PSI - Issue 69

Haofei Zhu et al. / Procedia Structural Integrity 69 (2025) 113–120

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Fig. 4. The SEM images of the aged samples after different solid-solution temperatures treatment:(a) 870 °C; (b) 900 °C; (c) 930 °C; (d) 960 °C.

To identify these spherical phases, TEM observations were performed. Fig. 5 shows the TEM images of the sample treated at 870 °C solid-solution and aging. Through EDS analysis and selected area diffraction (SAED) analysis, it was determined that the spherical particles are M 7 C 3 carbides with a hexagonal structure. These carbides have also been reported in AerMet100 steel [1]. According to the calculated phase diagrams ( Errore. L'origine riferimento non è stata trovata. ), the dissolution temperature of M 7 C 3 carbides is approximately 862 °C. When solid-solution temperature below 900 °C, the short holding time does not allow the system to reach equilibrium, and the M 7 C 3 carbides do not fully dissolve (Fig. 2a, b). The presence of M 7 C 3 carbides at the grain boundaries effectively suppresses the coarsening of PAG during solid-solution treatment. Above 900 °C, the M 7 C 3 carbides completely dissolve, and as a result, the growth rate of the PAG becomes faster in samples treated above 900 °C compared to those treated below 900 °C (Fig. 2e).

Fig. 5. The TEM images of the sample treated at 870 °C solid-solution and aging:(a) bright-field image of spherical carbide; (b) SAED of the spherical carbide. Fig. 6 illustrates the tensile and impact properties of the samples after solid-solution treatment (quenched) at various temperatures and subsequent aging (aged). In the quenched condition, the ultimate tensile strength (UTS) increases