| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系 === 89 === During the application of hypoeutectic Al-Si(-Mg) alloy, resonant vibration may be encountered to accelerate failure. Therefore, Al-xwt%Si alloys (x = ~4 to ~11), Al-xwt%Si-1wt%Mg alloys (x = ~1 to ~11) and Al-7wt%Si-0.3wt%Mg-xwt%Fe alloys (x = ~0.1 to ~1.0) were used as experimental materials to explore the damping capacity and behaviors of crack propagation under resonant vibration and how to improve its resonant vibration life by metallurgical factors in this study.
Experimental results indicate the deflection amplitude in resonance can be classified into three stages as a function of vibration cycle during resonant test. In stage I, the deflection amplitude increases with the number of vibration cycles. The hardness of specimen also increases with vibration cycles during this stage. In stage II, test specimens possess a distinct plateau stage of maximum deflection amplitude and the cracks propagate principally in this 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 amplitude 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. So, the number of vibration cycles at the onset of this final stage can be considered as resonant vibration life of material.
In resonant vibration test, increasing volume fraction of silicon particles can increase a material’s damping capacity, which may decrease deflection amplitude of material. A reduction of deflection amplitude can be considered as a factor which increases resonant vibration life of material. In as-cast samples, acicular silicon particles are easier to break in deformation processes, so increasing the volume fraction of silicon particles are detrimental to the resonant vibration life of material, and consequently, the factor of deflection amplitude don’t cause a significant effect. However, after spheroidizing eutectic silicon particles, the damping capacity indicated by the effect of deflection amplitude became influential and the resonant vibration life of material can be improved by increasing volume fraction of silicon particles. Additionally, compared the solution-treated and extruded specimens, the eutectic silicon particles mainly gathers in the interdendritic zone in solution-treated specimens, and are well distributed in -Al matrix in extruded specimens. Resonant vibration results indicate that the solution-treated specimens possess higher value of crack tortuosity than extruded specimens, that leads to a higher resonant vibration life in the solution-treated specimens.
For the influence of aging on resonant vibration behavior, in the natural and under-aging condition specimens, a major part of the crack extends along slip bands as crossing through -Al matrix and the feature of {111} crystal planes appears in the fracture surfaces. However, the feature of slip band cracking is absent in the peak and over-aging specimens. In addition, when -Al matrix with a fair amount of silicon particles, the damping capacity of material is mainly affected by volume fraction of silicon particles and the deflection amplitude of all specimens-aged indicates a similar values. On the basic of crack propagation mode, 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. Whereas, when -Al matrix without containing silicon particles, the deflection amplitude of all specimens-aged is significantly affected by aging conditions. The resonant vibration life can be correlated with the deformation resistance of matrix and deflection amplitude. Experimental results show a good linear relation between the resonant vibration life and the ratio of yield strength (or matrix microhardness)/largest deflection amplitude. The resonant vibration life increases with increasing the value of ratio.
Furthermore, in Al-Si-Mg alloys, after a critical iron content is surpassed, the needle -Al5FeSi starts to form, and this leads to the resonant vibration life reduced sharply. Experimental results indicate that the resonant vibration life obviously reduce when increasing iron content to 0.97wt% in Al-7wt%Si-0.3Wt%Mg alloy.
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