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Development of Innovative Co-based Oxide Dispersion Strengthened (ODS) Superalloys
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| Title: | Development of Innovative Co-based Oxide Dispersion Strengthened (ODS) Superalloys |
| Other Titles: | 革新的Co基酸化物分散強化(ODS)超合金の開発 |
| Authors: | 余, 浩 Browse this author |
| Issue Date: | 23-Mar-2017 |
| Publisher: | Hokkaido University |
| Abstract: | In order to develop Co-based oxide dispersion strengthened (ODS) superalloys with excellent mechanical properties and oxidation resistance at the temperature above 1000 ºC, a novel Co-20Cr-(5,10)Al (wt. %) ODS superalloys containing 2.4 wt. % Hf and 1.5 wt. % Y2O3 were designed and fabricated by mechanical alloying (MA), spark plasma sintering (SPS), hot rolling and the final annealing at 1200 ºC.
Based on the results of microstructure characterization, the superalloys were found to contain a Co solid solution phase with face-centered cubic (fcc) structure and a CoAl phase with body-centered cubic (B2) structure, which offers the guarantee for good ductility and favorable strength, respectively.
Composition and volume fraction of the both phases are almost consistent with the Co-Cr-Al ternary phase diagram computed with the Pandat software.
Fine Y-Hf oxides particles are densely distributed inside the fcc solid solution and the B2 phase.
Grain boundaries and phase boundaries are decorated with the oxide particles, which are beneficial to grains and phases stability at elevated temperature.
Moreover, a unique core-shell structure of Y2Hf7O17 in core and Y2Hf2O7 at periphery was observed, which is responsible for the coarsening of the Y-Hf oxides.
High-temperature oxidation behavior of the Co-based ODS superalloys was investigated by the isothermal oxidation test at 900 ºC and 1000 ºC in air.
With the increase of Al content from 5 wt. % to 10 wt. %, the oxidation resistance of the superalloys had been significantly improved by the formation of a single alumina scale, which is far better than conventional Co-based alloys.
A faster θ- to α-alumina transformation at higher temperature was found to be responsible for the smaller weight gain at 1000 ºC than at 900 ºC for the 10 wt. % Al specimens.
Besides, the effect of Cr and Y2O3 addition on the oxidation behavior of the Co-based superalloys was investigated, as well.
The results indicate that the addition of Cr is beneficial to the oxidation resistance by accelerating the formation of an exclusive Al2O3 scale, i.e., changing the oxide scales morphologies from the multilevel scales with an external CoO/CoAl2O4 and an internal Al2O3 to the single Al2O3 scale.
With Y2O3 addition, the grain size of Al2O3 scale was decreased, and even though the weight gain of Co-20Cr-10Al ODS superalloys has been slightly accelerated, the Co-based ODS superalloys possess far better oxide-scale adherence than that in free Y2O3 alloys.
Tensile strengths of two Co-based ODS superalloys with 5wt. % and 10wt. % Al addition were measured at 23 ºC, 250 ºC, 500 ºC, 700 ºC and 1000 ºC.
10Al shows less ultimate tensile strength (UTS) at low temperature since a brittle fracture caused by high B2 volume fraction.
5Al exhibits outstanding UTS of 2.85 GPa at room temperature, which is associated with the distribution of fine Y-Hf oxides and the nano-twins deformation during strain.
Along with the increasing of temperature, a drastic reduction in tensile strength was observed, which is associated with grain boundary deformation due to the ultrafine grains of 500 nm in the 5Al and 800nm in the 10Al Co-based superalloys.
Therefore, since the occurrence of brittle fracture in 10Al with high B2 volume fraction, less Al should be considered to add into the composition of the Co-based superalloys.
However, the 10Al superalloy possesses a better oxidation resistance than the 5Al to form a single alumina scale.
Hence, the composition optimization based on altering the Al content should be focused, and 6 or 7 wt.% was considered to be applicable in this system. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第12759号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 鵜飼 重治, 教授 三浦 誠司, 准教授 坂口 紀史 |
| Degree Affiliation: | 工学院（材料科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/68506 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （工学）
課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
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