Microstructure and Mechanical Property of 12Cr Oxide Dispersion Strengthened Steel

Microstructure observation

The relative density (the ratio of real density and theoretical density) of the 12Cr ODS steel produced by MA and hot isostatic pressing was greater than 98% by means of the Archimedes method. The microstructure of the 12Cr-ODS steel HIPed and heat treated was observed at JEOL 2100F and FEI TECNCI G²⁰. Figure 1 shows the TEM and EBSD images of the HIPed and heat-treated samples. It can be seen in Figure 1(a) and (b) that the grain size of the fabricated steels was very fine; the big grain diameter was several microns and the small grain diameter was hundreds of nanometers. Figure 1(c) and (d) gives the EBSD image of the 12Cr-ODS steel in HIPed and heat-treated states. No evident anisotropy was observed in the microstructure and the average grain sizes were around 701 nm and 512 nm, respectively, for the HIPed and heat-treated samples. Based on Hall–Petch relation, the smaller grain size means higher yield strength and guarantee the excellent high-temperature creep strength. The EBSD result proved that the HIP process could result in a uniform microstructure with small grain size. It was evident that the grain size of 12Cr-ODS steel was refined by heat treatment for getting close to equilibrium constitution due to the dynamic recrystallization. [16].

Figure 1:

(a), (b) TEM images of HIPed and heat-treated ODS steels, (c), (d). EBSD images of HIPed and heat-treated ODS steels.

Figure 2(a) and (b) shows that a high number density precipitates with size less than 30 nm could be observed in the TEM bright-field under-focused images. It shows that some oxide particles are sphere with 2–5 nm diameter. Based on results of M.K. Miller this oxide particles are non-stoichiometric complex Y-Ti-O nanoclusters which were coherent with the matrix [17]. The other precipitates with the Y/Ti ratio 1~2 are stoichiometric Y₂Ti₂O₇, which was semi-coherent or non-coherent with the matrix. This was confirmed by HRTEM analysis. A precipitate was indexed as Y₂Ti₂O₇ with a (0 1–1) zone axis, as shown in Figure 2(c). Two atom planes are (4 0 0) and (2 2 2) with an angle of 54°. No Y₂TiO₅ precipitates are found in this 12Cr-ODS steel. In general, Y₂Ti₂O₇ precipitates are larger than Y–Ti–O clusters. The formation of Y–Ti–O nanoclusters and precipitates depends on composition and processing parameters, but the comprehensive mechanism is not clear yet. Figure 2(b) reveals that these nanocomplex oxide particles were very stable and that the number of precipitates increased with the heat treatment. Figure 2(d) shows the size distribution of nanosized precipitates for the 12Cr-ODS sample in different states. This is the reason why 12 Cr-ODS steel has good mechanical properties at high temperature. High number density nanosized particles exerted a pinning force on grain boundaries to retard boundary migration. These stable fine precipitates were also expected to benefit the irradiation resistance [18].

Figure 2:

(a), (b) TEM images of precipitates of HIPed and heat-treated ODS steels, (c) HRTEM of the pyrochlore structure Y₂Ti₂O₇, (d) Precipitate size distribution in ODS steels with different states.

Tensile strength test

The stress–strain curves of 12Cr-ODS steel and normalizing and tempering samples at room temperature(RT), 400, 600 and 700°C were shown in Figure 3. The tensile properties of the tested specimens are summarized in Table 1. It is clear that 12Cr-ODS steel has excellent ultimate tensile strength of about 1,087 MPa at RT. The ultimate tensile strength value maintained at 336 MPa at 700°C. With the increasing temperature, the ultimate tensile strength of 12Cr-ODS steel decreased. The total elongation increased with the increasing temperature and reached 18.7% at 600°C. The very high tensile strengths at RT and high temperature make the low activation 12Cr-ODS steel promising for fusion reactor application. However, the fabricated steel needs higher strength and better toughness. Efforts, such as deformation works combined with heat treatment, are necessary for improving the ductility of the fabricated materials. It should be noted that a very high ultimate tensile strength (about 1,183 MPa) is achieved at RT and the total elongation reached 20.8% at 600°C by heat treatment. It was clear that the ultimate strength and the total elongation increase is due to the high density of nanoprecipitates with size less than 30 nm uniformly distributed in the microstructure; another reason is the small grain size. A similar feature observed in the HIPed and heat-treated samples is that the total elongation measured at 600°C has the highest value. At this temperature the plastic deformation is subjected to a viscoplastic response at a constant stress at an imposing constant strain rate. Otherwise in high temperature during deformation the damage of samples develops quickly and failure occurs with minimum tertiary creep above this temperature due to the dynamic recovery mechanism, commonly observed in α-Fe steels due to the annihilation of dislocations which leads to the steady-state substructure [19].

Figure 3:

(a), (b) Stress–strain curve of HIPed and heat-treated ODS steels at different temperatures.

Hardness test

Hardness test was carried out on a sample with dimension of 3 × 5 × 10 mm³. The hardness value of HIPed sample is 339±18 HV. The hardness value of heat-treated sample is 369±8 HV. It is clear that the hardness values increased by heat treatment. The hardness value increased, which also contributed to the number of precipitates that increased and the average grain size that decreased.