| Al-Zn-Mg-Cu alloy as an utra-high strength aluminium alloys with low density and high specific strength,good toughness and diffusion properties,and excellent fatigue resistance,which is widely used in the fields of aerospace and defense industry.With the successful test flight of C919 with independent intellectual property rights in May2017,China has made great breakthroughs in the independent research and development of aviation aircraft,which was a significant manifestation of China’s economic and technological strength.As the main structural material of aerospace vehicle,it is required to have good comprehensive mechanical properties and application value.Based on the flow characteristics of Al-Zn-Mg-Cu alloy during severe plastic deformation（SPD）processing,a new Al-Zn-Mg-Cu alloy（Zn content>12%）was used to carry out high-pressure torsion（HPT）experiment at room temperature under different torsion numbers.By applying a series of material characterization techniques（such as OM,XRD,TEM and EBSD）,the microstructures and secondary phase evolution under different deformation donditions were studied in-depthly.Based on theoretical knowledge,its evolution mechanism was analyzed and discussed extensively.Finally,according to the mechanical properties of alloy,a prediction strengthening model for this alloy under SPD processing is proposed.The results of microstructure show that the grain size of the initial sample with coarse grain is about 80μm.The HPT processing has a great grain refinement effect on the microstructure of alloy.After 5 turns of HPT,the grain size is significantly reduced from 80μm to 0.54μm because of the grain breakage and recrystallization.After 5turns of HPT processing,the proportion of large angle grain boundary increases from10.5%to 82.4%and the width of precipitation-free zone（PFZ）decreases from 2.1 um to 0.1 um compared with initials.In addition,the dislocation density is greatly increased and characteristic texture strength is obviously weakened,which means the uniformity of deformation material is significantly improved.The analysis of the composition,morphology and quantity of secondary phase shows that the secondary phase in the initial sample is mainly acicular semi-coherent second phase（Mg Zn2）.With the increasing of torsion numbers,the second phase gradually evolves into a non-coherent Mg Zn2 with the shape of circular-block.With the fragmentation and dissolution of the secondary phase,the content of the second phase decreases gradually.The secondary phase with the size of 50 nm is dispersed uniformly in the matrix when the turn of torsion comes to 5.The results of hardness and tensile test show that HPT processing can significantly improve the mechanical property of Al-Zn-Mg-Cu alloy.The microhardness and tensile strength increased with the increase of cumulative equivalent strain.After 5 turns of HPT processing,the microhardness and tensile strength of the specimens reach 212+12HV and 478 MPa respectively,which increased by 135.4%and 40%compared with that of initials.In addition,by calculating the deformation homogenization coefficient,it can be found that the uniformity of the material is significantly enhanced.The strengthening mechanism of Al-Zn-Mg-Cu alloy before and after deformation is discussed and analyzed.The results show that grain boundary strengthening,dislocation strengthening and precipitation strengthening are the main strengthening mechanism for HPT processed Al-Zn-Mg-Cu alloy at room temperature. |
