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Materials Science in Additive Manufacturing Y O influence in heat-treated LPBF IN718 composite
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below the critical diameter. In addition, due to the small Figure 11. A thorough quantitative analysis of the Orowan
volume fraction of the added nanoparticles, the fraction strengthening is challenging in heat-treated samples
of the nanoparticles that can contribute to strengthening since there are numerous types of nano-sized precipitates
is also small. (carbonitride precipitates, Y O nanoparticles, Y-Ti-O
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nanoparticles, γ′ and γ′′ precipitates, and δ and blocky
Furthermore, the previously calculated results of the
strengthening contribution are based on the CTE mismatch Laves phase) that each contributes to the strength of the
between the Y O nanoparticles and the IN718 matrix. material differently. Furthermore, the morphology of
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However, it has been pointed out that the nanoparticles the majority of the precipitates is not spherical and their
have a high tendency to combine with the carbonitride distribution in the matrix may not be homogeneous.
shell to form a complex precipitate. As such, it would be Thus, these factors make quantifying the contribution
more appropriate to compare the CTE mismatch between of the precipitates to the strength of the material highly
challenging. However, an appropriate qualitative analysis
the Y O nanoparticles and the carbonitride shell. Since to explain the differences in the material’s strength can
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the CTE of the shell is in the range of 6.65 × 10 K to
−1
−6
8.48 × 10 K −1[53] , which is much closer to that of the Y O still be carried out.
−6
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nanoparticles than IN718 is, the actual increase in strength First, the number of nano-sized precipitates in the
is again expected to be small. Due to these reasons, the Y O -reinforced sample is expected to be higher than in
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contribution of CTE mismatch strengthening in the the monolithic sample due to the formation of the Y-Ti-O
IN718/Y O composite will be omitted. precipitates after solutionizing treatment (Figure 10A).
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Finally, the load transfer mechanism, ∆ can be As such, these precipitates increase the strength of the
calculated using Equation V [42,54] . load IN718/Y O composite after heat treatment. Furthermore,
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these precipitates also contribute to grain refinement after
load 05. f p m (V) heat treatment since they act as nucleation centers for the
recrystallization of grains at high temperatures and at the same
Where  is the yield strength of the matrix. It can be time, suppress grain growth. Hence, this further enhances the
m
seen from Equation V that the contribution of the load material’s strength through grain boundary strengthening.
transfer mechanism is expected to be small as the volume Second, it is theorized that the difference in the Laves
fraction, f , of the reinforcement particles is very small. phase morphology at the grain boundaries between the
p
Coupling with the 0.5 coefficient, the load transferring
mechanism is believed to be not at play, which is also Y O -reinforced samples and the monolithic samples as
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shown in Figure 8 also contributes to the difference in
verified by Wang et al. [42]
the mechanical properties. However, after solutionizing
Thus, it can be seen that the majority of the strengthening treatment at 1075°C,the microstructural changes resulting
mechanisms do not result in a significant improvement in from additional aging treatment are not obvious. Thus, it
the strength of sample AP-Y. In fact, the strength of sample is challenging to accurately characterize the differences.
AP-Y is slightly lower than that of sample AP-0. This is As such, to accurately characterize the differences in
because the theoretical calculation does not consider the the microstructures that arise from aging treatment
agglomeration of the Y O nanoparticles. This further and to understand the reasons behind any changes in
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supports the observations made in Section 3.1 in which mechanical properties after aging treatment, both samples
there is a high concentration of Y O in the interdendritic are solutionized at 1275°C before aging. This is because
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regions. As a result, this overestimated the strengthening when the samples are solutionized at this temperature,
contribution of the Orowan mechanism. Furthermore, the diffusion of different alloying elements is significantly
several other factors could potentially overestimate the enhanced . As such, the movements of the alloying
[55]
Orowan strengthening mechanism such as not all the elements during heat treatment can be visualized.
particles being spherical and the mean particle diameter SEM analysis of sample 1275A-Y shows that the addition
values being inaccurate to represent the entire size of Y O has resulted in grain refinement. This is evident in
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distribution. Thus, due to the similar grain sizes, the Figure 12 showing a significantly higher number of smaller
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small volume fraction of reinforcement particles, and equiaxed grains in sample 1275A-Y compared to sample
the presence of carbonitride precipitates in sample AP-0, 1275A-O. It is also observed that the segregated regions
the strengths of the as-printed samples only differ slightly. (red color) are thinner in sample 1275A-Y than those in
However, in this study, the value added of the sample 1275A-0. This further reinforced the phenomenon
reinforcement particles in strengthening IN718 in which the addition of Y O reduces the segregation at
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can only be realized after heat treatment as seen in the grain boundaries as shown in Figure 8.
Volume 1 Issue 4 (2022) 11 https://doi.org/10.18063/msam.v1i4.25