| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系 === 87 === During the application of hypoeutectic Al-Si-Mg alloy, resonant vibration may be encountered to accelerate failure. Therefore, the mechanisms of crack propagation of hypoeutectic Al-Si-Mg alloy under resonant vibration and how to improve its cracking resistance by metallurgical factors are necessary to be explored.
The experimental results indicate the deflection in resonance can be classified into three stages as a function of vibration cycle during resonant test. In stage I, the deflection increases with the number of vibration cycles. The hardness of specimen also increases with vibration cycles during this stage. Except for few specimens with faster crack growth rate, the test specimens possess a distinct plateau stage of maximum deflection and the cracks propagate principally in the second stage. As the cracks are sufficiently long, the resonant frequency of the specimen decreases. Therefore, the specimen diverges from the resonance condition. As the crack continues to advance, the degree of divergence increases and the deflection decreases with increasing vibration cycles in stage III. Moreover, the critical crack length for the onset of this final stage is not a function of microstructure change.
In current study, the crack propagation behavior of Al-7Si-0.3Mg alloy with various aging conditions was investigated under resonant vibration. The aging conditions used in this study can be divided into two groups. The natural and under-aging specimens belong to one group and peak and over-aging specimens are the other. Between these two groups, although they exhibit the different mechanisms of crack propagation, their resistances to crack propagation increase with the yield strength and microhardness. In the natural and under-aging conditions, a major part of the crack extends along slip bands as crossing through a-Al matrix and the feature of {111} crystal planes appears in the fracture surfaces. Under this cracking mode, the crack path is meandering, so the cracking resistance increases. In some regions of silicon particles clustering, the crack goes through silicon particle/matrix interfaces or broken particles. Comparing with the natural aging Al-1Si-0.3Mg alloy, which contains no eutectic silicon, the eutectic silicon shows little effect on crack propagation path under the crack extension mode with slip band cracking. However, the feature of slip band cracking is absent in the peak and over-aging specimens. In this group, the eutectic silicon heavily influences the cracking path and the crack shows the more preferred trend toward the zone of silicon particles clustering. As the crack propagates in this zone, the cracking mode is still particle/matrix interface decohesion or through broken particle. From the comparison result between Al-7Si-0.3Mg and Al-1Si-0.3Mg alloys at the over-aging condition, the eutectic silicon can cause the cracking resistance to decrease.
According to the result of the comparison between as-cast and solution-treated unmodified Al-7Si-0.3Mg specimens, the solution treatment can enhance the cracking resistance. Also, the longer solution time can further increase the resistance. Besides the solution-treated specimens affected by natural aging, the level of spheroidization of eutectic silicon increases with postporning the solution time is the other reason. So, Al-7Si alloy possessing no aging behavior is used to investigate the effect of eutectic silicon. The experimental results indicate the solution treatment can alter the morphology of eutectic silicon to more round. While the solution time increases, the level of spheroidization of eutectic silicon increase and the trend of cracking toward the zone of silicon particles clustering decreases. Therefore, the resistance to crack propagation increases. As crossing through the clustering zone, the cracking path of as-cast specimen dominantly goes through broken particles. After the solution treatment, the fraction of interface decohesion increases with solution time. By using the extrusion process for the unmodified alloy, the silicon particles change to particulate type and their distribution becomes more uniform. This feature causes the material to possess the higher elongation. However, its cracking path is smoother, so the extrusion process can not improve the cracking resistance. After the Sr-modification treatment, the effect of crack deflection and branching on the as-cast specimen are higher. However, the crack almost extends through the zone of silicon particle clustering. So, combining these two effects, the cracking resistance of as-cast modified specimen is slightly higher than unmodified one. After short solution time for the modified alloy, the deflection and branching effect on crack path decreases but the preferential trend through the clustering zone still persists. Thus, its cracking resistance is lower than the unmodified alloy.
To sum up the abovementioned results about hypoeutectic Al-Si-Mg alloy, under the consideration of same strength or microhardness, the under-aging condition, which exhibit the higher elongation and strength (microhardness) simultaneously is the better choice in the aging condition during the application necessary for higher cracking resistance under resonant vibration. Among the results of the influence of eutectic silicon, long solution-treating time and Sr-modification in as-cast condition can both improve the cracking resistance.
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