Grain Boundary Segregation and Fracture in Al-Li Alloy.

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An experimental study was made on the origins of low ductility of age-hardenable Al-Li binary alloy. The experiment includes the measurement of room temperature mechanical properties of hydrogen-charged alloy, the measurement of low temperature tensile properties of non-charged alloy and chemical analysis of fracture surfaces by SIMS(Secondary Ion Mass Spectrometry). Results are as follows.(1) Mechanical properties of Al-2.6wt%Li alloy are influenced by cathode-charging; proof stress increases and elongation decreases, the tendency being most pronounced in underaged conditions.(2) SEM(Scanning Electron Microscope) observation of fracture surfaces revealed a frequent occurrence of transgranular brittle fracture in the cathode-charged alloy in underaged conditions.(3) Strain contrasts accompanied with bubbles are observed by TEM(Transmission Electron Microscopy) along grain boundaries of the charged alloy, that are believed to be due to the stable <delta> -phase with hydrogen bubbles.(4) At low temperatures, the as-quenched alloy is ductile and fractures in grains. While, grain boundary brittle fracture is predominant in the aged alloy, corresponding to the much reduced elongation.(5) By the SIMS analysis, appreciably high conconcentrations of Na and K were detected on the fracture surfaces. This is more pronounced for the aged alloy. But no considerable segregation of S and Li was detected for all the aging conditions for all the test temperatures.The above experimental results strongly suggest the detrimental effects of <H_2> , Na and K on the mechanical properties of Al-Li alloy ( 3wt%Li) in the temperature range tested.