Improved High Duty Alloys through Control of Grain Boundary Character

Purpose and goal

It is well established that special boundaries such as twins have different properties than random boundaries with regard to precipitation and corrosion. A methodical investigation of how process parameters influence grain size and the proportion of special (CSL) boundaries in the nickel base alloy X750 was carried out. Two different concepts have been tested. These were either repeated cold rolling followed by recrystallisation annealing or commencing with a large grain size from a high temperature anneal with subsequent moderate rolling and then recrystallisation annealing.

Results and expected effects

The results show that it is possible to achieve an advantageous microstructure containing an enhanced proportion of special grain boundaries. With a small deformation of around 4% and subsequent recrystallisation annealing, the proportion of special CSL boundaries increased from about 45% in the initial material to over 70%.

Approach and implementation

The material´s ability to resist precipitation of Cr-rich second phases was analysed by a sensitisation heat treatment. The investigation showed that no particles were present in the 3 twins. In 5 and 9 boundaries some ambivalence was found with indications of precipitation, but that precipitation took place in all other high angle grain boundaries. Evaluation of grain boundary attack in corrosion tests using a combination of SEM-EBSD and confocal microscopy showed that the same boundaries that had resisted precipitation were also not susceptible to attack during corrosion.