Research On Grain Boundary Plane Distributions In Recrystallized High Purity Al After Cold Forming

As key engineering materials,Al and its alloys exhibit grain boundary failures in many cases.To better the grain-boundary-dominant performances of A1 and its alloys through grain boundary engineering(GBE)is becoming one of the most promised research directions.However,it is quite different from those low to medium stacking fault energy face-centered cubic metals,Al and most its alloys are not applicable to twin-induced GBE processing due to their high stacking fault energy.In order to optimize the grain boundary character distribution(GBCD)so as to remarkably better the properties of Al and its alloys,it is necessary to study the grain boundary plane distributions(GBPDs).In this work,high purity(99.99%)A1 specimens,which were prepared by multi-directional forging followed by recrystallization annealing resulting in a homogeneous microstructure with averaged grain size around 20 ?m,were separately processed by equal channel angular pressing(ECAP)and direct rolling(DR)at room temperature followed by a recrystallization annealing at 330?360?.Then,the GBPDs were characterized by five-parameter analysis(FPA)based on stereology method and electron backscatter diffraction(EBSD),and the effects of deformation strain,deformation modes and pre-recovery treatment on the recrystallization GBPDs discussed as well.The major results are as follows.The deformation strain has an obvious influence on the recrystallization GBPDs of high purity Al.Two specimens,which were processed by 1-pass and 2-pass ECAP,respectively,were annealed at 330?.EBSD and FPA analyses indicate that more than 75%(length percentage)out of the entire grain boundaries in the primary recrystallized microstructure were high angle grain boundaries(mis-orientation angle more than 15°).During grain growth,the GBPD of the 1-pass ECAP specimen evolves intrinsically from ?3 3 2} to ?1 1 1}.While,those of 2-pass ECAP specimen evolves from {3 3 2} to {1 1 1} and {2 2 3}.It could be attributed to the different substructures developed in the two specimens before annealing.Such difference of substructure may affect both behaviors of recrystallization and grain boundary migration.Pre-recovery treatment after ECAP exerts a remarkable effect on the recrystallization GBPDs of high purity Al.A specimen of 1-pass ECAP was recovered at 240? for 0.5h and then undergone a recrystallization annealing at 330?.FPA analysis shows that its GBPDs orient on {1 1 1} but scatter to higher-energy {3 3 1?.However,in the same specimen but without recovery treatment,its GBPDs just orient around {1 1 1}.Furthermore,for some specific boundaries such as those with[1 11]/20?60° mis-orentations,the GBPD in the specimen with recovery treatment usually scatter around {111} but that in the pecimen without recovery treatment orient on the exact {111} position.These effects may be originated from the fact of dislocation annilation and substructure adjustment during recovery which have impacts on the behaviors of recrystallization nucleation and grain boundary migration.Deformation modes also affect the recrystallization GBPDs of high purity Al.Two parallel specimens were separately processed by ECAP and DR with true strain??2 followed by a recrystallization annealing at 360? for 8?90min.The results show that the grain boundary planes of the specimens as processed mainly orient on{1 1 1},mostly corresponding to the<1 1 1>twist high angle boundaries.It is due to the energy minimum of {1 1 1}.The primary difference of grain boundary plane distributions between ECAP and DR specimens lies in the behaviors of grain boundary planes orienting onto {1 1 1}.For ECAP specimens,it is slow the grain boundary planes orienting onto {1 1 1}.However,for DR specimens,it is quite easy the grain boundary planes orienting onto {1 1 1}.Discussions pointed out,compared with ECAP deformation,DR deformation is more efficient for grain boundary plane orienting onto {1 1 1} in the subsequent recrystallization annealing and thus is more in favor of the optimization of grain boundary character distribution.It could be attributed to the development of<1 1 0>//ND textures during DR deformation which results in the fast grain growth in the subsequent recrystallization annealing.