| In order to varify the origin of the anisotropy of mechanical properties particularly in short-transverse (S-T) orientation, the micromechanism of fracture processes involved in void and microcrack formation was identified in detail using an in-situ SEM fracture technique. For the S-T oriented specimen, microcracking occurs at the coarse grain boundary particles at the relatively low stress levels. Since the crack propagation path in the S-T oriented fracture nearly coincides with the elongated grain boundary, fracture proceeds readily along the grain boundary already weakened from cracking of coarse δphases, and a very low elongation is recorded. For fracture in the L orientation, on the other hand, fracture occurs along a tortuous path mainly along the slip band because the grains are smaller and of an equiaxed form, resulting in excellent mechanical properties. In this case, the coarse phases along the grain boundary do not have a large effect on the fracture path, according to the fact that a crack propagating along the slip band leads to intergranular fracture only when it meets the grain boundary directly. Therefore, the most important factor affecting the anisotropy of mechanical properties in the 8090 Al-Li alloy plate is the grain shape because fracture mode is largely determined by the grain shape. |
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