The addition of Al to ODS ferritic steels sometimes softens their creep and tensile properties. Figure 2544 shows the effects of Al addition and Cr content on the

tensile strength of high Cr-ODS steels. A decrease in UTS versus Al content is obvious in 16Cr-ODS steels; this dependency becomes weaker at higher tempera­ture. The effect ofCr concentration on the UTS is not so obvious between 13.7 and 17.3 wt% at 450 and 700 °C.

In the case of 9Cr-12Cr-ODS steels, high — temperature strength is considerably enhanced by the uniform dispersion of Y-Ti complex oxide (Y2Ti2O7) particles. In Al-added high Cr-ODS steels, however, Y-Al complex oxides and/or Al oxides are formed rather than Y-Ti complex oxides, which leads to larger oxide particles, causing a degradation of high — temperature mechanical strength. Therefore, Hf or Zr, which form thermodynamically stable oxides, were added to form Y-Hf or Y-Zr complex oxide particles rather than Y-Al complex oxide. The process of manufacturing these materials is exactly the same as that used for 9Cr-ODS ferritic steels: MA by an attri­tion type ball mill and hot extrusion at a nominal temperature of 1150 °C. The extruded bars were provided for mechanical tests. The creep rupture prop­erties of Al-added high Cr-ODS steels are summar­ized in Figure 26.44 The creep strength of the standard steel is generally lower than that of Al-free steel. On the other hand, the addition of Zr and Hf induces an improved creep strength which approaches that of Al-free steel. Furthermore, it was found that their

fracture elongation and reduction of area are slightly higher than those of Al-free steel. Under microstruc­tural observation by TEM, oxide particles consisting of Y3A5O12, YAlO3, and Al2O3, were observed in typical Al-added ODS steel, whereas this Y-Al complex oxide can be changed to Y2Hf2O7 in Hf-added ODS steel. The improved creep rupture strength in Hf-added ODS steel could be attributed to the nanosize disper­sion of the Y2Hf2O7 complex oxide.