| 其他摘要 | In this dissertation, single crystal superalloys with different contents of Ru addition (0, 1.5, 3.0wt.%) were employed to study the influence of Ru addition on the as-solidified microstructure, structural evolution during heat treatment and long term aging, tensile and stress rupture properties, and oxidation behavior by using scanning electron microscope (SEM), transmission electron microscope (TEM), differential scanning calorimetry (DSC), energy dispersive spectroscopy (EDS) and some other research methods.
All the three kinds of alloys with different Ru content were directionally solidified with a 6mm/min withdrawal rate and the microstructures of the alloys were dendritic structures. β-NiAl phase particles with blocky morphology formed in the alloy with 3wt.% Ru content. With the increment of Ru content, the dendrite arm spacing and the volume fraction of γ/γˊ eutectics decreased. The size of γˊ phase after solidified and solution and aging treatment also decreased. The liquidus and solidus of the alloys did not change a lot. But the incipient melting temperature and γˊsolvus temperature of the alloys dropped.
The investigation of segregation behavior for alloying elements showed that Ru was apt to segregation to dendritic core region. The segregation of other alloying element was apparently influenced by addition of Ru. With 1.5wt.% Ru addition, the accumulation of elements Al, Ta, etc. towards interdendritic area became smaller and refractory elements like W, Re towards dendrite core were enhanced. When Ru addition further increased to 3wt.%, the segregation trends of Al, Ta turned reversely. But the segregation trends of Re became more significant with the rise of Ru content in the alloy.
With more Ru content in the alloy, an apparent inverse distribution of alloying elements was observed, which was more γˊ forming elements distributing to the matrix and more matrix forming elements distributing to the γˊ phase. During long term aging, µ phases precipitated in the Ru-containing alloys much later than Ru-free alloy. Ru can effectively suppress the precipitation of TCP phases. But Ru addition accelerated the coarsening of γˊ phase.
The tensile test showed that Ru has revealed pronounced strengthening effect on the tensile properties. All the three alloys with different Ru content reached their peak tensile strength at 760℃. When the tensile tested temperature increased to 800℃, the tensile strengths of three alloys decreased a little, the alloy with 3wt.% Ru content showed the most obvious decreasing amplitude. At 1000℃, the tensile strengths of three alloys decreased dramatically, and the alloy with less Ru addition showed slightly higher strength than that with more Ru content. In the stress rupture tests, the Ru free alloy possessed almost the same stress rupture life with the alloy of 3wt.% Ru content at 1100℃/180MPa, while the alloy of 1.5wt.% Ru content had a much lower rupture life. However, at 1000℃/310MPa, the Ru free alloy and 1.5wt.% Ru containing alloy possessed similar rupture lives, while the alloy with 3wt.% Ru had an apparently lower stress rupture life and a higher elongation. When m phase precipitates in the alloy during stress rupture tests, they can act as substrate to promote the nucleation of pores, and further accelerate the connection of micro-cracks or even induce the formation of cracks. With 1.5wt.% Ru addition, the nucleation of m phase can be retarded, so the pore sizes were reduced. With 3wt.% Ru addition, the precipitation of m phase can be completely eliminated, which was beneficial to obtaining higher stress rupture life. However, Ru destabilized the raft like microstructure, which was deleterious for the mechanical properties of single crystal superalloys. It was considered that the influence of Ru on the mechanical properties of single crystal superalloys depended on the competition of the above beneficial effect and deleterious effect.
Same as Ru free single crystal superalloys, Ru containing alloys had the same oxidation kinetic curves which obeyed the parabolic rule. Comparing with Ru free alloy, the Ru containing alloys had higher oxidation rates at 1000℃ and 1100℃. At 900℃, the dendrite core region and interdendrite region revealed different oxidation behavior during the early stage of oxidation in Ru containing alloys. Ru addition increased the apparent activation energy of oxidation.
After oxidation at 1100℃, the Ru free alloy and Ru containing alloys had the same phases distribution in the oxide films along the specimen surface towards the interior of the alloys, which were NiO (outer layer), CrTaO4, (Ni,Co)(Cr,Al)2O4, (Ni,Co)Ta2O6, and NiWO4 spinel phase (intermediate), a-Al2O3 (inner layer). The oxide films formed at 900℃ and 1000℃ were almost same as that at 1100℃, but they did not contain NiWO4 oxide in the intermediate. Besides, all the alloys had some extent of inner nitride under the three selected oxidation temperatures. The hot corrosion resistance of the single crystal superalloys has been decreased for some extent by the addition of Ru. The corrosion products at early stage were composed by NiO in the outer layer and (Ni,Co)Al2O4, NiTa2O6 in the inner layer. |
修改评论