| With the rapid development of science and technology, the properties of the traditional metallic structural materials hardly met the requirements in practical applications. The strength, plasticity, high temperature corrosion resistance, and wear properties of the materials are expected to be improved immediately. Especially, some metallic structural materials which are employed in complex environment such as high temperature, high stress or high restriction, may endure oxidation or plastic deformation, which results in damage and failure. Rare earth element, due to their special physical and chemical characteristics, has been paid great attentions in recent years. The construction and mechanical properties of the alloys will be effectively influenced by adding even a trace amount of rare earth element. Based on such background, two kinds of metallic structural materials of Ni-22Cr-14W-2Mo alloy and Ti-29Nb-13Ta-4.6Zr alloy with different rare earth additions were employed for the present study. The effect of rare earth on the microstructure characteristics and properties of the alloys are investigated and the effect mechanism was also discussed. The results of the study are expected to provide the new method for microstructure modification, as well as improving the strength, fatigue limit, and oxidation resistance of metallic structural materials.The precipitate characteristics and existing form of trace rare earth La in Ni-22Cr-14W-2Mo alloy were investigated and the effect of rare earth La on grain size, carbides morphology, and tensile properties of the alloy were studied. The results show that only a small amount of La dissolves in the matrix. When La content is0.026%, La-S phases preferentially form because of its high melting point and low forming energy. The disperced La-S phases which formed before M6C carbides could be the heterogeneous nucleus of M6C carbides, thus the nucleation ratio increase. Therefore, large M6C carbides are refined by La-S phases, and the plasticity of the alloy is improved. Furthermore, La-S phases can promote the fine M6C precipitation which have coherent {111} plans with the matrix around large M6C, which results in the improvement in the strength of the interface between M6C and the matrix. However, more La-Ni phases with lager size precipitate when La content is higher than0.048%, which could deteriorate the mechanical properties.The high-temperature oxidation behaviors of the Ni-22Cr-14W-2Mo alloys with different La contents were investigated in static air at the temperature range from1173K to1373K, simulating the servicing environment of the alloy. The mechanism of rare earth La effect on the improvement of the oxidation resistance of the alloy was discussed. The results show that the effect of La on the improvement of the oxidation resistance is dependent on the amount of soluted La, when the alloy was oxidized at1173K or1273K. Dispersed LaCrO3from soluted La segregates on the grain boundaries of oxides, thus prevent the diffusion of Mn2+、Cr3+through the oxides grain boundary. The diffusivity of O2-becomes higher, and the O2-diffusing inward dominates the whole oxidation process. Meanwhile, the dispersed oxide particles LaCrO3act as nucleation sites for MnCr2O4-Therefore, there are fine and dispersed MnCr2O4distributed in the Cr2O3oxide-film, and the adhesiveness of the oxide-film is improved. However, when the alloy was oxidized at1373K, the effect of La on the improvement of the oxidation resistance of the alloy not only ascribes to soluted La, but also the amount of La-Ni intermetallic phases. On one hand, the dispersed La2O3with small size from the soluted La on the grain boundary of Cr-oxides can reduce the oxidation rate and refine the grain size of Cr-oxides; on the other hand, the aggregated La-0oxides from the large amount of La-Ni phases increase the whole oxidation.The effect of Y2O3on the microstructure refinement of Ti-29Nb-13Ta-4.6Zr alloy was studied. In order to reduce the cost, TiB2were added to the alloy instead of Y2O3, and the effect of TiB2additions on the microstructure of the alloy was also investigated. It is found that the grain size of the alloy can be refined by adding Y2O3or TiB2. However, the mechanism of the effect of Y2O3or TiB2on the microstructure refinement is different. Dispersed Y2O3with high melting point can act as nucleation sites for the β phase and increase the nucleation rate. Therefore, the grain size of the β phase in the alloys containing Y can be reduced. The effect of Y on the grain refinement is dependent on the size and the distribution of Y2O3particles. While, secondary TiB phases cannot act as the nucleation sites (3phase, because primary β grains first nucleate from the liquid during the solidification. The solid solubility of B in Ti is limited, thus B is rejected by the solidifying P phase and precipitates as TiB along the solid/liquid interface, which results in constitutional supercooling. As solidification progresses, constitutional supercooling causes instability at the liquid/solid interface and provides an additional driving force for the nucleation of more fine β grains ahead of the solid/liquid interface. Furthermore, the barrier formed by excess B in the solid/liquid interface reduces the growth rate of P phase. Therefore, the effect of TiB2additions on the grain refinement is dependent on the amount of B in the alloy.Young’s modulous, tensile properties, and fatigue properties of the Ti-29Nb-13Ta-4.6Zr alloys added with Y2O3or TiB dispersions were studied. The results show that the Young’s modulous of Ti-29Nb-13Ta-4.6Zr alloys with different B or Y contents are maintained low, and are almost similar to that of the alloy without adding. Although the tensile properties are not improved drastically, the fatigue strength is significantly enhanced by Y2O3or TiB dispersions. Especially, the fatigue limit of the alloy with0.10%Y increases by64%, as well as the fatigue limit of the alloy with0.10%B is improved by67%. The plasticity of the alloy decrease, with the volume fraction of TiB phase increase. However, the elongation does not deteriorate by Y2O3additions. The fatigue limit of the alloys with different Y2O3or TiB dispersion firstly increases and then decreases with the increase in Y or B content. On one hand, the sliding of the dislocations can be blocked by the stiff Y2O3or TiB particles, thus the second phases may have resistance to the fatigue crack initiation, reduce the crack propagation rate, change the crack propagation direction, and increase the crack propagation route. On the other hand, if the Y2O3or TiB particles are large enough, the stress concentration is easily generated in the interface between the Y2O3or TiB particles and the matrix. Y2O3or TiB particles can easily break or the crack may easily initiate from the interface between the second-phase and the matrix, if the interface is weak. In addition, the small crack between the second-phase and the matrix provide the priority way for the crack propogation. Therefore, the improvement in fatigue properties of Ti-29Nb-13Ta-4.6Zr alloy combined with Y2O3or TiB dispersions is dependent on the two ways discussed above.The proper La content in Ni-22Cr-14W-2Mo alloy is between0.026%and0.048%, and the proper Y content in Ti-29Nb-13Ta-4.6Zr alloy is between0.10%and0.20%. |
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