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Materials Science in Additive Manufacturing Carbon addition in IN738LC

A B C
1 1 1

A B C
2 2 2

A B C
3 3 3

Figure 4. Backscattered electron imaging of specimens after the proposed three-step HT at different magnifications for (A –A ) pure IN738LC, (B –B )
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1
1
3
IN738LC with 0.1 wt.% carbon, and (C –C ) IN738LC with 0.3 wt.%.
1 3
are mainly observed within the interdendritic/cellular decreased. This lack of sufficient γ’ is expected to diminish
boundaries, which can be ascribed to the high partitioning the high-temperature performance of IN738LC with an
coefficient of carbon within the matrix gamma phase. additional 0.3 wt.% carbon. Yet, for room-temperature
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With the adoption of a conventional two-step HT, the applications, the presence of more carbides contributes
addition of 0.1 wt.% carbon did not seem to alter the γ’ to a higher hardness, as shown in Figure 7A. The similar
morphologies to a large extent (Figure 3A and B ). The microstructures after the two-step HT in Figure 3A and B
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size and distribution of γ’ are rather similar to the pure yielded similar hardness results around 490 HV. With the
IN738LC, which underwent the same two-step HT process. increase in carbide density, the hardness property of the
The density of MC carbides did increase with the addition IN738LC added with 0.3 wt.% carbon reaches about 520
of carbon, but γ’ was still the dominant precipitate within HV, yielding a 30 HV increment.
the sample (Figure 3B ).
3 The higher solution and aging HT temperatures in the
However, the same cannot be said for the alloy with three-step HT process promote the growth of primary γ’, but
0.3 wt.% carbon addition. After the standard two- reduce the size of those secondary γ’ (Figures 3 and 4). Prior
step HT, the number density of the larger γ’ was greatly literature suggests that the presence of fine γ’ (< 100 μm) is
reduced, estimated to be less than 50% of those in the beneficial toward both the room temperature and high-
pure IN738LC by visual inspection (Figure 3B ). Under temperature (up to 850°C) strengths of IN738LC, which
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a higher magnification, those smaller γ’ can no longer is also the rationale for the design of the existing three-
be detected (Figure 3C ). Instead, smaller carbides were step HT. Yet, our preliminary hardness results show the
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observed. It is shown that the MC carbide within LPBF- opposite trend (Figure 7A). In general, the samples after
built IN738LC is mainly made of titanium, which has the the three-step HT are about 50 HV to 80 HV, softer than
same atomic concentration as its carbon content. 13,28 Thus, their counterparts experiencing the two-step HT, despite
when more carbides are formed through acquiring the that the three-step HT produced much finer secondary
titanium solutes within the supersaturated gamma matrix, γ’ precipitates. There are two potential explanations for
the availability of titanium for γ’ formation is sharply the observed phenomenon. First, during the growth of

Volume 3 Issue 1 (2024) 7 https://doi.org/10.36922/msam.2264