| 其他摘要 | TEM, SEM, fast-fast coincidence positron annihilation spectra, dynamic mechanical analysis(DMA), thermal hydrogen charging and tensile tests at different temperatures have been used to study the hydrogen embrittlement mechanism of two types of FeNi based r'phase-strengthened austenitic alloys. The morphology, number and size of precipitates, and their influences on the hydrogen embrittlement of the alloys have been investigated. The effects of experimental conditions such as, hydrogen concentration, tensile rate on the hydrogen embrittlement of the alloys, and the effect of last pass forging deformation on the mechanical properties have been investigated as well.
The results of positron annihilation experiments show that hydrogen charging decreases the number of vacancy and increases the number of vacancy cluster in the solution-treated alloy. In the aged alloy, when r'phase and r matrix are in coherency, the r'-r interfaces are not traps for hydrogen atoms. However, the loss of coherency due to the growth of r' phase leads the r'-r interfaces to become traps for hydrogen atoms. In the overaged alloy, hydrogen atoms are trapped by a significant number of grain-boundary carbides, resulting in a dramatic loss of plasticity and intergranular fracture. Experimental alloys aged at 720 ℃ for different time validates that, the amount of grain-boundary carbide increases with the increasing of aging time, which results in increase tendency of the loss of plasticity and intergranular fracture with the aging time.
Internal friction measurements on DMA indicate that hydrogen induced an internal friction peak in the experimental alloy. The height of the peak decreased as the degassing of hydrogen. The calculated activation energy suggests that the peak is caused by the interaction between hydrogen atom and grain boundaries which are decorated with a significant amount of eta phases and a few of carbides. It is revealed that the grain boundaries are strong traps for hydrogen atoms, and the accumulation of hydrogen atoms at the grain boundaries causes plasticity loss and intergranular fracture. Addition of boron can suppress the precipitation of eta phases at grain boundaries. The hydrogen-induced internal friction peak for the alloy with boron appears at a lower temperature and has smaller activation energy. It is implied that due to the introduction of boron, the interactions between hydrogen and the grain boundaries decrease, and less hydrogen atoms accumulate at grain boundaries.
Testing conditions such as hydrogen concentration, predeformation, strain rate and test temperature can remarkablely influence hydrogen embrittlement of the alloy. As the hydrogen concentration increases, the stress needed to cut particles for dislocation is decreased and the mobility of dislocation is increased, and coplanar slipping is enhanced while cross slipping is weakened, resulting in the decrease of hydrogen-induced plasticity loss. When the predeformation increases, the density of dislocation goes up, hydrogen reduces the starting stress of dislocation movement, leading to the increase of plasticity and the decrease of hydrogen-induced plasticity loss.
The last pass forging deformation should be controlled in the range of 10-15% in order to improve the room-temperature and high-temperature properties of the alloy. When the last pass forging deformation is less than 10%, the grain size is not homogeneous and super coarse grains still exist, while the last pass forging deformation is larger than 15%, a significant number of twin boundaries and carbides appear within grains, and all of those are harmful to properties of alloy. |
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