EXFOLIATION CORROSION RESISTANCE OF NEAR PEAK-AGED AA7449 AS A FUNCTION OF QUENCH SPEED AND AGEING TREATMENT

摘要

The influence of variations in processing parameters on the sensitivity to exfoliation corrosion (EC) of 7x49 has been addressed in a programme of controlled quenches followed by a variety of near-peak ageing treatments on 38 mm plate and two gauges of extrusion. Detailed microstructural characterisation (optical microscopy and transmission electron microscopy) has been performed on selected product-quench-ageing combinations in order to study the critical microstructural features for resistance to EC of AA7x49. For these near-peak ageing treatments the following conclusions can be drawn: the EXCO performance is better for a product that has undergone a faster quench and a lower temperature ageing treatment; there is no simple relationship between sensitivity to exfoliation corrosion and electrical conductivity; for an identical quench and ageing treatment, the EXCO ratings can be ranked in order of grain structure alignment. These observations are consistent with an underlying mechanism of EC depending on both the degree of alignment of the grain structure and the degree of solute segregation in the vicinity of the grain boundary. However the TEM observations demonstrate that there is no simple relationship between PFZ width or intergranular precipitate size and sensitivity to EC.