Low-cycle Faigue Behavior Of Al-7Zn-2Mg-1.5Cu-0.1Zr(-0.2Sc) Deformation Aluminum Alloys At Room And Low Temperatures

As a typical age-hardenable alloy,Al-Zn-Mg-Cu series alloys processed in the form of rolling,extrusion or forging,have been widely used for the key structural components of aerospace and transportation in virtue of their properties of high-specific strength and low density.Most properties of aluminium alloys can be controlled through appropriate alloying,processing,heat treatment or a combination of them.With the rapid development of modern industry and the requirements of lightweight design,the demand for the performances of aluminum alloys as the structural materials is putting forward higher.The mechanical property of Al-Zn-Mg-Cu series alloys can be further reinforced through grain refinement obtained by hot extrusion.And in practice,many applications,such as the airplane frame and automobile body structures,will be subjected to alternating loads during service,which will eventually lead to fatigue damage.It has been generally accepted that the fatigue fracture is one of main fracture modes for engineering components.In order to ensure the security of components in service,the fatigue performances need to be further studied.However,the influence of Sc,as one of the most effective elements in improving performances of aluminium alloys,on the low-cycle fatigue performances of aluminium alloys has been seldom reported.Moreover,the researches for the cyclic deformation properties of aluminum alloys under different environments conditions have been also rarely performed,which mainly focus on elevated temperature and hydrogen embrittlement environments.As is well known,in high altitude environments or some regions and seasons,the components may work in temperature below0°C,thus the fatigue and fracture characteristics can be affected by such temperature environment.Obviously,it is necessary to understand the low-cycle fatigue performance of Al-Zn-Mg-Cu series alloys with Sc at low temperature,and can also provide the theoretical foundation for the anti-fatigue design and reasonable use of these alloys.Hence,the microstructure and low-cycle fatigue behavior for extruded Al-7Zn-2Mg-1.5Cu-0.1Zr（-0.2Sc）alloys under room and low（0^oC,-20^oC,-40^oC）temperature environments were investigated,and the effects of Sc element and temperature on the microstructure and low-cycle fatigue properties of alloys were determined.The microstructural observations show that in Al-7Zn-2Mg-1.5Cu-0.1Zr（-0.2Sc）alloys,the mainly precipitated phases are GP zones andη’phase.Some Al₃（Sc,Zr）particles with the bean-like shape can be seen inside theα-Al matrix due to the addition of Sc element.Al₃（Sc,Zr）particle exhibits an excellent thermal stability and plays the dominant role in pinning the dislocation and subgrain boundaries,which can make the alloy retain the organization structure of extrusion state after T6 treatment.The tensile results at room temperature show that the aging strengthening tendencies of Al-7Zn-2Mg-1.5Cu-0.1Zr（-0.2Sc）alloys exhibit the similar feature,which depend on the aging precipitation sequence.By the combined effects of grain boundary strengthening and dispersion strengthening caused by Al₃（Sc,Zr）particle,the ultimate tensile strength and yield strength of Sc-containing alloy are significantly improved under the same aging time,but the elongation to failure is relatively reduced.The low-temperature tensile tests show that with the decrease of temperature,the ultimate tensile strength and yield strength of two alloys increase and the elongation decreases.The observations on tensile fracture surfaces show that at room temperature,the fracture mode of two alloys is mainly ductile fracture.With the decrease of temperature,the mixed ductile-brittle fracture can be observed.The results of low-cycle fatigue tests at room and low temperature show that two alloys exhibit different cyclic stress response behaviors depended on the applied total strain amplitude.For the Al-7Zn-2Mg-1.5Cu-0.1Zr alloy with T6 state,the cyclic stress response behaviors present cyclic stabilization during the whole process of fatigue deformation under the strain amplitudes of 0.4%and 0.5%.At the strain amplitudes of 0.6%～0.8%,the initial cyclic stabilization can be generally discovered.As the number of cycles increase,the cyclic hardening occurs,and finally the cyclic softening appears at the later stage of fatigue deformation.For the Al-7Zn-2Mg-1.5Cu-0.1Zr-0.2Sc alloy with T6 state,under the conditions of strain amplitudes from 0.4%to 0.8%,the cyclic stress response behaviors exhibit cyclic stabilization at all temperatures.Under the conditions of same temperature and total strain amplitude,the addition of Sc can significantly improve the stress required for cyclic deformation of the alloy.The stress amplitudes of two alloys increase with the decrease of temperature.In terms of low cycle fatigue life,at the same total strain amplitude,the low-cycle fatigue lives of Al-7Zn-2Mg-1.5Cu-0.1Zr（-0.2Sc）alloys with T6 state decrease with the decrease of temperature.And the low-cycle fatigue life curves of two alloys intersect at the same temperature.Compared with the Al-7Zn-2Mg-1.5Cu-0.1Zr alloy,the low-cycle fatigue lives of Al-7Zn-2Mg-1.5Cu-0.1Zr-0.2Sc alloy are longer at lower strain amplitudes,while are shorter at higher strain amplitudes.At all temperatures and total strain amplitudes,The relations of plastic and elastic strain amplitudes with reversals to failure for two alloys obey the Coffin-Manson and Basquin formulas,respectively.And based on hysteresis energy theory,the relationship between temperature and low-cycle fatigue life is deduced.The observations on fatigue fracture surfaces show that for two alloys,the fatigue cracks initiate transgranularly at the surface of fatigue samples and propagate in a transgranular way under all temperatures and total strain amplitudes.Temperature has no effect on the initiation and propagation of fatigue crack.The observations on microstructure near fatigue fracture show that the cyclic plastic deformation mechanism of Al-7Zn-2Mg-1.5Cu-0.1Zr（-0.2Sc）alloys with T6 state exhibits the plane slip under the conditions of low-cycle fatigue loading at room temperature and low temperature.For the Al-7Zn-2Mg-1.5Cu-0.1Zr alloy,at lower strain amplitudes,there exists a large number of dislocation debris in the fatigue deformation zone.And at higher strain amplitudes,the dislocation configuration is shown as dislocation tangle.For the Al-7Zn-2Mg-1.5Cu-0.1Zr-0.2Sc alloy,the dislocation configuration is mainly shown as dislocation arrays at lower strain amplitudes.And the dislocation arrays with different directions can interact and form the dislocation networks at higher strain amplitudes.