Development of oxide dispersion strengthened ferritic steels with and without aluminum

Abstract

Pure Fe, Cr, Al, Ti elemental powders and pre-alloyed Y₂O₃ powder were processed by high energy mechanical milling. The compositions of the mixed powders are designed as Fe-18Cr-0.2Ti-0.35Y₂O₃ and Fe-18Cr-5Al-0.2Ti-0.35Y₂O₃ in weight percent. The as-milled powders were consolidated by hot extrusion at 1423 K. The dispersed oxide particles were identified to be titania+ yttria for Al-free oxide dispersion strengthened (ODS) steel and alumina+ yttria for Al-added ODS steel, respectively. The ultimate tensile strength of Al-free ODS steel was higher than that of Al-added ODS steel over the temperature range of 298–973 K, because of the difference in number density and size of thermally stable oxide particles dispersed in both steel matrices. The strength in the longitudinal direction was lower than that in the transverse direction, probably due to anisotropy of the microstructure with elongated grains in the hot-extrusion direction for the 18%Cr-ODS steels with and without 5%Al.

Keywords

oxide dispersion strengthened (ODS) steel / milling / extrusion / aluminum / yttria

Introduction

Oxide dispersion strengthened (ODS) ferritic steels have been recently developed as fuel cladding for next generation nuclear systems and blanket material for fusion power systems [1-4]. It was found that the ODS steels possess superior tensile and creep strength at evaluated temperatures and enough resistance to high temperature corrosion and radiation embrittlement. In addition, nano-sized oxide particles dispersed in the steel matrix are expected to improve their strength and irradiation resistance, because the dispersed oxide particles are very stable even at a temperature close to the melting point of the steel and can be effective barriers to the motion of dislocations during tensile deformation [1,5].

Mechanical alloying (MA) is nowadays one of the most widely applied methods for the production of composites reinforced by thermally stable metal oxides [6,7]. The process is performed in a high-energy ball mill, making possible the introduction of fine metal oxide particles into a relatively soft metal matrix. The mechanically alloyed powders are then consolidated by hot plastic working. From the viewpoint of metal oxides, they are dissolved into the metal matrix during MA and such dissolved oxides are re-formed as complex metal oxides in the bulk metal during heat treatment [8]. This kind of precipitation significantly improves the tensile strength at high temperatures and the irradiation resistance.

In this study, 18% (in wt.) Cr-ODS ferritic steels with and without 5% (in wt.) Al were prepared by MA and then consolidated by hot extrusion. Alloying element Ti was added in the ODS steels in order to decrease the size of the oxide particles and increase their number density in the metal matrix. Grain morphologies of the fabricated ODS ferritic steels were observed by field emission scanning electron microscopy (FE-SEM). Microstructures and chemical composition of oxide particles dispersed in the steels were determined by transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDS). Mechanical properties were measured in tensile tests from room temperature up to 973 K.

Experimental

Pure elemental metal powders of Fe, Cr, Al, Ti and pre-alloyed Y₂O₃ powder were used as raw materials. The purity of all powders was higher than 99.9%. The particle sizes of the metal powders were below 12 μm, while the average particle size of yttria was 2 μm. The compositions of the mixed powders with and without Al were designed as Fe-18Cr-0.2Ti-0.35Y₂O₃ and Fe-18Cr-5Al-0.2Ti-0.35Y₂O₃ (in wt. %), respectively.

The fabrication process of ODS ferritic steels is represented in Fig. 1. The MA was conducted by a high-energy planetary ball mill. The mixed powders were milled for 48 h under a pure Ar atmosphere. The ball-to-powder weight ratio was 15 to 1 and the milling intensity was adjusted to a rotational speed of 250 r/min. The mechanically alloyed powders were then canned and degassed at 673 K in a 133.322 mPa (10^-3 Torr) vacuum for 2 h. The hot-extrusion was conducted at 1423 K to fabricate a cylindrical rod 25 mm in diameter and 210 mm in length. Finally a homogenization heat treatment was performed at 1323 K for 1 h. Table 1 lists the chemical compositions of 18Cr-ODS steel and 18Cr5Al-ODS steel produced by MA and hot-extrusion.

The surface morphologies of as-milled powders were observed by a scanning electron microscopy (SEM). The microstructure and chemical composition of dispersed oxide particles were examined using a TEM with EDS. The miniaturized tensile specimens, 5 mm in length, 0.25 mm in thickness, and 1.2 mm in width, respectively, were cut out from the hot extruded bar in both longitudinal and transverse directions. Tensile tests were conducted at various temperatures ranging between room temperature and 973 K, using a strain rate of 6.7 × 10^-4 s^-1 in an argon atmosphere.

Results and discussion

Figure 2 shows the surface morphologies of as-milled powders with and without Al powder. Obvious deformation is observed on their surface and there is almost no agglomeration in both as-milled powders. The mean diameter and particle-size distribution of as-milled 18Cr-ODS powder (Fig. 2(a)) are approximately 18 μm and 3-56 μm, respectively. On the other hand, the mechanically alloyed 18Cr5Al-ODS powder (Fig. 2(b)) has a mean diameter of approximately 21 μm and multimodal size distribution ranging from 5 to 62 μm. It was well known that the actual process of MA starts with mixing the raw powders in the targeted alloy compositions and loading the powder mixture into the mill along with the grinding steel balls. The powder mixture is milled for a desired period (here, 48 hours) and then the as-milled powder is consolidated into a bulk shape. Thus, important factors of the MA process are the raw powders and the process variables. Note that the manufacturing process of the ODS ferritic steels in this paper was referred from Ukai’s report for alloying design of ODS ferritic steels [9].

The grain morphologies of Al-free ODS and Al-added ODS steels with 18% Cr are depicted in Fig. 3. Their microstructures substantially consist of the bamboo-like grain structure parallel to the extruded direction. The mean grain diameters of 18Cr-ODS steel (Fig. 3(a)) and 18Cr5Al-ODS steel (Fig. 3(b)) measured in a plane perpendicular to the extrusion axes are 0.91 and 1.58 μm, respectively. On the other hand, their average grains measured in a plane parallel to the extrusion axes are 3.57 and 2.43 μm for Al-free and Al-added steels, respectively. It is considered that the oxide particles tend to become aligned along the extrusion, making a favored growth direction. Alternatively, in the absence of particle alignment, the anisotropic columnar growth can be stimulated by recrystallizing in a temperature gradient [10]. Thus, the major peculiar feature of ODS steels consolidated by hot-extrusion tends to recrystallize into the anisotropic columnar grain structure.

The distribution of the oxides dispersed in ODS steels is extremely important since their high-temperature strength is associated with the existence of oxide particles in the matrices. In Fig. 4 bright field TEM images show the distribution of the nano-sized oxide particles in 18Cr-ODS steel (Fig. 4(a)) and 18Cr5Al-ODS steel (Fig. 4(b)). These images indicate that various sizes and kinds of precipitates likely exist in both steel matrices. The quantitative chemical analyses of the oxide particles in 18Cr-ODS steels with and without 5Al are summarized in Table 2. It was found that most of the oxide particles are compounds consisting of titania+ yttria for Al-free ODS steel and alumina+ yttria for Al-added ODS steel, respectively. The atomic ratio of yttrium to titanium (Y/Ti) and yttrium to aluminum (Y/Al) indicates that these compounds have a wide constitution range. Although corresponding chemical formations for Y-Ti complex oxides, Y₂TiO₅ (Y₂O₃-TiO₂) or Y₂Ti₂O₇ (Y₂O₃-2TiO₂), in Al-free ODS steels and Y-Al complex oxides, YAlO₃ (0.5Y₂O₃-0.5Al₂O₃) or Y₃Al₅O₁₂ (1.5Y₂O₃-2.5Al₂O₃), in Al-added ODS steels can be found in Refs. [11,12], the results obtained in this paper are not always consistent with the ratio of 2 for Y₂TiO₅, 1 for Y₂Ti₂O₇ and YAlO₃ and 0.6 for Y₃Al₅O₁₂, respectively. These suggest that most of the complex oxides in these ODS steels are non-stoichiometric. The size distributions of the oxide particles in both ODS steels are presented in Fig. 5. For the oxide particles in 18Cr-ODS steel and 18Cr5Al-ODS steel, the mean diameters are 3.2 and 6.7 nm, the total number densities are 9.28 × 10²² and 1.72 × 10²² m^-3, and the inter-particle spacings are 57.9 and 93.3 nm, respectively. These results mean that the addition of Al causes a considerable increase in the size of oxide particles and markedly decrease in the number density, which lead to degradation of the strength of ODS steels.

The results of tensile tests of the 18Cr-ODS steels with and without 5Al in both longitudinal and transverse directions are displayed in Fig. 6 after testing at room temperature. The addition of 5%Al results in a decrease in the tensile strength. As shown in Fig. 4 and Table 2, TEM observations revealed that the structure and dispersion morphology of the oxide particles were different between the steels with and without Al addition. The average diameter of oxide particles in the Al-added steel was approximately twice as big as that in the Al-free steel. The number density of oxide particles was reduced by approximately 80%. The structure of oxide particles was also changed by the Al addition. The fine oxide particles in the Al-free steel were mainly Y₂TiO₅ and Y₂Ti₂O₇, while those in the Al-added steel were mostly rather larger YAlO₃ and Y₃Al₅O₁₂. Thus, the difference in tensile properties of the Al-free ODS and Al-added ODS steels is attributed to the difference in number density and size of dispersed oxide particles which contribute to their strengthening. Interestingly, the 18Cr-ODS and 18Cr5Al-ODS steels in the transverse direction exhibit an ultimate tensile strength (UTS) of approximately 1260 and 1050 MPa, respectively. In the longitudinal direction the values are approximately 5% lower, which means that the ODS steels are stronger in the transverse direction than in the longitudinal one. A similar behavior was reported by Oksiuta et al. [13] for the 14Cr2W-ODS steel. It is considered that this lower strength in the longitudinal direction is likely caused by the anisotropy of their microstructure with elongated grains in the hot-extrusion direction.

The temperature dependence of the UTS and total elongation of 18Cr-ODS and 18Cr5Al-ODS steels in the longitudinal direction is given in Fig. 7 for temperatures ranging from room temperature to 973 K. The strength of Al-free ODS steel is higher than that of Al-added ODS steel in the temperature range of 298-973 K, indicating that this is because the oxide particles dispersed in the Al-free steel are smaller and these oxide particles are stable over the whole investigated temperature range. Note that a more pronounced decrease in strength and increase in elongation with increasing temperatures are observed above 673 K. It is well known that the decrease in strength is generally attended by an increase in ductility. The occurrence of strength relaxation is more evident at intermediate temperatures, where both temperature and strength may be high enough for creep strain to cause some elastic unloading of the specimen. Romanoski [et al. 14] reported that above the temperature of 773 K, carbide precipitation begins to degrade the strength of the steel matrix. Therefore, resistance to plastic flow becomes more dependent on the dispersed oxide particles in the ODS ferritic steels, and refinement to their microstructure likely contributes to the increase in strength for the Al-free and Al-added ODS steels with 18 %Cr.

Conclusions

18 %Cr-ODS ferritic steels with and without 5%Al were prepared by MA and hot extrusion. It is confirmed that most of the oxide particles are non-stoichiometric compounds consisting of titania+ yttria for Al-free ODS steel and alumina+ yttria for Al-added ODS steels, respectively. The ultimate tensile strength of Al-free ODS steel is higher than that of Al-added ODS steel in the temperature range of 298-973 K, due to the difference in number density and size of thermally stable oxide particles dispersed in both steel matrices. For both 18Cr-ODS and 18Cr5Al-ODS steels, their strength in the longitudinal direction is lower than that in the transverse direction, because of the anisotropy of their microstructure with elongated grains in the hot-extrusion direction.

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