In recent years AI -Li alloys have become very popular in the aircraft industry. These alloys are characterized by a high specific strength and rigidity which increases substantially the load ratio and fuel efficiency of the parts. As a rule, Al -Li alloys are used in an underaged state, which provides a high ductility over the height of the semifinished products. However, the threshold stress (Ocr) in corrosion cracking in this state does not usually exceed 25 -50 N/ram 2, and the yield strength is not always satisfactory. The results of a study of the mechanical properties and corrosion resistance of some AI-Li alloys are described and the ways for optimizing these properties are described on the basis of the phase transformations in the alloys.

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The load-bearing structure of modem airframes requires monolithic members that are produced from large forged and rolled aluminum-alloy semifinished products. These members are designed in accordance with the principle of "safe service life," and critical parts are designed in accordance with the principle of "safe vulnerability to damage." For this reason, the characteristics of static and cyclic crack resistance (Ki,., RGFC, nel G~r, and others) have become very important and some of them are used as control parameters in standard documentation. These parameters characterize the operating capacity of the material of a structure with a crack and take into account the magnitude and direction of the applied load. If the crack front deviates from the given direction to another plane (for example, from a transverse plane to a heightwise plane), the process of crack propagation becomes unpredictable and uncontrollable. The present paper is devoted to the special features of fracture in large semifinished products made of aluminum alloys.