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Li et al. J Mater Inf 2024;4:27 https://dx.doi.org/10.20517/jmi.2024.44 Page 15 of 20

Discussion
The strengthening mechanisms of RAFM steels primarily include boundary strengthening (∆σ ), solid
bs
solution strengthening (∆σ ), dislocation strengthening (∆σ ), and precipitation strengthening from both
ss
ds
MX-type (∆σ ps-MX ) and M C -type precipitates (∆σ ps-M23C6 ),which are calculated by [48,54-57] :
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(5)

(6)

(7)

(8)

(9)

-1.5 [58]
where k is a constant (0.62 MN ) ; d is the average size of prior austenite grains; the hardening
pa
h-p
constant k for the main solute atoms W and Cr are 75.79 MPa at.% and 9.95 MPa at.% , respectively ;
[54]
-3/4
-3/4
i
C is the atomic fraction of element i (i.e., W or Cr); the exponent z is 3/4 for substitutional solid solution ;
[59]
i
M is the Taylor factor (3) ; G is the shear modulus (84 GPa) ; b is the Burgers vector (2.5 × 10 m) ; ρ is
[60]
[61]
[60]
-10
the dislocation density; d and d M23C6 are the average sizes of MX and M C precipitates, respectively; α is
MX
6
23
[62]
strength factor [α ≈ 0.1757 ln(2.7013d M23C6 )] ; and N M23C6 is the number density of M C precipitates.
23 6
Table 3 summarizes the microstructural features of 1# and 2# steels, together with literature data of the
conventional RAFM steels [6,20,21,27,51,63-66] . The strengthening contributions at room temperature, calculated by
Equations (5)-(9), are also included in Table 3. The comparison between 1# and 2# steels and conventional
RAFM steels indicates that:
i. Compared to 2# steel, 1# steel has a larger grain size but a higher solute atom concentration, leading to
enhanced ∆σ . In contrast, 2# steel exhibits finer grains and achieves the higher ∆σ . The conventional
bs
ss
RAFM steels show intermediate values for both aspects.
ii. Compared to other strengthening contributions, dislocations exhibit the highest strengthening effect in
1#, 2#, and conventional RAFM steels. The higher dislocation density in 2# steel (3 × 10 m ) raises its ∆σ
-2
14
ds
to 272 MPa, compared to 223 MPa in 1# and conventional RAFM steels with dislocation density of 2 × 10
14
m .
-2
iii. For 1# and 2# steels, the particle sizes of MX precipitates are 24.2 and 22.5 nm, with calculated V of
MX
0.49% and 0.42%, respectively. Consequently, 1# and 2# steels exhibit ∆σ ps-MX of 128 and 127 MPa,
respectively, which are ~70-110 MPa higher than those of conventional RAFM steels.
iv. The M C precipitates in 1# steel have a size of 48.4 nm and a number density of 2.32 × 10 m ,
20
-3
23
6
contributing to a ∆σ ps-M23C6 of 180 MPa, which is higher than ~98-154 MPa calculated in conventional RAFM
steels.
The above analysis indicates that compared to conventional RAFM steels, the superior room-temperature
strength of 1# steel primarily stems from increased ∆σ ps-MX and ∆σ ps-M23C6 . For 2# steel, its enhanced room-
temperature performance is primarily driven by the improvement of ∆σ , ∆σ , and ∆σ ps-MX . At elevated
bs
ds
temperatures, the strengthening effects from grain boundaries, dislocations, and M C decrease
23
6
significantly, while the nanoscale MX precipitates with excellent thermal stability can exert effective pinning