Unraveling the formation mechanism of abnormally large grains in an advanced polycrystalline nickel base superalloy

Schematic showing of grain size evolution of three typical regions within an individual specimen under hot compression and heat treatment. (a) The pristine microstructure corresponding to hot extruded alloy after annealing at 1050 °C for 1 h, showing the uniform grain size throughout the specimen; (b) Inhomogeneous grain size distribution is observed inside the specimen due to the effect of strain gradients; the grain size distribution in the “end region” is akin to that of pristine microstructure, in the “quarter region”, the grains are elongated and surrounded by a low density of DRX grains, obvious DRX occurs in the “center region” and results in a uniform and fine grain size distribution. (c) The abnormally large grains take place in the “quarter region”.336