A Study On Near Singular Boundary In 7A85 Aluminum Alloy

Super-high strength aluminum alloy is an important aerospace material.However,the fairly low resistance to intergranular corrosion limits its applications,and how to significantly improve this property is a key issue in the research of such alloy.Taking grain boundary itself as the point-cut,the near singular boundaries(NSBs)in a home-made 7A85 aluminum alloy was studied by five-parameter analysis(FPA)method based on electron back-scattered diffraction(EBSD)and stereology.Then,the effect of pre-stain,annealing time and pre-aging treatment on the NSBs were investigated.The main result are summarized as follows.7A85 aluminum alloy with uniform microstructure and orientation was obtained by repeated multi-directional forging followed by recrystallization annealing at 520? for 15 minutes.The grain boundary plane distribution(GBPD)of the alloy as processed was mainly distributed on {1 1 1}.The frequency of the boundary with a grain boundary inter-connection(GBIC)as {1 1 1}/{1 1 1} was the highest amoung all boundaries.Scanning electron microscopy(SEM)observation revealed that such boundary has discontinuous precipitates,and it has higher intercrystalline corrosion resistance than general boundaries.Hence,the boundaries with {1 1 1}/{1 1 1} GBIC is confirmed to be a kind of easily formed NSBs in 7A85 aluminum alloy.The amount of pre-strain(cold rolling)has a significant effect on the formation of {1 11}/{1 1 1}-NSBs in 7A85 aluminum alloy.When the pre-strain was 20%,the GBPD after recrystallization at 520? for 15 minutes was mainly distributed on {1 1 1},and the frequency of {1 1 1}/{1 1 1}-NSBs reached a peak value of 4.62%(length fraction out of the entire grain boundaries under two dimensional measurement).It was dicussed that the recrystallization of 7A85 aluminum alloy after 20% pre-strain was governed by a mechanism of so-called continuous recrystallization.The movement of the intrinsic dislocations to the grain boundary region resulted in a local sdjustment and grain boundary reconstruction(GBR).Such GBR promoted the grain boundary re-orientation on {1 1 1}and it contributes to the formation of {1 1 1}/{1 1 1}-NSBs.The annealing time has an obvious effect on the formation of {1 1 1}/{1 1 1}-NSBs in7A85 aluminum alloy.During the recrystallization annealing at 520? after 20% pre-strain,the frequency of {1 1 1}/{1 1 1}-NSBs in the 7A85 aluminum alloy was increasing at first and then decreasing.The peak vslue was 4.62% corresponding to an annealing holding time of 15 minutes.Further discussion pointed out that the intrinsic dislocation movement towards grain boundaries during continuous recrystallization is dependent of time,and there is a critical time for grain boundary plane re-orientation onto {1 1 1} via grain boundary reconstruction.After critical time,the behaviors of triple junctions and quadruple nodes may have adverse effects on the formation of {1 1 1}/{1 1 1}-NSBs.This is a issue deserved further investigations.The pre-aging treatment has an adverse effect on the formation of {1 1 1}/{1 11}-NSBs in 7A85 aluminum alloy.Compared to the samples which were directly cold rolled and annealed,the frequency of {1 1 1}/{1 1 1}-NSBs in the samples with pre-aging treatment at 120? for 8h-32 h before cold roling and annealing is obviously lower.It indicates the second phase particles developed during pre-aging have pinning effect on the movement of intrinsic dislocations during cold rolling and recrystallization annealing.Such effects have negative influence on the continuous recrystallization,grain boundary local adjustment,grain boundary reconstruction,thus negative influence on the formation of {1 11}/{1 1 1}-NSBs.