A Study On The Formation Of Near Singular Boundary And Its Precipitation And Corrosion Behavior In A Recrystallized Al-Cu Alloy

Aluminum-copper alloys are widely used in aerospace and automobile manufacturing industries owing to their high strength,light weight,good ductility and weldability.However,due to its insufficient resistance to grain boundary corrosion,the use of Al-Cu alloys is limited to a certain extent.According to the studies of traditional grain boundary engineering,the introduction of a large number of coherent twin grain boundaries(that is singular grain boundaries)can effectively improve the grain boundary corrosion resistance of medium and low stacking fault energy face-centered cubic metals.However,Al-Cu alloy is a high stacking fault energy face-centered cubic metal,and its ability to form singular grain boundaries is very weak.Related previous research results indicate that among the high-purity aluminum with high stacking fault energy face-centered cubic structure,the grain boundary with grain boundary inter-connections of {1 1 1}/{1 1 1} is a kind of near singular boundary with high corrosion resistance and strong generation ability.It is worthwhile to study the formation and corrosion resistance of such near singular boundaries in Al-Cu alloys.Therefore,this paper uses the five-parameter grain boundary plane distribution characterization based on the electron backscatter diffraction technique and the principle of stereology,the quantitative characterization of the grain boundary inter-connections,the transmission electron microscope observation,the O-lattic theory analysis to study {1 1 1}/{1 1 1} near singular boundary formation and its precipitation and corrosion behavior in the rolling deformation Al-Cu alloy recrystallized,providing the theoretical and experimental basis for the research of aluminum alloy grain boundary engineering based on near singular boundary.The main research results of this paper are as follows:(1)Rolling deformation and rolling temperature have a significant effect on the formation of {1 1 1}/{1 1 1} near singular boundaries in Al-Cu alloy recrystallized.The alloy sample is hot-rolled at 350? and deformed by 20%-80%(thickness reduction),and then recrystallized and annealed at 400? for 30 minute.The ratio of {1 1 1}/{1 1 1} near singular boundaries shows a slight decrease first and then a sharp increase with the increase of rolling deformation before annealing.Among the samples that were annealed after 80% rolling,the proportion of near singular boundaries was as high as 5.9%.Compared with hot rolling deformation at 350?,cold rolling at room temperature and annealing under the same conditions(400? for 30 minute),the ratio of {1 1 1}/{1 1 1} near singular boundaries is significantly lower.(2)The precipitation and corrosion behavior of {1 1 1}/{1 1 1} near singular boundaries in Al-Cu alloy is different from that of singular boundaries and random boundaries.The precipitated phases at the near singular boundaries are distributed discontinuously,no precipitated phases are observed at the singular boundaries.While the precipitated phases of random boundaries are continuously precipitated.Although corrosion resistance of near singular boundaries are not as good as that of singular boundaries,it is significantly better than random boundaries.(3)The structure of {1 1 1}/{1 1 1} near singular boundaries in Al-Cu alloy is different from singular boundaries and random boundaries.The singular boundary is coherent twin boundary,and its structure is that the atoms on both sides of the boundary are distributed in mirror symmetry with {1 1 1} as the common atomic plane.Meanwhile,there is no dislocation and free volume at the grain boundary,and the boundary energy is low.However,the structure of random boundary is disorderly arrangement of atoms,so there are a large number of aperiodic dislocations and the boundary energy is relatively high.The near singular boundary observed in this paper is a special structure separated by periodic dislocations in the good matched area of periodic atoms.This structure may have a specific impact on the segregation energy of grain boundaries and the thermodynamics/kinetics of precipitation,resulting in the precipitation and corrosion behavior of near singular boundaries different from singular boundaries and random boundaries.(4)Further research on the formation mechanism of {1 1 1}/{1 1 1} near singular boundaries to achieve active control of such grain boundaries can significantly improve the grain boundary corrosion resistance of Al-Cu alloys and expand its application range,which has universal significance for all high stacking fault energy face-centered cubic structure aluminum alloys.