The development of the “necklace” structure in a powder-produced nickel-base superalloy

This paper describes the nucleation and propagation of recrystallization in a powder-produced nickel-base superalloy. It is shown that recrystallization is initiated by a sub-grain coalescence mechanism, on coarse γ′ precipitates associated with pre-existing grain boundaries, leading to the formation of the so-called “necklace” structure. No intragranular nucleation of recrystallization is observed, owing to the inherent stability of the recovered matrix regions. It is also shown that the networks of MC-type carbides which often delineate the prior-particle boundaries do not radically affect the recrystallization process.