| 其他摘要 | Due to energy crisis and environmental pollution, magnesium alloys are getting more and more widely used in automobile industry，electronic products, air and space and other fields because of their low density, high specific strength, high thermal and electrical conductivity and reclamation property. Although their properties are better than casting magnesium alloys, wrought magnesium alloys are got far less used than casting alloys. Wrought magnesium alloys have a more promising future in vehicle parts. Since many mechanically loaded parts are often subjected to prolonged cyclic stresses in an active medium, corrosion fatigue failure becomes an important problem. But comparison to other metals such as steels, aluminum alloys, there is much less fatigue research about magnesium alloys (especially for corrosion fatigue). So it is of significant scientific and practical interest to study corrosion fatigue of magnesium alloys.
Rolled AZ31B magnesium alloy was chosen. The fatigue behavior under different conditions was systematically studied by acoustic emission technique. Some important progresses have been obtained as follows.
Fatigue behavior in several environmental systems was investigated. The results indicated that crack of β phase was responsible for the crack initiation site of rolled AZ31B in air. While in NaCl solution, crack initiated from surface damage which was produced by pit corrosion or filiform corrosion. The effects of pre-immersion on the fatigue life in 1.0% NaCl solution with different pH values were also studied. Pre-immersion greatly reduced the fatigue life. The fatigue life of the specimens pre-immersed in NaCl solution with pH＝3 was shortest. The fatigue life of the specimens pre-immersed in NaCl solution with pH＝6 was longest. The fatigue life of the specimens pre-immersed in NaCl solution with pH＝12 was in-between. The results were related to the pre-damage during pre-immersion process. In NaF solution, the corrosion fatigue life was higher than that in the same molar concentration of NaCl solution due to the form of MgF2 film. The fatigue life increased with the increase of NaF concentration.
The parameter of AE count was chosen to study corrosion fatigue process of rolled AZ31B magnesium alloy. Representative curves of cumulative count as a function of fatigue cycles had similar characteristics. It was found the curves had three stages, representing crack initiation, slow and fast crack propagation. Through these curves, the cycle to crack initiation and to rapid crack propagation can be identified. Cumulative AE counts can serve as a parameter to express the fatigue damage. A mathematical model was based on the parameter to express fatigue damage.
There were mainly four types of signals found in the corrosion fatigue process. Two types of AE signals corresponded respectively to anodic dissolution and cathodic hydrogen evolution appeared in a whole loading cycle. The signals of anodic dissolution belonged to the sources in plate produced extensive waveform, while the signals of cathodic hydrogen evolution belonged to the sources out of plate produced flexural waveform. Another two types of AE signals were corresponded to mechanical signals. One was the signals of plastic deformation which were continuous signal and appeared at the certain stress stage in a loading cycle. At the stage of crack propagation, crack propagation signals appeared at high stress loading part of each cycle.
Experiments showed the mechanical factors (stress amplitude, frequency, waveform) could remarkably affect the fatigue behavior of magnesium alloy. In NaCl aqueous system, magnesium alloy did not have fatigue limit. With the increase of loading stress amplitude, the fatigue life and cycles to crack initiation decreased. The frequency effect appeared during corrosion fatigue process. The fatigue life decreased with the decrease of loading frequency over a certain range of frequency. The fatigue life and cycles to crack initiation decreased in the order of triangle waveform(1-9s), triangle waveform(9-1s) and trapezoidal waveform(1-4-1-4s).
The AE signals can express the corrosion fatigue damage well. The AE counts at higher stress amplitude were more than those at lower stress amplitude. And the AE amplitude at higher stress amplitude was higher than that at lower stress amplitude. The AE counts in one loading cycle at lower frequency were more than those at higher frequency. And the AE amplitude at lower frequency was higher than that at higher frequency. The corrosion during holding phase remarkably promoted the production of plastic deformation signals.
The influence of solution concentration, pH values, and applied potential to the corrosion fatigue behavior of rolled AZ31B magnesium alloy was studied. With the increase of Cl- concentration, the fatigue life and cycles to crack initiation decreased. With the increase of pH value, the fatigue life and cycles to crack initiation increased. The applied anodic potential decreased the fatigue life and cycles to crack initiation, while the applied cathodic potential had certain protective role.
With the increase of solution concentration or the decrease of pH values, the stress in loading phase for plastic signals first to appear decreased. Compared to free corrosion potential, the stress in loading phase for plastic signals first to appear decreased under applied anodic potential or cathodic potential. In loading phase, the stress for plastic deformation signals first to appear was lower than the yield strength. |
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