Microstructure Evolution In Uniformity Regulation And Deformation Of Rare Earth Microalloyed Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-Cu alloy with low density,high specific strength,good toughness and excellent weldability has been widely concerned and studied since its appearance.It has been very mature to improve the microstructure and properties of Al-Zn-Mg-Cu alloy by changing the main alloying elements and adjusting the heat treatment process.Microalloying has become the most promising method to further improve the microstructure and mechanical properties of aluminum alloys.In this paper,a microalloyed Al-Zn-Mg-Cu-Sc-Zr alloy was prepared by adding 0.2wt% scandium.The alloy was subjected to homogenization annealing and equal channel angular extrusion(ECAP)deformation.Combined with the microstructure characterization methods,the microstructure transformation of the alloy as-cast and homogenized as well as the distribution of alloy elements,the changes of grains,dislocations and the characteristics of second phase precipitation after deformation were studied.0.2wt% Sc is added to Al-Zn-Mg-Cu-Zr alloy to prepare Al-Zn-Mg-Cu-Sc-Zr alloy.The results show that the addition of 0.2wt% Sc helps to suppress dendrite segregation,reduce non-equilibrium eutectic structure,and refine the as-cast grain size to 55~80?m(75% of the whole).After two-stage homogenization of the alloy,it is observed that a large amount of T(Al Zn Mg Cu)phase distributed at the grain boundary is completely transformed into S(Al2Cu Mg)phase after being kept at 465? for 24 hours.After the temperature is increased to 480? for 24 hours,the S phase at the grain boundary disappeared.At this time,Zn,Mg,and Cu elements are evenly distributed inside the alloy,but there are still some Cu elements remained at the grain boundary due to the presence of a small amount of impurity phase(Al7Cu2Fe).There are coarse grain boundary preciptates(GBP)and large width precipitation free zone(PFZ)at the grain boundaries.A large number of L12-type Al3 Sc phases with a size of 40 nm are precipitated in the alloy,which are completely coherent with the ?-Al matrix.These precipitated second phases are pinned at the grain boundaries or act as heterogeneous nucleation cores to promote grain growth and hinder grain coarsening,resulting in an average grain size of 40?m.Compared with the as-cast hardness of 143.5HV,the hardness of the alloy after homogenization reaches 181.2HV,which is increased by 26%.Difficult deformation zone and severe deformation zone appeared in the cross section of ECAP deformed sample.The grain size in the difficult deformation zone is unchanged,but there are large compounds remaining at the grain boundary.The alloy grains in the severely deformed zone are significantly shortened.Compared with homogenization,the long axis of the grains is basically unchanged and the short axis is shortened by 56%,showing an elliptical shape after a single deformation(90° corner extrusion).After two passes of deformation(according to Bc path),the short axis of grains is shortened by 31% on average,showing willow leaf shape and the coarse compound between grains is significantly reduced.After two passes of deformation,the Al3(Sc,Zr)phase,Al3 Zr phase,rod-like ?? phase and ? phase with a size of 25 nm are dispersed in the alloy grains,which has a strong pinning effect on grain boundaries and dislocations,and inhibits the coarsening of grains and the recovery and recrystallization during deformation.In addition,the dislocation density in the alloy continues to accumulate under ECAP deformation.The grain boundary orientation changes from small angle grain boundary to large angle grain boundary,and the wave-striped dislocations are concentrated between the second phases.These dislocations present as dislocation couples with the same size of the Berkshire vector but opposite directions.At the same time,large grains composed of many subgrains or dislocation cell structures were observed,which had high dislocation density.Some precipitates had different depth shadows and different structure stacking faults,which made them present network distribution.The above research lays the foundation for the subsequent aging treatment and provides certain technical support for the improvement of the comprehensive performance of rare earth aluminum alloy.