The dependence of precipitate morphology on the grain boundary types in an aged Al-Cu binary alloy

A solid-solution treated Al-Cu binary alloy ingot containing 2.42 wt% copper was subjected to a multi-directional forging at ambient temperature followed by a recrystallization annealing at 400 degrees C and aging at 175 degrees C. Then, the dependence of precipitate morphology on grain boundary types was studied by field emission scanning electron microscopy, electron backscatter diffraction and grain boundary trace analysis. The results show that no precipitate is found at the coherent twin boundaries (singular boundaries). Discontinuous or necklace-like precipitates are observed at the near singular (vicinal) boundaries with grain boundary inter-connections of {1 1 1}/{1 1 1} (excluding coherent twin boundaries), but continuous precipitates are developed at the random boundaries. A surface etching result indicates that the coherent twin boundaries are immune to, while the near singular boundaries are more resistant to intergranular corrosion compared to random boundaries.O-lattice theory analysis indicates that the near singular boundaries usually have specific atomic structure of periodic good matching sites isolated by the periodic dislocations. This could be the origin of discontinuous precipitates and higher resistance to intergranular corrosion in comparison with random boundaries. Based on the fact that coherent twin boundaries are hard to be introduced due to the high stacking fault energy, it is suggested that to purse a grain boundary character distribution containing high frequency near singular boundaries would be a promised approach to higher the resistance to intergranular corrosion for aluminum and its alloys in view of grain boundary engineering.