetry between tension and compression at ambient temperature and high strain rates. The dynamic tensile strength of extruded Mg-Gd-Y magnesium alloy is 208 MPa and the dynamic compressive strength of extruded Mg-Gd-Y magnesium alloy is 535 MPa. The ratio of dynamic tensile fracture strength to dynamic compressive strength is 0.37. { 10 12 }< 1120 > tensile twinning causes the dynamic tension-compression asymmetry of extruded Mg-Gd-Y magnesium alloy along ED direction.The dyniamc compressive deformation mechanisms at ambient temperature of extruded Mg-Gd-Y magnesium alloy are: twinging is the main mechanism at relative low strain rates while it is a combination of slipping and twinning at higher strain rates. The higher strain rates are, the more percents of slipping are. Slipping is non-basal-plane slip and has the trend to turn to basal-plane slip. During the dynamic deformation, recrystallization and adiabatic shearing happen. The dyniamc compressive deformation mechanism at high temperatures of extruded Mg-Gd-Y magnesium alloy is a combination of slipping and twinning. Slipping is the main mechanism. During the dynamic deformation, adiabatic shearing, recover and strong recrystallization happen. The dyniamc tensile deformation mechanism at ambient temperature of extruded Mg-Gd-Y magnesium alloy is a combination of slipping and twinning. Slipping is the main mechanism. During the dynamic deformation, adiabatic shearing happens.The microstructure evolution of extruded Mg-Gd-Y magnesium under dynamic compression at ambient temperature is that the amount of twinning firstly increases and then decreases with the increase of strain rates. At higher strain rates, recrystallized grains form the adiabatic shear bands. Under the strain rate from 1000s-1 to 2500s-1 and strain from 8% to 15%, the adiabatic shear band forms.The formation process of the adiabatic shear band can be divided into three stages. In the first stage, under the impact loads a great deal of twins and recrystallized grains form and the deformation mechanism is twinning. In the second stage, discontinuous ASB forms due to twins adjusting the directions of located grains and the deformation mechanisms are twinning and non-basal slip. In the last stage, non-basal slip turns to basal slip. Strong thermal softing effect happens and leads the continuous ASB form.The dynamic compressive fracture performance of extruded Mg-Gd-Y Magnesium alloy at ambient temperature along different directions is mix-fractured quasi-cleavage fracture and is not sensitive to the loading direction. The dynamic compressive failure mechanism of extruded Mg-Gd-Y Magnesium alloy at ambient temperature along different directions is muti-crack propagation mechanism. Cracks initiate in the grain boundary and extend along the grain boundary. In some area there are special melting areas. The dynamic compressive fracture performance of extruded Mg-Gd-Y Magnesium alloy at high temperature is also mix-fractured quasi-cleavage fracture. The dynamic compressive mechanism of extruded Mg-Gd-Y Magnesium alloy at high temperature is muti-crack propagation mechanism. Cracks initiate in the grain boundary and extend along the grain boundary. With the increasing temperature, special melting areas become more. The dynamic tensile fracture performance of extruded Mg-Gd-Y Magnesium alloy at ambient temperature is brittle cleavage fracture. The dynamic tensile mechanism of extruded Mg-Gd-Y Magnesium alloy at ambient temperature is muti-crack propagation mechanism. Pits form and connect together under dynamic tensile stress which causes the breakage of the specimen.