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Page 16 of 20 Li et al. J Mater Inf 2024;4:27 https://dx.doi.org/10.20517/jmi.2024.44

Table 3. Comparison of microstructural features and calculated strengthening contributions at room temperature for 1#, 2#, and
conventional RAFM steels
Microstructural parameters 1# 2# Conventional RAFM [6,20,21,27,51,63-66]
Prior austenite grains Size (m) 18.8 9.7 ~10-60
∆ σ (MPa) 143 199 ~80-196
bs
Solute atoms W, Cr (at.%) 0.393, 9.234 0.078, 8.954 ~0.3-0.6, ~8.5-9.8
∆ σ (MPa) 90 63 ~80-107
ss
-2 14 14 14
Dislocations Density (m ) 2 × 10 3 × 10 ~2 × 10
∆ σ (MPa) 223 272 ~223
ds
MX Size (nm) 24.2 22.5 ~15-30
-3 20 20 19 20
Density (m ) 6.57 × 10 7.04 × 10 ~10 -10
V MX (%) 0.49 0.42 ~0.002-0.15
∆ σ ps-MX (MPa) 128 127 ~12-55
M C Size (nm) 48.4 - ~70-200
23 6
19
-3
Density (m ) 2.32 × 10 20 - 10 -10 20
∆ σ (MPa) 180 ~98-154
ps-M23C6
RAFM: Reduced activation ferritic-martensitic.

effects. Therefore, the excellent high-temperature tensile properties of 1# and 2# steels are mainly related to
the improved strengthening effect of MX precipitations. Moreover, according to previous studies [21,67-69] , the
1# and 2# steels with high V can exhibit excellent creep resistance. To further evaluate their suitability for
MX
fusion environments, long-term creep tests under various stress levels, temperature gradients, and
environmental conditions are recommended. A more comprehensive creep constitutive model will also be
developed to better predict material behavior and support the life assessment of fusion reactor structural
materials.

CONCLUSIONS
In this study, a new design strategy was presented to develop RAFM steels with targeted microstructures
and tensile properties using ML and CALPHAD methods. This strategy centers on utilizing microstructural
and forward models to screen compositions and processing parameters suggested by the inverse model,
meeting specific structure and performance criteria.

The key microstructural model, consisting of four sub-models, was developed to predict microstructural
attributes based on CALPHAD data. It achieved an accuracy of >85% in predicting the presence of δ-ferrite
and coarsening phases (i.e., Laves and Z-phase), and R of > 0.9 for predicting V and V M23C6 . Validated by
2
MX
CALPHAD and experiments, this microstructural model demonstrates strong reliability in guiding the
microstructural optimization of RAFM steels.

An integrated design model was developed by combining the microstructural model with forward and
reverse models, optimizing the compositions and processing for desired microstructures and tensile
properties. Using this model, two novel RAFM steels with high V and excellent tensile properties were
MX
designed and experimentally analyzed. In both steels, no δ-ferrite, Laves, and Z-phase were observed, and
the V (0.49%, 0.42%) and V M23C6 (1.38%, 0%) met the structure requirements. Tensile testing at 600 °C
MX
revealed a ~100 MPa increase in UTS compared to conventional RAFM steels. These experimental results
are almost consistent with the targeted microstructures and tensile properties, confirming the effectiveness
of our design strategy.