| 其他摘要 | Investigation on effect of elements and mechanical properties of M951 alloy
Zhou Pengjie (Materials Science)
Supervised by Prof Sun Xiaofeng, Prof Guan Hengrong and Prof Yu Jinjiang
In this dissertation, the role of boron, zirconium and yttrium on the microstructure and mechanical properties of M951 alloy are investigated. In addition, the creep and the stress rupture properties, as well as the low cycle fatigue and the high cycle fatigue of M951 alloy are studied. The main results are summarized as following:
The stress rupture life of M951 alloy at 1100℃/40MPa is improved by adding an optimized amount of boron. The rupture life increases with increasing B content first, till 0.024%, then decreases. The tensile strength at 1000℃ drops with increasing B content, while the ductility improves. The boron segregates to grain boundaries, which facilitates the dislocation movement to the adjacent grain. As a consequence, the stress concentration there decreases, preventing the early microcrack initiation in grain boundaries. Finally it results in longer rupture life and decreased tensile strength. The impact toughness at room temperature can also be improved with the addition of boron. The flaky boride may form in the microstructure with excess boron doping. That is the origin of microcrack and deleterious to the mechanical properties. B depresses the incipient melting temperature and increases the volume fraction of γ/γ＇eutectic.
Adding an appropriate amount of zirconium not only improves the rupture life of M951 alloy at 1100℃/40MPa, but also increases the tensile strength and the ductility at 1000℃. Zr slightly depresses the impact toughness at room temperature. There are two remarkable modifications when Zr is added. Firstly, Zr inhibits the formation of Chinese script-like carbide in grains and changes the morphology of carbide from relatively continuous rod-like or platelet-like to discrete blocky morphology. Secondly, Zr increases the volume fraction of γ/γ＇eutectic.
The stress rupture life of M951 alloy at 1100℃/40MPa increases by Y doping. The optimum Y content is about 0.013%. Y improves high temperature tensile strength slightly and has insignificant effect on the ductility. The morphology of carbide can also be modified from continuous rod-like or flaky to the discrete blocky by the Y doping. The modification is beneficial to the mechanical properties. Y drastically increases the volume fraction of γ/γ＇eutectic. In the Y-free alloy, the γ/γ＇eutectic can be scarcely observed. The volume fraction of eutectic increases more significantly when both B and Y are added. The rise of eutectic volume fraction results in the increment of boride. After 1000h exposure at 850℃, a body-centered cubic Ni6Al2Y3 phase precipitates in the alloy with more than 0.03% Y doping.
Zr and Y both possess large atomic radius and positive segregation coefficient. In the final stage of solidification, they are enriched in the residual liquid. It results in slower diffusion rate of alloying elements, aggravating the imbalance of elements in the mushy zone. This is apt to the formation of eutectic. In addition, these elements are strong carbide former, which modify the composition of primary MC carbide. This modification increases the lattice distortion, and results in the extra increment of system free energy, finally leads the carbide morphology to blocky, which has smaller free energy.
At 700℃, the main deformation mechanisms of creep are dislocations shearing γ＇precipitates forming twinning and formation of stacking faults. The morphology of γ＇do not occur directional coarsening after long time creep, but changes to irregular morphology. At 900℃, dislocations move in the matrix channel, and are obstructed by the interface of γ/γ＇, forming dislocations tangles and dislocations networks. Only at the temperature over 900℃, γ＇occurs directional coarsening, and its orientation depends on the character of grains.
The life of low cycle fatigue increases with decreasing of the total strain range (△εt/2), increases with decreasing temperature. At 700℃, the low cycle fatigue of M951 alloy exhibits cyclic hardening. At 900℃, the stress response displays cyclic softening. The deformation mechanism of fatigue at 700℃ is mainly dislocation planar sliding, and its deformation is inhomogeneous. Both slip bands and grain boundaries impede the dislocation movement. At 900℃, the mechanism changes to wavy slip. Most of the dislocations move within slip band, and forms mass dislocation tangles when slip bands intersect.
The high cycle fatigue life of smooth specimens at a certain temperature decreases with increasing stress. The fatigue strength at 700℃ is slightly higher than that of 900℃. M951 alloy exhibits no fatigue notch sensitivity at 700℃. The notch sensitivity at 900℃ is also low. All the fracture surfaces at 900℃ exhibit brittle fracture characteristic and the surface is perpendicular to the applied stress axis. The fatigue crack originates from inner of the specimen, in addition, the crack initiation site changes from single-site to multi-site with increasing stress. At 700℃, the fatigue crack initiates from the inner defects in specimen when the stress is high. If the stress is low, the crack starts from the surface and subsurface of specimens. |
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