Preparation, Microstructure And Mechanical Properties Of Nano-structured ODS Steels

Nano-structured oxide dispersion strengthened (ODS) steels exhibit excellent high-temperature creep strength and radiation resistance due to their extremely high-density nanosized precipitates and sub-micrometer grains, thus nano-structured ODS steels have been considered to be the most promising candidates of fuel cladding materials for the next generation (Generation Ⅳ) fission reactor. The conventional manufacture process route of nano-structured ODS steels needs long time mechanical alloying (MA) to realize complete dissolving of all alloy elements into Fe matrix, resulting in low production efficiency and high production cost. In addition, the pollution from atmosphere and MA media is inevitable during the MA process. These pollution will reduce the performance of nano-structured ODS steels. Therefore, a new manufacture process route of nano-structured ODS steels was announced in this thesis, in order to increase the production ability and improve the properties of nano-structured ODS steels. This new manufacture process route includes the production of supersaturated solid solution alloy powers by metal atomization, short time milling and hot isostatic pressing (HIP), and it has obtained an invention patent. The microstructural evolution and performance optimization of nano-structured ODS steels produced by new method were studied deeply in this thesis:The morphologies, phases and particle sizes of atomized alloy powders and milling alloy powders with different milling time were observed; The microstructure and mechanical properties of nano-structured 9Cr-ODS produced by atomized alloy powders were researched; the influences of milling time and HIP temperature on the microstructure and mechanical properties were also characterized; Small angle X-ray scattering (SAXS) was applied to study the size distribution and number density of nanosized precipitates distributed in nano-structured ODS steels, and illustrated the effect of Y/Ti atomic ratio, HIP temperature, and milling time on the size distribution and number density of nanosized precipitates in nano-structured ODS steels. The conclusions have been obtained as follows:(1) The shape of supersaturated solid solution alloy powders produced by gas atomization technology is spherical, and the mean particles size and grain size are 47μm and 2μm, respectively. Alloy elements have been dissolved into Fe matrix after the atomization process.(2) The mean particle size of atomized alloy powders rises firstly with increasing milling time, and the biggest value of mean particle size is 111μm appears at 2 h, after that the mean particle size decreases with milling time increases until 30 h. The mean grain size of atomized alloy powders decreases with increasing milling time, and it reaches to 10.8 nm after 20 h. After that, the mean grain size does not change obviously. The variation tendency of lattice strain and hardness with milling time is completely opposite to grain size. Comparing to the milling process of purity metal elements powders with Y2O3 powders, the milling time is obviously shortened by using atomized alloy powders, and the production efficiency is also improved.(3) The density of nano-structured 9Cr-ODS steels HIPed by short time (20 h) milling atomized alloy powders is close to theoretical density of ferrite. The microstructure consists equiaxial ferrites and a small amount of martensites. The grain size changes from hundreds nanometer to several micrometer, and the mean grain size is 0.7μm. The extremely high-density Y-Ti-0 enriched nano-clusters distribute uniformly in matrix, and the distirbution density of nano-clusters is 3.5×1022/m3. Besides nano-clusters, a small amount of Y2Ti207 (number density is 1.57×1020/m3) with size of 10-30 nm and Al2O3, CrTi2O5 with size of about 100 nm have also been found in the nano-structured 9Cr-ODS steels. The ultimate tensile strength of nano-structured 9Cr-ODS steels produced by atomized alloy powders at room temperature,550℃ and 650℃ is 1318 MPa,960 MPa and 251 MPa, respectively. The experimental results manifest the feasibility of manufacturing nano-structured ODS steels by using atomized alloy powders.(4) The electron backscattered diffraction (EBSD) and high annular dark field (HAADF) imaging results illustrate that the grain size of nano-structured 9Cr-ODS steels produced by atomized alloy powders decreases from 3.49μm to 0.47μm, meanwhile the number density of Y, Ti, O elements enriched nanosized precipitates increases from 6.17×1020/m3 to 1.2×1022/m3, when milling time increases from 0 h to 20 h. The SAXS results illustrate that the number density of Y-Ti-O nano-clusters in nano-structured 9Cr-ODS steels produced by milling 20 h sample reaches to 6.59×1023/m3, and the mean size of Y-Ti-O nano-clusters is 2.7 nm, but the number densities of Y-Ti-O nano-clusters in nano-structured ODS steels produced by milling 0 h,8 h samples are very low. The number density of in nano-structured 9Cr-ODS steels produced by MA 8 h sample is higher than that in nano-structured 9Cr-ODS steels produced by MA 0 h and MA 20 h samples, and the number density and mean size are 1.03×1022/m3 and 7.5 nm, respectively. Prolonging milling time is beneficial to improve the ultimate tensile strength of nano-structured 9Cr-ODS steels. The nano-structured 9Cr-ODS steels produced by 20 h milling atomized alloy powders exhibite higher ultimate tensile strength than other two samples at all test temperatures. While the nano-structured 9Cr-ODS steels produced by 0 h MA atomized alloy powders exhibite lowest ultimate tensile strength.(5) The experimental results illustrate that the HIP temperature has obvious effect on microstructure, distribution characteristics of Y, Ti,O elements enriched nanosized precipitates and tensile properties of nano-structured ODS steels. The grains of nano-structured 9Cr-ODS steels produced by atomized alloy powders become coarser as HIP temperature raise, meanwhile the number density of Y, Ti, O elements enriched nanosized precipitates decreases. The number density of Y, Ti, O elements enriched nano-sized precipitates decreases from 3.94×1022/m3 to 8.66×1021/m3, and the mean diameter increases from 4.5 nm to 7.8 nm, if HIP temperature raises from 900℃ to 1200℃. Reducing HIP temperature is beneficial to improve the ultimate tensile strength of nano-structured 9Cr-ODS steels. The ultimate tensile strength of nano-structured 9Cr-ODS steels HIPed at 900℃ is higher than that of the samples HIPed at 1100℃ and 1200℃ in the test temperature range of 25℃ to 600℃, the ultimate strength at room temperature is 1509 MPa; For the nano-structured 14Cr-ODS steels produced by conventional manufacture process, the number density of Y-Ti-O nano-clusters in nano-structured 14Cr-ODS steels HIPed at 1100℃ is highest among three samples, and reaches to 5.60×1024/m3. While the number density of Y2Ti2O7 in nano-structured 14Cr-ODS steels increases with HIP temperature raises, and reaches to 8.13×1021/m3 in nano-structured 14Cr-ODS steels HIPed at 1200℃, which is higher than other two samples.(6) For the nano-structured 9Cr-ODS steels produced by MA accompany with HIP, Y/Ti atomic ratio has obvious effect on the distribution characteristics of Y, Ti, O elements enriched nanosized precipitates. When Y/Ti atomic ratio is 0.4, Y, Ti, O tend to form Y-Ti-0 nano-clusters, the number density of Y-Ti-O nano-clusters is 5.39×1024/m3, and the mean size is 1.4 nm; When Y/Ti atomic ratio is 1, the number density of Y-Ti-O nano-clusters decreases, while the number density of Y2Ti2O7 increases to 1.46×1022/m3, and the mean size of Y2Ti2O7 is 7.9 nm. The SAXS experiment results also manifest the refining effect of Ti element on nanosized precipitates. The mean size of nano-clusters in 0.3% Ti and 0% Ti samples are 1.4 nm and 2.5 nm, respectively. The number density of nano-clusters in 0.3% Ti and 0%Ti samples are 5.39×1024/m3 and 1.1×1024/m3, respectively.