Page 16 - MSAM-1-4

Materials Science in Additive Manufacturing Y O influence in heat-treated LPBF IN718 composite
2 3

optimized parameters and solutionized at 1075°C so that increases the size of the carbonitride precipitates. Thus, the
the best mechanical properties could be obtained. This is increase in Orowan strengthening is small.
because at this temperature, the Laves phase is dissolved Second, the strength of the as-printed samples is
almost completely and the remaining Laves phase has a also affected by the grain size through grain boundary
particulate morphology instead of a long chain morphology. strengthening or the Hall-Petch strengthening effect.
Furthermore, the amount of δ phase precipitates in the However, due to the small difference in the grain size of
microstructure is kept relatively low as this phase also has samples AP-0 and AP-Y as shown in Table 6, the difference
several negative impacts on the mechanical properties . in the Hall-Petch strengthening effect is also negligible.
[37]
The strength and elongation values after each heat
treatment stage are shown in Figure 11B to D. In addition to the stated strengthening mechanisms,
other mechanisms such as the coefficient of thermal
It can be seen that sample AP-Y has higher elongation expansion (CTE) mismatch strengthening mechanism
than that of sample AP-0 (27.6% compared to 24.1%) or the load transfer mechanism can also strengthen the
while the strength of sample AP-Y (yield strength material. The CTE mismatch strengthening, ∆ , can be
[YS] and ultimate tensile strength [UTS] are 856.8 and calculated using Equation III . CTE
[42]
1189.3 MPa, respectively) is slightly lower than that of
sample AP-0 (YS and UTS are 879.8 and 1212.4 MPa, 12” T( ) f p
p
m
respectively). After being solutionized, a significant CTE kGb bd 1 ( f ) (III)
decrease in strength is observed in the monolithic sample p p
in which YS and UTS both decrease by 29.9% and 11.5%, Where k is a constant approximately equal to 1.25,
respectively. However, the extent of the reductions in YS ∆T is the difference between the processing and testing
and UTS in the Y O -reinforced sample is lesser, which temperatures, a and a are the coefficient of thermal
3
2
p
m
are 25.6% and 6.4%, respectively. After both solutionizing expansion of matrix and reinforcement particles, respectively.
and aging treatments are done, the strength of the Y O - The theoretical volume fraction of the nanoparticles will
2
3
reinforced sample (YS and UTS are 1099.3 and 1385.5 be used for calculation. The processing temperature for
MPa, respectively) continues to be higher than that of the LPBF can be taken to be 2100 K , thus ∆T = 2100 – 300
[43]
monolithic sample (YS and UTS are 1015.5 and 1284.3 = 1800 K. Using the theoretical vol% of the added nano-
MPa, respectively).
Y O particles, d = 65 nm , a = 13.1 × 10 K −1[44] ,
−6
By considering the contribution of different 2 3 pY O( 2 3 ) m
strengthening factors, the minor differences in the strength and ) . 81 10 6 K 1 [45] , the contribution of ∆a CTE in
pY O( 2
3
of the as-printed samples can be explained as follows. First, sample AP-Y is calculated to be 207.85 MPa.
as nanoscale carbonitride precipitates are also present in the
base material, it is reasonable to also include their Orowan Thus, based on the calculated results, CTE mismatch is
strengthening contribution. The contribution of Orowan indeed one of the important strengthening mechanisms in
strengthening,  Orowan , is calculated using Equation II . the IN718/Y O composite due to the significant increase
[38]
3
2
in the sample strength. This is also verified by Wang et al.
[42]
0 538. Gb f p d in which the calculated contribution of CTE mismatch
Orowan d p ln b 2 p (II) strengthening is the highest. However, it has been reported

that the inclusion of CTE mismatch strengthening often
Where G is the shear modulus and b is the Burgers overestimates the actual strength of the sample. As
a such, several papers agreed that the activation of CTE
[40]
vector. For IN718, G = 63 Gpa and b = 110 , where
[39]
2 mismatch strengthening is questionable and thus could
a is the lattice parameter of IN718, which a = 0.359 nm . be negligible [46–50] . Furthermore, it has also been shown
[41]
Thus, b = 0.2539 nm. that CTE mismatch strengthening can only be significant
when the reinforcement particles are above a certain size
[51]
However, the increase in strength due to Orowan and volume fraction . The critical diameter, d can be
[52]
strengthening in sample AP-Y is calculated to be only calculated using Equation IV. c
12.34 MPa higher than that of sample AP-0.As such, due
to the presence of the carbonitride precipitates in sample d b (IV)
c

AP-0, the contribution of Orowan strengthening in sample T
AP-Y is only significant if it is noticeably higher than the In this case, the d of the nano-Y O particles is
2
c
3
contribution of the carbonitride precipitates. However, it calculated to be 91.4 nm. As the mean diameter of the
can be seen that the addition of reinforcement particles Y O nanoparticles is 65 nm, the majority of them are
3
2
Volume 1 Issue 4 (2022) 10 https://doi.org/10.18063/msam.v1i4.25