Effect of Mg treatment and cooling rate on primary carbides in GH3128 alloy

The banded aggregation of primary carbides in GH3128 alloy severely affects the thermo-plasticity and creep life of the alloy. Therefore, the effects of Mg treatment and cooling rate on primary carbides in the alloy were investigated in a resistance furnace. To differentiate the cooling rate, after alloying, the molten alloy per heat was air-cooled and furnace-cooled, respectively. The results indicate that neither Mg treatment nor increasing cooling rate can change the composition and type of primary carbides. However, as the Mg content increases from 0 to 0.0130%, the primary carbides are gradually dispersed and reduced in size and increased in number due to the refinement of dendrite structures by Mg treatment. Meanwhile, the irregular skeleton-like primary carbides are interrupted into relatively regular blocks and short bars. Compared with the furnace-cooled samples, the primary carbides in the air-cooled samples are finer in size because the refinement of dendrite structures inhibits the remote diffusion of carbide-forming elements. However, the rapid nucleation of primary carbides in the air-cooled samples results in an increase in their number. Consequently, compared with the furnace-cooled sample without Mg addition, the size of primary carbides in the air-cooled sample with 0.0130%Mg addition is reduced by about 76%, while the number of primary carbides is increased by 6.3 times.