Effects Of Alloying Elements On Fracture Behavior Of A7N01S-T5 Aluminum Alloy Used In High Speed Trains

A7N01S-T5 aluminum alloy belongs to the Al-Zn-Mg alloy series,which can be enhanced by heat treatment.It has been widely used in high-speed train bodies due to its high strength and low density.Although extensive studies on one or two elements have been made to affect various mechanical properties and microstructural changes in Al-ZnMg alloys,published work on multiple elements of the same is rather limited.With the development of high-speed train,the comprehensive properties of body material put forward higher requirements.So it has important engineering value and practical significance to optimize the composition of A7N01S-T5 alloy on multiple elements and the effect of alloy elements on the various properties study,especially the fracture mechanical properties.Four kinds of A7N01S-T5 aluminum alloys were designed by orthogonal method.The effect of elemental composition on the tensile properties,impact property,hardness,fracture toughness,fatigue crack growth behavior and corrosion fatigue crack growth behavior has been investigated.The alloys with different compositions were tested by tensile,impact,hardness,three point bending,fatigue crack growth and corrosion fatigue crack growth tests,and the range analysis results showed that the compositions of Zn and Mg were the main factors that affect the conventional mechanical properties and fracture mechanical properties of the alloys.The testing results showed that the#1 and#2 alloys had good fracture mechanical properties.However combined with the conventional mechanical properties experimental results,the#1 alloy had the best combination properties.The tensile strength,yield strength and elongation,impact energy,hardness,fracture toughness,threshold in fatigue crack growth,threshold in corrosion fatigue crack growth and corrosion current density of Al-4.34Zn-1.43Mg-0.27Mn-0.13Cr-0.12Zr0.07Ti(#1)alloy were 415 MPa?378 MPa?13.49%?12.3 J?123.07 HV3?23.37 kJ·m2?7.85 MPa?m?6.43 MPa?m and 1.60×10-6 A/cm2,respectively.The microstructure,grain size and orientation,compositions of constituent particles and corrosion product and fracture characteristics of the alloys were investigated by optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive X-ray(EDX).backscattered electron diffraction(EBSD)and transmission electron microscopy(TEM).The results indicated that the strength is mainly determined by the volume fraction,size and distribution of precipitated ?'(MgZn2)phase.The ?' phases distributed dispersion with a larger amount and smaller size.The more significant the precipitation strength effect is,the higher the strength of the alloy is.Moreover,the fracture toughness,resistance of fatigue crack growth and corrosion fatigue crack growth are determined by grain size and orientation and microstructure of grain boundary.The fine grain size,grain orientation difference small between adjacent grains,without strip of a wide range of the same or similar orientation direction and discontinuous distribution and big size of ?(MgZn2)phase played important roles to obtain high fracture mechanical properties.