Fig. 3.

Schematic illustrations of abnormal grain growth (AGG) phenomenon. a In high-temperature cycles (HTC) (900/500 °C), the subgrain structure formed in association with α-phase precipitation at 500 °C remains after resolution of α-phase in heating. At 900 °C after HTC, some grains start AGG by consuming the surrounding subgrains, and this continues until the abnormal grain (AG) comes in contact with a neighboring AG, where the dominant driving pressure for grain boundary (GB) migration is the sub-boundary energy. AGG can repeatedly occur if HTC is repeated. b In multiple low-temperature cycles (LTCs) (740/500 °C), the sub-boundary energy increases due to the increasing misorientation between subgrains. As a result, the GB migration rate at 900 °C becomes faster and a super-large crystal can be realized