Study On The Low Cycle Fatigue Behavior Of Cu-Cr-Zr Alloy

The rapid development of electrification roads towards heavy loads and high speeds,which has placed higher requirements for the various performance of contact wires.The high-performance Cu-Cr-Zr alloy,as a new generation of contact wires material,is becoming increasingly wide-spread used in high-speed rail networks.However,based on the actual service environment of high-speed railways contact wires such as repeated cyclic loading and high-tension installation,there are few scholars have conducted in-depth studies on the low-cycle fatigue behavior of alloys in different states in combination with the specific preparation process of Cu-Cr-Zr alloys.Regarding the issue above,in this work,the hot-extruded Cu-Cr-Zr alloy is taken as the research object.From the two dimensions of cold deformation and heat treatment,their effects on the monotonic tensile properties and low-cycle fatigue properties of the selected materials at room temperature were systematically studied,and combined with many advanced detection methods,mainly made the following aspects of research:(i).The evolution law of microstructure and properties(including tensile properties and low cycle fatigue properties)of alloy during deformation and heat treatment were discussed;(ii).The effect of cold deformation and solution-aging treatment on low cycle fatigue life were analyzed and clarified,and revealed the relationship between fatigue life and various influencing factors;(iii).The low-cycle fatigue behavior of alloys in different states were compared and evaluated,combined with the fatigue fracture and cyclic deformation structure under different applied total strain amplitudes,a correlation model between"prepared process-microstructure-low cycle fatigue properties"was constructed.As the amount of cold deformation applied to the hot-extruded Cu-Cr-Zr alloy increases,the tensile strength of alloy continues to increase,but the low-cycle fatigue life does not necessarily show a gradual increasing trend,a greater degree of cold deformation can significantly improve the transition life N_t of Cu-Cr-Zr alloy.The Cu-Cr-Zr alloys in the three cold deformation states all exhibited the Bauschinger effect at a low total strain amplitude(??_t=0.4%)and the strength is proportional to the degree of cold deformation,this is also the first discovery in Cu-Cr-Zr alloy.Also,for Cu-Cr-Zr alloy that has undergone the most severe cold deformation,the Bauschinger effect can also be observed in the cyclic deformation process of??_t=0.45%,but its strength is much lower than that when??_t=0.4%.The alloys in the three cold deformation states exhibit cyclic softening under all the total strain amplitudes.A higher strain amplitude will promote a faster reduction of the internal dislocation density of the alloy,resulting in poorer structural stability.Solution-aging treatment can significantly improve the low cycle fatigue life of the cold-drawn state Cu-Cr-Zr alloy under high total strain amplitude(??_t/2>0.6%),the Hall-Petch formula is not completely applicable to the grain size effect during cyclic deformation.Grain coarsening brought by solution-aging,the value of total strain amplitude and precipitation strengthening effect significantly affect the low cycle fatigue life of the material,when??_t/2>0.6%,the enhancement effect of precipitation strengthening on fatigue life is higher than the weakening effect of grain coarsening,When??_t/2?0.6%,the weakening effect due to grain coarsening will be higher than the enhancement effect by precipitation strengthening.The observation of the fatigue fracture of the aging state Cu-Cr-Zr alloy shows that it is in sharp contrast to the fatigue fracture surface when??_t/2=0.4%,??_t/2=1.0%was characterized by multiple fatigue sources,more obvious fatigue striations and more river-like patterns.Moreover,the friction phenomenon of the fracture surface was more serious,a sawtooth-like feature formed and the smaller proportion of dimples indicate that the fracture mode was mainly brittle fracture.Besides,the cyclically deformed microstructure of the alloy showed a significant difference due to the difference in the total strain amplitudes,and the dislocation density is positively correlated with the value of??_t/2.This work introduces a low cycle fatigue behavior evaluation system combined with the specific preparation process of Cu-Cr-Zr alloy,a unique mechanical phenomenon was discovered in the course of the experiment,namely the"Bauschinger effect",and its formation mechanism was discussed in depth.This work has perfected the theoretical database of Cu-Cr-Zr alloy on the cyclic deformation,and provided the necessary theoretical basis for its practical application in the high-speed railway contact network.