The effect of Ni on microstructure, elemental partition behavior, γ ′ phase solvus temperature, lattice misfit between γ and γ ′ phases, and mechanical properties of the Co-8Ti-11V-xNi alloys was investigated. The result shows that the lattice misfit in the alloys decreases from 0.74% to 0.61% as the Ni content increases from 0 to 10%, and the average sizes of the cuboidal γ ′ phase were measured to be 312.10 nm, 112.86 nm, and 141.84 nm for the Co-8Ti-11V, Co-8Ti-11V-5Ni, and Co-8Ti-11V-10Ni, respectively. Ti, V, and Ni exhibit a slight tendency to partition into the γ ′ phase, while Co shows a slight tendency to partition into the γ phase. The solvus temperatures of the γ ′ phase were measured to be 1167°C, 1114°C, and 1108°C for the Co-8Ti-11V, Co-8Ti-11V-5Ni, and Co-8Ti-11V-10Ni alloys, respectively, by using differential scanning calorimetry (DSC). Moreover, the yield strength and ultimate strength of the Co-8Ti-11V, Co-8Ti-11V-5Ni, and Co-8Ti-11V-10Ni alloys were investigated, and the yield strength and ultimate strength of the 10Ni alloy were highest, at 219 MPa and 240 MPa. After compression at 1000°C, the dislocations cannot effectively shear the γ ′ phase in the 0Ni and 10Ni alloys, resulting in a relatively high compressive strength of the 0Ni and 10Ni alloys. However, the γ ′ phase of the 5Ni alloy is no longer visible, and its strength is the lowest.
The influence of relatively high-frequency, low-amplitude vibrations on the low-cycle fatigue (LCF) behavior and its fractographic feature for a GH36 Fe-Ni alloy at 600°C is studied. It is found that the influence is rather complicated and significant. The nature and extent of the influence depended upon the ratio of the amplitude of high-cycle to low-cycle stress range, Q. As Q approaches Qc (the critical value of Q), there is no appreciable influence on the total LCF life; as Q < Qc, it will prolong the lifetime slightly; and as Q > Qc, the fatigue life will decrease progressively. The variation of fatigue life was ascribed to the occurrence of some considerable interactions between creep, high-cycle fatigue (HCF), LCF, and environmental effect under different values of Q at elevated temperature, and the change of damage mechanism of the material. The total LCF life, especially crack propagation life, will be markedly reduced by the high-cycle vibrations when Q is much greater than Qc. Meanwhile, the mixed failure of creep, HCF, LCF, and environmental attack will gradually change into combined cycle fatigue failure with increasing Q. The fractographic analysis is in agreement with these results.
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