Microstructural characterization of novel in-situ Al-Be composites

The microstructure of cast and extruded in-situ Al-Be alloys, of compositions of Be-37Al-3Ni (wt pct) and Be-34Al-2Ni-2Ag-2Si (wt pct), was investigated using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The study indicates that both the Be and Al phases are continuous. The Be phase has a coarse dendritic structure in the as-cast material. Fractographic analysis of failed tensile specimens tested at room temperature revealed basal-plane cleavage failure of the Be phase and ductile failure of the Al phase. A significant number of deformation twins were observed in the Be phase when the tensile loading axis was parallel to the Be dendrite growth axis. An additional fracture mode was observed in the samples tested at elevated temperatures. At elevated temperatures, decohesion of the Al-Be interface was observed on the fracture surface. This phenomena was observed to increase as the test temperature increased from 150 °C to 315 °C. A high density of dislocations with a tangled morphology were observed in the Al phase after the tensile test. These were determined to be associated with easy slip of 1/2〈101〉-type dislocations. The limited ductility of the Be phase was attributed to the predominant basal slip of 〈a〉-type dislocations, b = 1/3〈1120〉, and the lack of dislocations with 〈c〉 components. However, a significant number of dislocations with 〈c〉 components were found in localized areas of the Be phase after extrusion.