MECHANICAL-PROPERTIES OF AL-LI ALLOYS .1. FRACTURE-TOUGHNESS AND MICROSTRUCTURE

Mechanisms influencing the ambient temperature mechanical properties of commercial Al-Li alloys 2090, 8090, 8091, and 2091 are examined as a function of plate orientation, with specific emphasis on the role of microstructure. In Part 1, results on the uniaxial tensile and plane strain fracture toughness properties are presented and the behaviour is discussed in terms of the role of the matrix and grain boundary precipitates, associated precipitate free zones (PFZs) and the occurrence of short-transverse delamination. It is seen that in general peak aged microstructures show an excellent combination of strength and toughness (L-T, T-L), equal to or exceeding that shown by traditional 2000 and 7000 series high strength aluminum alloys. The superior toughness of peak aged compared with naturally aged microstructures seems to be associated with widespread matrix precipitation of platelike precipitates (T1 in Al-Li-Cu alloys and S in Al-Li-Cu-Mg alloys), β′ -dispersoids and second phase particles which promote ductile (void coalescence) fracture, and with secondary cracking (through thickness delamination) caused by poor short transverse properties.