Papers by Keyword: Oxide Dispersion Strengthened (ODS)

Abstract: The oxidation resistance of 18%Cr-oxide dispersion strengthened (ODS) ferritic steels with and without 5%Al has been investigated in air at 700900 °C for time period up to 540 h. The oxidation rate of ODS steels is significantly dependent on the oxidation time and temperature. Compared to Al-containing ODS steel, the finer grains of Al-free ODS steel are due to the formation of smaller coherent oxide particles which suppress the steel's grain growth. The grain refinement of ODS steels is expected to allow rapid segregation of Cr or Al to the steel surface, so that the continuous Fe-Cr spinel or alumina layer is formed quickly in comparison to the alloys without oxide particles dispersion. Therefore, the excellent oxidation resistance of ODS steels is owing to the formation of continuous, protective oxide layers which correlate with oxide nanoparticles and grain refinement.

Abstract: Oxidation tests of 18%Cr-oxide dispersion strengthened (ODS) steels with and without 5%Al were carried out in air at 700900 °C for time period up to 540 h. No minor alloying elements affect the oxidation behavior and the Al concentration between these ODS steels is a main difference. Cr₂O₃ and (Fe,Cr)₃O₄ spinel oxides exist on the surface of 18Cr-ODS steel; however, the surface oxide of 18Cr5Al-ODS steel is comprised of only Al₂O₃. Oxidation resistance of the ODS steels exposed at 700 °C is much better than Incoloy800 of which the Cr content is larger and their oxidation behavior doesnt follow the aluminum content. These results suggest that oxide particles dispersion and grain refinement play a more critical role than alloy composition in the high-temperature oxidation resistance.

Abstract: 18%Cr-oxide dispersion strengthened (ODS) ferritic steels with and without 5%Al have been produced by mechanical alloying and hot-extrusion. The microstructure of the ODS steels has been characterized by means of electron microscopy (SEM, TEM), showing that in the Al-added ODS steel, the semi-coherent and coherent oxide particles are about 75% and 10%, respectively. It was found that the coherency of oxide particles depends on the size of dispersed particles. Tensile tests performed between room temperature and 973 K denote that the ultimate tensile strength of Al-free ODS steel is higher than that of Al-added one. The ductility values of both materials are sufficiently high. Impact tests reveal that the ductile-to-brittle transition temperature of Al-free ODS steel are higher than that of Al-added ODS steel; however, the upper shelf energy of 18%Cr-ODS steel is substantially smaller in comparison to the Al-added one. It is considered that the difference in mechanical properties between Al-free and Al-added ODS steels is caused by the smaller, stable titania + yttria complex oxides dispersed in the Al-free ODS steel.

Abstract: Oxide dispersion strengthened (ODS) ferritic alloys of composition Fe-19Cr-0.5Y₂O₃ have been prepared by mechanical alloying at various heat-treatment conditions to produce a range of grain sizes and dispersed oxide particle size. Fine oxide particles appear to pin grain boundaries and result in inhibition of grain growth in the alloy matrix. Particle strengthening is shown to be a larger component of the material strength, rather than grain boundary strengthening or matrix strength, indicating that the finely dispersed oxide particles contribute very significantly to the total strength of 19Cr-ODS ferritic alloy.

Abstract: The oxide dispersion strengthened (ODS) ferritic steel and non-ODS reduced-activation ferritic (RAF) steel were irradiated at 773 K by means of a dual-beam ion irradiation technique to a dose of 0.4 dpa with simultaneous helium implantation up to 1000 appm. Microstructural changes were investigated by transmission electron microscopy. The RAF steel showed a preferential formation of cavities at grain boundaries, precipitate interfaces and dislocations. In contrast, the ODS ferritic steel showed a homogeneous and fine distribution of cavities in the matrix. This paper discusses the superior resistance of the ODS ferritic steel against development of cavities in terms of the effects of nano-oxide particles dispersed in the matrix.

Abstract: Pure titanium has good specific properties i.e. low density of 4.5g/cm3, extremely high resistance for corrosion and good elongation. However, its mechanical properties are not enough to be employed as structural parts and components. Accordingly, titanium alloys are often applied to industrial fields due to their high specific strength. However, the application is limited to high-performance products because of their expensive material cost and poor plastic formability at low temperature. In the present study, from a view point of cost reduction, pure titanium was used as a starting material. The materials design by oxide dispersion strengthening (ODS) was basically applied to improve the poor mechanical strength of pure titanium. TiO2 powders were used as reinforcement dispersoids because of their easily obtainable and low material cost. Powder metallurgy (P/M) method was applied to fabricate TiO2 particles reinforced pure titanium composite. Pure titanium powder and TiO2 particles were elementally mixed by conventional mixing process. Their elemental mixture powders were consolidated by using spark plasma sintering (SPS) equipment to serve a high density compact billet. Subsequently, hot extrusion process was applied to the billet to prepare a full density rod specimen. The evaluation of mechanical properties at room temperature showed high tensile strength of 1040 MPa and good elongation of 25 % when the composite included 1.5mass% TiO2 particles.

Abstract: Nb, Al and Y2O3 powders were mechanically alloyed together with 5 wt% stearic acid. The heavy plastic deformation of the powders by mechanical alloying led significant hardening to 970 Hv and the reduced grain size to 10 nm. Nb-Al base ODS alloys consolidated by HIP at 1500 °C and 150 MPa for 0.5 h gave the dual phase of Nb solid solution and Nb3Al compound. The oxide particles are of the hexagonal type YAlO3 (YAH), with the size of 50 nm to 200 nm. The high-temperature ductility at 1200 °C and capability of the grain growth at 2000 °C were confirmed.