Tensile test and fracture toughness test at cryogenic temperature were carried out on AA 2014-T6 and AA 2219-T87. The serrated flows observed at rather low temperatures on various aluminum alloys were attributed to the dislocation trapping with interstitial atoms created from the comlex reactions between dislocations. Since the fracture toughness should be determined from the standpoint of the initiation of the crack growth, AE technique which is one of the most useful method for detection of the initiation of the crack growth was used in the present analysis. It seems that the K-values obtained from AE behaviors are strongly affected with the deformation mechanism, because the ratio between Kc and σy is nearly independent of the temperatures.
