In this study, we investigate the effect of the duration of mechanical alloying on the microstructures and mechanical properties of ODS ferritic/martensitic steel. The Fe(bal.)-10Cr-1Mo pre-alloyed powder and Y2O3 powder are mechanically alloyed for the different mechanical alloying duration (0 to 40 h) and then constantly fabricated using a uniaxial hot pressing process. Upon increasing the mechanical alloying time, the average powder diameter and crystallite size increased dramatically. In the initial stages within 5 h of mechanical alloying, inhomogeneous grain morphology is observed along with coarsened carbide and oxide distributions; thus, precipitate phases are temporarily observed between the two powders because of insufficient collision energy to get fragmented. After 40 h of the MA process, however, fine martensitic grains and uniformly distributed oxide particles are observed. This led to a favorable tensile strength and elongation at room temperature and 650oC.

In this study, we investigated the effects of precipitates and oxide dispersoids on the high-temperature mechanical properties of oxide dispersion-strengthened (ODS) Ni-based super alloys. Two ODS Ni-based super alloy rods with different chemical compositions were fabricated by high-energy milling and hot extrusion process at 1150℃ to investigate the effects of precipitates on high-temperature mechanical properties. Further, the MA6000N alloy is an improvement over the commercial MA6000 alloy, and the KS6000 alloy has the same chemical composition as the MA6000 alloy. The phase and microstructure of Ni-based super alloys were investigated by X-ray diffraction and scanning electron microscopy. It was found that MC carbide precipitates and oxide dispersoids in the ODS Ni-based super alloys developed in this study may effectively improve high-temperature hardness and creep resistance.

This study investigates the oxidation properties of Fe-14Cr ferritic oxide-dispersion-strengthened (ODS) steel at various high temperatures (900, 1000, and 1100°C for 24 h). The initial microstructure shows that no clear structural change occurs even under high-temperature heat treatment, and the average measured grain size is 0.4 and 1.1 μm for the as-fabricated and heat-treated specimens, respectively. Y–Ti–O nanoclusters 10–50 nm in size are observed. High-temperature oxidation results show that the weight increases by 0.27 and 0.29 mg/cm2 for the asfabricated and heat-treated (900°C) specimens, and by 0.47 and 0.50 mg/cm2 for the as-fabricated and heat-treated (1000°C) specimens, respectively. Further, after 24 h oxidation tests, the weight increases by 56.50 and 100.60 mg/cm2 for the as-fabricated and heat-treated (1100°C) specimens, respectively; the latter increase is approximately 100 times higher than that at 1000°C. Observation of the surface after the oxidation test shows that Cr2O3 is the main oxide on a specimen tested at 1000°C, whereas Fe2O3 and Fe3O4 phases also form on a specimen tested at 1100°C, where the weight increases rapidly. The high-temperature oxidation behavior of Fe-14Cr ODS steel is confirmed to be dominated by changes in the Cr2O3 layer and generation of Fe-based oxides through evaporation.

15Cr-1Mo base oxide dispersion strengthened (ODS) steel which is considered to be as a promising candidate for high- temperature components in nuclear fusion and fission systems because of its excellent high temperature strength, corrosion and radiation resistance was fabricated by using mechanical alloying, hot isostatic pressing and hot rolling. Torsion tests were performed at room temperature, leading to two different shear strain routes in the forward and reverse directions. In this study, microstructure evolution of the ODS steel during simple shearing was investigated. Fine grained microstructure and a cell structure of dislocation with low angle boundaries were characterized with shear strain in the shear deformed region by electron backscattered diffraction (EBSD). Grain refinement with shear strain resulted in an increase in hardness. After the forward-reverse torsion, the hardness value was measured to be higher than that of the forward torsion only with an identical shear strain amount, suggesting that new dislocation cell structures inside the grain were generated, thus resulting in a larger strengthening of the steel.

Fe-based oxide dispersion strengthened (ODS) powders were produced by high energy ball milling, fol- lowed by spark plasma sintering (SPS) for consolidation. The mixed powders of 84Fe-14Cr-2Y2O3 (wt%) were mechanically milled for 10 and 90 mins, and then consolidated at different temperatures (900~1100o C). Mechani- cally-Alloyed (MAed) particles were examined by means of cross-sectional images using scanning electron micros- copy (SEM). Both mechanical alloying and sintering behavior was investigated by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). To confirm the thermal behavior of Y2O3, a replica method was applied after the SPS process. From the SEM observation, MAed powders milled for 10 min showed a lamella structure consisting of rich regions of Fe and Cr, while both regions were fully alloyed after 90 min. The results of sintering behavior clearly indicate that as the SPS temperature increased, micro-sized defects decreased and the den- sity of consolidated ODS alloys increased. TEM images revealed that precipitates smaller than 50 nm consisted of YCrO3.

Alloy 617, Ni-22Cr-12Co-9Mo base oxide dispersion strengthened alloy was fabricated by using mechanical alloying, hot isostatic pressing and hot rolling. Uniaxial tensile tests were performed at room temperature and at . Compared with the conventional Alloy 617, ODS alloy showed much higher yield strength and tensile strength, but lower elongation. Fracture surfaces of the tensile tested specimens were investigated in order to find out the mechanism of fracture mode at each test temperature. Grain adjustment during tensile deformation was analyzed by electron backscattered diffraction mapping, inverse pole figures and TEM observation.

Yttrium oxide is one of the most thermo-dynamically stable materials, so that it is generally used as a dispersoid in many kinds of dispersion strengthed alloys. In this study, a nickel-base superalloy is strengthened by dispersion of yttrium oxide particles. Elemental powders with the composition of Ni-22Cr-18Fe-9Mo were mechanically alloyed(M.A.) with 0.6 wt% . The MA powders were then HIP(hot isotactic press)ed and hot rolled. Most oxide particles in Ni-22Cr-18Fe-9Mo base ODS alloy were found to be Y-Ti-O type. The oxide particles were uniformly dispersed in the matrix and also on the grain boundaries. Tensile test results show that the yield strength and ultimate tensile strength of ODS alloy specimens were 1.2~1.7 times higher than those of the conventional X(R), which has the same chemical compositions with ODS alloy specimens except the oxide particles.

Two kinds of oxide-dispersion-strengthened (ODS) 316L stainless steel were manufactured using a wet mixing process(wet) and a mechanical alloying method (MA). An MA 316L ODS was prepared by a mixing of metal powder and a mechanical alloying process. A wet 316L ODS was manufactured by a wet mixing with 316L stainless steel powder. A solution of yttrium nitrate was dried after being in the wet 316L ODS alloy. The results showed that carbon and oxygen were effectively reduced during the degassing process before the hydroisostatic process (HIP) in both alloys. It appeared that the effect of HIP treatment on increase in impact energy was pronounced in the MA 316L ODS alloy. The MA 316L ODS alloy showed a higher yield strength and a smaller elongation, when compared to the wet 316L ODS alloy. This seemed to be attributed to the enhancement of bonding between oxide and matrix particles from HIP and to the presence of a finer oxide of about 20 nm from the MA process in the MA 316L ODS alloy.

Austenitic oxide-dispersion-strengthened (ODS) stainless steel was fabricated using a wet mixing process without a mechanical milling in order to reduce contaminations of impurities during their fabrication process. Solution of yttrium nitrate was dried after a wet mixing with 316L stainless steel powder. Carbon and oxygen contents were effectively reduced by this wet processing. Microstructural analysis showed that coarse yttrium silicates of about 150 nm were formed in austenitic ODS steels with a silicon content of about 0.8 wt%. Wet-processed austenitic ODS steel without silicon showed higher yield strength by the presence of finer oxide of about 20 nm.

1. The oxidation resistance of containing Ni-base alloy is superior to that of the alloy without . 2. The appearance of oxides of Ni-20Cr-20Fe-5Nb- alloy is similar to that of oxides in commercial PM1000 and MA754 alloy. 3. The oxides in ODS alloy are grown mainly at particle boundaries.

NiAl based ODS (Oxide Dispersion Strengthened) intermetallic alloys have been produced by mechanical alloying (MA) process and consolidated by hot extrusion. Subsequent thermomechanical treatments have been applied to induce secondary recrystallization in an attempt to improve creep resistance in this material. The creep behavior of secondary recrystallized MA NiAl has been investigated and compared with those of as-extruded condition. Minimum creep rate were shown to be approximately two orders of magnitude lower than that in as-extruded condition. The improvement in creep resistance is believed due to the grain coarsening, restricting of dispersoid coarsening as well as increase in grain aspect ratio. Creep threshold stress behavior, below which no measurable creep rate can be detected, has been discussed on the basis of particle-dislocation interaction theory. The threshold stress becomes negligible after secondary recrystallization in MA NiAl, presumably due to dispersoid coarsening and a decrease in grain boundary area during secondary recrystallization.

NiAI기 산화물 분산강화(Oxide Dispersion Strengthende:ODS)합금을 기계적 합금화 (Mechanical Alloying: MA)방법으로 제조하였으며, 열간압축방법으로 성형하였다. 연이어 단순항온처리에 의한 정상결정립성장과 특성조건에서의 thermomechanical treatment 에 의한 이차재결정화를 유도하였다. 결정립 조대화된 ODSD MA NiAI의 creep 성질 및 이에 조대화된 미세조직은 creep 성질이 저하된 반면, 이차재결정화된 MA NiAI의 creep성질은 크게 향상되었다. 이 creep 성질의 향상은 이차재결정화의 특성인 급격한 결정립의 조대화, 분산상의 성장억제 및 grain aspect ratio의 증가에 기인한 것으로 사료되었다. 이차재결정화된 ODS MA NiAI의 creep또는 glide controlled dislocation creep임을 제시하지만, 전체 creep속도가 결정립 크기 및 grain aspect ratio의 영향을 크게 받은 것을 볼 때, 결정립계 미끄럼기구가 주 creep 기구와 조합되어 MA NiAI의 전체 creep기구에 영향을 준 것으로 추정할 수 있었다.

NiAl powders containing oxide dispersoids have been produced by mechanical alloying process in a controlled atmosphere using high energy attrition mill. The powders have been consolidated by hot extrusion and hot pressing followed by isothermal annealing to induce microstructure coarsening to improve high temperature properties. Grain growth and dispersoid coarsening kinetics have been investigated as functions of annealing time and temperature. Coarsening of dispersion strengthen NiAl and dispersoid has been discussed. Some clues of secondary recrystallization have been investigated. Mechanical property measurements have been also made and correlated with the microstructures.