4.08.3.2.1 Mechanically alloyed powder characterization

The computed phase diagram of the Fe-0.13C-2W — 0.2Ti system without Y2O3 is shown in Figure 6 with respect to carbon content. For a carbon content of 0.13 wt%, a single austenite g-phase containing TiC carbide exists at a normalizing temperature of 1050 °C. The equilibrium g/g + 8-phase boundary at this temperature corresponds to a carbon content of 0.08 wt%, beyond which 8-ferrite is not stable. The specimens without and with 0.1 wt% Y2O3 exhibit the full martensite structure, whereas the specimens with 0.35 and 0.7 wt% Y2O3 exhibit a dual phase compris­ing both martensite and ferrite phases. Digital image analyses show that the area fraction of the ferrite phase is ^0.2 for specimens with 0.35 and 0.7 wt% Y2O3. High-temperature X-ray diffraction measurement at 950 °C showed a considerable difference; the specimen without Y2O3 shows diffraction peaks that correspond only to the austenite g-phase, whereas specimens with 0.35 and 0.7 wt% Y2O3 show diffraction peaks corresponding not only to an austenite g-phase but to a
ferrite phase as well. The austenite g-phase transforms to the martensite phase, but the ferrite phase remains unchanged by quenching. Considering that the ferrite phase is formed only in the specimens containing 0.35 and 0.7 wt% Y2O3, and that four types of ODS steels have an identical chemical composition except for Y2O3 content, the Y2O3 particles could suppress the a—g reverse transformation.

Figure 722 shows the results of dilatometric mea­surement when 9Cr-0.13C-2W-0.2Ti is heated without and with 0.35 wt% Y2O3. In the case of the specimen without Y2O3, the linear thermal expansion begins to decrease from an Ac1 point of850 °C to an Ac3 point of 880 °C, due to the reverse transformation of a-g-phase, which corresponds reasonably well with the computed phase diagram. The addition of 0.35 wt% Y2O3 induces an increase up to an Ac3 point of 935 °C. By comparing both curves, it was found that the specimen with 0.35 wt% Y2O3 exhibits a smaller degree of reduction in linear thermal expansion during the reverse transformation of the a-g-phase; this obser­vation indicates that the entire a-phase could not be transformed to a g-phase. This untransformed ferrite phase was designated as a residual ferrite.