Effects of Microstructure on Crack Initiation in AA6451 and Crack Propagation in AA3xxx
Yoo, Yung Suk
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Aluminum alloys have been enjoying the spotlight in recent years as the next generation alloy for a wide variety of applications. Their potentially waste-free recyclability, excellent corrosion resistance, and desirable balance in physical properties—low density and high strength-to-weight ratio—makes them an ideal candidate material for efficient and environmentally-friendly products. Mechanical properties of aluminum alloys can be engineered to suit the requirements for different functions by controlling the microstructural features. Naturally, the variety of alloying elements, microstructural features, and thermomechanical processes produce complex microstructures that deform heterogeneously under different mechanical loading conditions. To get a better understanding of the failure mechanism of aluminum alloys, this dissertation will explore the effects of dispersoids, a type of second phase particle, on the crack initiation and propagation behaviors. A multiscale electron microscopy-approach was employed to characterize different aspects of the microstructure and their localized deformation behavior. This work is divided into two parts. The first part will delve into the crack initiation mechanism of AA6451 during three-point bending and the influence of microstructural features on each step of the process. It will also discuss the effects of variation in alloying elements and tempering conditions on the microstructure evolution and localized deformation behavior of AA6451. The second part involves studying the crack propagation behavior of deep drawn and necked AA3xxx. The dispersoid effects on crack growth direction will be discussed in depth. These findings will ultimately help scientists gain a better mechanistic understanding of defect interactions during extreme stress.