Structure and mechanical properties of the directionally solidified Al-Cu-Mg eutectic

The ternary eutectic is located at Al-33.1 pct Cu-6.25 pet Mg and consists of an aluminum-rich phase, CuAl₂, and CuMgAl₂. In this work its morphology, crystallography, interfacial dislocation arrangements, and mechanical properties have been studied. The scale of the phases is proportional to the negative one-half power of the growth rate. The phases have preferred growth directions and, in many cases, preferred interfacial planes. Interfaces between the aluminum-rich and the CuMgAl₂ lamellar phases are semicoherent, having a/2 〈110〉 {111} aluminum misfit dislocations spaced 300Å apart. The observed dislocations are imaged only in the aluminum and appear to be aluminum-phase slip dislocations. The deformation of the composite to failure in tension is macroscopically elastic, and the failure strength depends on growth rate. It is shown that Griffith brittle fracture theory may be applied to the failure process, with cracks developed in the CuMgAl₂ phase during loading acting as the required Griffith cracks. A specific failure mechanism is proposed and related to observations of the fracture surface.