| This work studied the strengthening mechanism of the Final Thermal-Mechanical Treatment?FTMT?in 7005 alloys with two different kinds of aging states?one was peak-aged with GP?'precipitates,and the other one was over-aged with?+?'precipitates?,the alloys were all cold-rolled heavily with 80%deformation and then undergo a re-aging treatment at 120°C with 30h/120h.After detail test and analyses on the tensile mechanical properties and the microstructure of all samples during the FTMT,it was found that there were big differences in the strengthening mechanism between peak-aged and over-aged 7005 alloys during FTMT.The main conclusions are followed:?1?After being cold-rolled heavily with 80%deformation,the yield strength of peak-aged7005 alloys increased obviously by 67%than before rolling,and the yield strength of over-aged7005 alloys increased by 77%than before rolling,the increasing rate of the latter one was bigger.After rolling,during the re-aging at 120°C,the changes in yield strength of alloys being rolled were different:the yield strength of rolled peak-aged 7005 alloys slightly decreased 24MPa after 30h re-aging,and then regained 30MPa after 120 re-aging;while the yield strength of rolled over-aged 7005 alloys clearly decreased 53MPa after re-aging without regain.?2?The cold rolling refined the grain sizes and improved the dislocation densities greatly in materials.The GP?'precipitates were sheared while the?precipitates were bypassed by dislocations after deformation.And the Zn atoms segregated on the holes of dislocations,which came into Cottrell atmospheres and strengthened the materials.The 120°C re-aging wouldn't change the grain morphology and dislocation densities,but it would make Zn atoms and other solute atoms flow to precipitates and make them have some interesting transformations,and also caused the Zn atoms disappearing from the holes of dislocations,which means the strengthening effect of Cottrell atmospheres would disappear.The coherent GP?'precipitates transformed to semi-coherent?p precipitates because of the segregation of Zn,which changed the precipitation sequence of 7005 to alloys with higher Zn/Mg atom ratio?such as 7055 or7150 alloys?,the precipitate evolution would strengthen the materials again.While the?precipitates surrounded by holes of dislocation loop would grow into spherical with the solute atoms came into the holes,and Cottrell atmospheres strengthening disappeared too.But due to the incoherent?precipitates were thermostable previously,the 120°C heat treatment wouldn't strengthen?precipitates anymore,and the precipitate evolution wouldn't strengthen the materials again.?3?The FTMT has different impacts on the fracture mechanism of alloys with different aging states,the fracture mechanism of samples with peak-aged+cold rolled?R80%?+120°C re-aging 120h treatment was intergranular fracture+dimple fracture and shear deformation,while the fracture mechanism of samples with over-aged+cold rolled?R80%?+120°C re-aging120h treatment was dimple fracture and half dislocation multiplication deformation+half shear deformation.This work revealed the strengthening mechanism of the FTMT in 7005 alloys with different precipitates,and studied detailedly in the mechanical properties and microstructure of materials during the FTMT,especially in the change of precipitates,and found that the precipitation sequence changed in 7005 alloys during the FTMT.This work can have some guidance for developing the mechanical property in 7005 alloys by optimizing the technology of deformation or thermal treatment. |
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