On the formation of serrated grain boundaries and fan type structures in an advanced polycrystalline nickel-base superalloy

The formation of serrated grain boundaries and fan-type structures in an advanced polycrystalline nickel-base superalloy has been extensively studied and linked to the thermal history. The development of grain boundary serrations has been quantitatively linked to the cooling rate from the solution heat treatment temperature. Slower cooling rates were found to result in more convoluted grain boundaries formed as a result of impingement of growing secondary γ′. Subtle differences were also noted in the γ′ distributions in the vicinities of the grain boundaries, in particular they exhibit differences in size, morphology and chemistry as a function of location within the grain, all of which are reported in the paper. Fan-type structures were found to form in the alloy, irrespective of the cooling rate employed, although their size and volume fraction increased with a decrease in cooling rate. The presence of these structures is shown to impact upon the total length of grain boundary and the degree of serration. The fan-type structures exhibit a typical dendritic morphology and were found to nucleate on minor phases present within the alloy.