Microstructural Evolution Of Al-4.0wt.%Cu Alloy During Aging And Its Effect On Mechanical Properties

The heat-treatable aluminum alloys usually exhibit remarkable rapid hardening in the initial stage of artificial aging,which is related to the hindering of the rapidly precipitated coherent second phases on the movement of dislocations.The natural aging before artificial aging promotes the formation of atomic clusters,which will provide a large number of nucleation sites and diffusible solute atoms for the formation of second phases during subsequent artificial aging,further affecting the evolution of the precipitates and the corresponding mechanical properties.However,less investigation on the effect of natural aging on microstructural evolution and mechanical properties of Al-Cu alloys has been conducted.Therefore,in this paper,Al-4.0wt.%Cu alloy is selected as the research object,and the microstructural evolution of Al-4.0wt.%Cu alloy aged naturally and its effect on mechanical properties in the early stage of artificial aging are systematically studied by the measurement of internal friction spectrum,the observation of TEM and uniaxial tensile tests.The aim is to provide the references for the drawing-up of heat treatment processes of aging hardening aluminum alloys with the optimum mechanical properties.For solution-treated Al-4.0wt.%Cu alloys aged artificially for 1～5 h at 160℃,namely,one-step aging,with the prolonging of aging time,the shape of the dominant precipitates was changed from small needle-like to disc-like,finally to longer needle-like.The needle-like precipitates were θ//phase,whereas a disk-like precipitate was actually a region formed by the aggregation of many fine needle-like θ//phases distributed almost in the same direction.With the extending of aging time,most of fine needle-like θ//phases in a disc-like region were redissolved and a few θ//phase grew,and the content and size of needle-like θ//phases increased.During tension,the dislocations were mainly piled up nearby the needle-like θ//phases,and the larger the size of θ//phases was,the more obvious the piling-up of dislocations nearby θ//phases was.In comparison to needle-likeθ//phases,there was no obvious piling-up of the dislocations nearby the disc-like regions,and some dislocations were observed in the disc-like regions,which indicates that fine needle-like θ//phases in the disc-like regions had a certain hindering on dislocation movement,but the dislocations could eventually slip through the regions.In addition,a large number of fine granular second phases were precipitated on the dislocations during tension,namely,the strain aging occurred.These granular second phases produced the obvious pinning on the dislocations,further leading to the bowing-out of these dislocations,meanwhile,some dislocations cut through granular second phases were also observed.The combination of the above three kinds of hindering the movement of the dislocations encountered led to the gradual increase in strength with aging time.After Al-4.0wt.%Cu alloy aged naturally for 24 h was artificially aged for 1～13 h at 160℃,namely,two-step aging,the precipitates were needle-like θ//phase in different treated specimens,and their shapes were still needle-like and disc-like regions formed by the aggregation of many fine needle-like θ//phases,However,with the prolonging of artifical aging time,the growth of needle-like θ//phases in a disc-like region was accelerated,so that the content of needle-like θ//phase increased and their distribution became more uniform.As aging time was 13 h,the coarsening of needle-like θ//phases led to their evolution to semi-coherent θ/phases.This is related to the partial dissolution and the migration to {100} planes of the Cu atom clusters segregated on {111} planes during natural aging,which provided more nucleation sites and Cu atoms for the formation of the precipitates during subsequent artificial aging,the evolution of the precipitates was thus accelerated.As artificial aging time is 1 h and 3 h at 160℃,the strength of one-step aged specimens was higher,whereas the strength of two-step aged specimen was significantly increased comparing to that of one-step aged specimen when aging for 5 h;with the extending of artificial aging time for two-step aged specimens,yield strength σYS increased and reached the maximum value when aging for 10 h,and then decreased;whereas tensile strength σUTS first increased to 161.7 MPa when aging for 5 h,and then decreased to 143.1 MPa when aging for 7 h.As the aging time was continuously extended to 10 h and 13 h,σUTS increased to about 160 MPa again,and then decreased.The uniform strain ε was basically close and about 18.5%when aging for 1 h and 3 h,and then continuously decreased to a lowest value when aging for 10 h,with the continuous prolonging of aging time,ε increased to about 11.0%,but the value was lower than that of specimen aged for 7 h.Therefore,σYS of Al-4.0wt%Cu alloy treated by natural aging for 24 h and artificial aging for 5 h at 160℃ remained basically the highest σYS of 65.3 MPa obtained by one-step aging for 16 h at 160℃,and σUTS was increased from 139.2 MPa of one-step aging specimen to 161.7 MPa of two-step aging specimen,and the corresponding ε was increased from 8.2%to 15.27%.Therefore,the two-step aging significantly shortens the artificial aging time,meanwhile the tensile properties are largely improved,which is related to the precipitation and growth of needle-like θ//phases.