FRACTOGRAPHY AND TRANSMISSION ELECTRON-MICROSCOPY ANALYSIS OF AN AL-LI-CU-MG-ZR ALLOY DISPLAYING AN IMPROVED FRACTURE-TOUGHNESS

Al-2.3 wt % Li-1.8 wt % Mg-0.15 wt % Zr alloys with 0.1 and 0.45 wt % Cu were studied. The materials were cast, homogenized at 530 degrees C for 24 h, extruded at 400 degrees C with a reduction ratio of 30 : 1, solution treated at 528 degrees C for 0.5 h and then artificially age d at 190 degrees C to an overaged condition. The concerted application of light-, transmission electron- and scanning electron microscopy to gether with short rod fracture toughness measurements allowed the infl uence of low level Cu together with 0.15 wt % Zr on the fracture tough ness of this alloy system to be studied. It was found that the additi on of 0.1 wt % Cu together with 0.15 wt % Zr not only affected the del ta' precipitate nucleation and their size and size distribution throug h Li-Cu-v triple complexes but also affected the beta' dispersoid nucl eation through Zr-Cu-v triple complexes. This in turn affected the siz e and size distribution of the delta'/beta' duplex precipitates. This causes an increase in the fracture toughness in underaged material art ificially aged at 190 degrees C. This effect was probed in an SEM frac tographic investigation and by mechanical testing.