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Materials Science in Additive Manufacturing Carbon addition in IN738LC

A B

C

Figure 5. X-ray crystallography results of the IN738LC samples mixed with different amounts of carbon (from 0 to 0.4 wt.%) fabricated with a 130 W laser
power input in the (A) as-built, (B) two-step heat treatment (HT), and (C) three-step HT conditions. The orange rectangle indicates the γ/γ’ phase, while
the blue circle and yellow represent the MC and M C carbides.
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those exceptionally large primary γ’ under the three-step IN738LC and IN738LC with 0.1 wt.% carbon additions but an
HT, they absorbed all available adjacent solutes, as shown obvious grain growth for the 0.3 wt.% carbon-added sample.
in the empty white space in Figure 4A . These areas are Prior literature suggests that the carbide within nickel-base
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unable to resist the propagation of slip dislocations during superalloys serves dual purposes during recrystallization. On
plastic deformation, hence weakening the material. the one hand, carbides act as nucleation sites for recrystallized
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Second, the carbides under the three-step HT have become grains, termed as particle-stimulated nucleation; on the other
much larger in size and are primarily confined within hand, they inhibit recrystallized grain boundary migration
the high-angle grain boundaries (Figure 4B and C ), through the Zener pinning effect. It is postulated that when
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possibly due to the lowering of grain boundary energies the carbide density increases, the grain nucleation during
through solute segregation during HT. The lack of fine recrystallization is initially promoted (Figure 6C ), but as
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intragranular carbides will then reduce the material’s the carbides along the high-angle grain boundaries reach a
hardness. The hardness results from Figure 7A corroborate critical size (measured to be 1.8 ± 0.6 μm from Figure 4C ),
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this hypothesis, as all three samples under the three-step the grain can no longer grow further (Figure 6C ).
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HT have similar hardness results, and the difference in The same phenomenon was also observed for the
their carbon content (thus carbides) did not make any 0.1 wt.% carbon-added sample under the three-step
discernible difference in the hardness property. HT procedure (Figure 6B ). The only specimen that
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While the influence of carbon content on the γ’ can transform from a columnar-shaped grain into an
characteristics might not be too obvious, at least for the 0.1 equiaxed grain is the pure IN738LC under the three-step
wt.% sample, its effect on recrystallization was much more HT (Figure 6A ). These results highlight the sensitivity
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pronounced. All as-built specimens have a similar as-built of carbon content within the IN738LC alloy during the
grain morphology comprising columnar grains (Figure 6). recrystallization process. Even with only 0.1 wt.% carbon
The standard two-step HT yielded little change for the pure addition, grain growth is severely limited.

Volume 3 Issue 1 (2024) 8 https://doi.org/10.36922/msam.2264