Study On The Near-singular Boundaries In High Purity Aluminum After Deformation And Recrystallization

{1 1 1}/{1 1 1} near-singular boundaries are more resistant to corrosion than random grain boundaries.Increasing the proportion of such grain boundaries is an important way to significantly improve the corrosion properties of aluminum and its alloys.In order to understand the intrinsic behavior of {1 1 1}/{1 1 1} near-singular boundaries during deformation and recrystallization of aluminum and its alloys,this paper selects high-purity aluminum as the research object.Based on the quantitative characterization method of grain boundary inter-connection matching,the grain boundary characteristic distribution of the samples with homogenized structure was measured,and the effects of rolling deformation and recrystallization as well as grain growth on {1 1 1}/{1 1 1} near-singular grains were studied.The formation mechanism of such grain boundaries was discussed based on the related principles of metallography and off-line in-situ analysis methods.The atomic structure of such grain boundaries was revealed by O lattice computational simulation and transmission electron microscopy observation,which provides theoretical and experimental basis for further active regulation of such grain boundaries.The main research results of this paper are as follows:(1)After repeated multi-directional forging and recrystallization annealing at 370℃ for 45 min,a sample with a homogenized structure was obtained.The grain boundary characteristics distribution show that the grain boundary with {1 1 1}/{1 1 1} inter-connection features occurs at the highest frequency,reaching 5.43%,which is a kind of near-singular grain boundary that is easily generated in high-purity aluminum.(2)The rolling temperature has a significant effect on the formation of {1 1 1}/{1 1 1}near-singular grain boundaries during subsequent recrystallization annealing of high-purity aluminum.The samples with homogenized structure were unidirectionally rolled with 80%deformation at 25℃,150℃ and 210℃,and then recrystallized at 370℃ for 45 min.The results show that the ratio of {1 1 1}/{1 1 1} near singular grain boundary increases first and then decreases with the increase of rolling temperature,reaching a maximum value of 5.93% at150℃.The deformation substructure formed by unidirectional rolling at 150°C occurs continuous recrystallization during subsequent recrystallization annealing,which is beneficial to the formation of near-singular grain boundaries.(3)The rolling deformation has a significant effect on the formation of {1 1 1}/{1 1 1}near-singular grain boundaries during subsequent recrystallization annealing of high-purity aluminum.The homogenized samples were unidirectionally rolled at 150℃ with deformations of68%,56% and 44% and recrystallized at 370°C for 45 min.The analysis shows that the proportion of {1 1 1}/{1 1 1} near-singular grain boundaries decreases with the decrease of rolling deformation.The ratio of {1 1 1}/{1 1 1} near singular grain boundary reaches a minimum of 3.03% when the deformation is 44%.Under medium and small deformation,the strain energy storage is uneven,and some grains grow abnormally.In the process of grain boundary migration,some {1 1 1}/{1 1 1} near singular grain boundaries are swept away.(4)Grain growth has a significant effect on the formation of {1 1 1}/{1 1 1} near-singular grain boundaries during subsequent recrystallization annealing.The samples with a homogenized structure were kept at 500°C for different times to make them grow further on the basis of recrystallization.It was found that the proportion of near-singular grain boundaries increased with the increase of grain size.The fraction of {1 1 1}/{1 1 1} near singular boundaries is 3.91%when the averaged grain size is 38μm,while it increases to 6.56% as the averaged grain size reaches 77μm.Off-line in-situ electron backscatter diffraction coupled with grain boundary trace analysis indicate that the {1 1 1}/{1 1 1} near singular boundary is formed mainly via the encountering of two growing grains having <1 1 1>/θ misorientation relationships(θ is the rotation angle around <1 1 1>).Meanwhile,the {1 1 1}/{1 1 1} near singular boundary is also formed via the re-orientation of the grain boundaries having <1 1 1>/θ misorientations.(5)High resolution transmission electron microscope observation reveal that the {1 1 1}/{11 1} near singular boundary possesses disclinations and the degree of atomic ordering of such boundary is much higher than that of the random boundaries,the O lattice structure indicates that there is a periodic matching area in{1 1 1}/{1 1 1}near-singular grain boundaries,which is the reason why the sub-grain boundaries are more resistant to corrosion than the general grain boundaries.