CONDITION AND WAYS FOR RAISING CRACK RESISTANCE OF HIGH-STRENGTH ALUMINUM ALLOYS

Abstract

In the 1960s metallurgists worked extensively on the problem of raising the crack resistance of structural aluminum alloys in connection with the development of airplanes that required a long service life and high reliability. Crack resistance is understood as the capacity of a material to resist propagation of cracks generated in service by microscopic inhomogeneities (intermetallics, oxide scale), treatment-induced scratches, and structural stress concentrators (rivet holes, etc.) under the action of static and dynamic stresses. The characteristics of crack resistance are not related in a one-to-one manner to static strength and ductility ( r , 0.2 , ). Some structural parameters of the material that do not or very little affect the characteristics of static strength considerably influence crack resistance. In this connection, researchers who study and control structural alloys, aluminum ones in particular, operate with special structural strength parameters that characterize the behavior of the material in actual structures. These parameters include the fracture toughness, which is evaluated in terms of the critical stress intensity factor at the tip of the crack in plane deformation KIc (used to determine the fracture toughness of massive semiproducts like forgings, stampings, plates, or heavy shapes), the critical stress intensity factor at the tip of the crack in a planar stressed state Kc or K c f , the resistance to low-cycle fatigue (LCF)2 evaluated in terms of the number of cycles before failure under cyclic tests of flat specimens with a stress concentrator Kt = 2.3 – 2.5, and the rate of fatigue crack propagation (RFCP) in mm kcycle. Numerous studies of crack resistance characteristics performed in the last decades in Russia and other countries [1 – 7] have shown that crack resistance depends on many factors, the most important of which are (1) the composition and structure (not recrystallized, recrystallized fine-grained, coarse-grained) of the solid solution of the alloy; (2) the inclusions of eutectic intermetallics preserved after homogenization of the ingot, their deformation, and subsequent heat treatment, the size of which commonly fluctuates from several tenths of a micron to several microns; (3) secondary segregations of hardening phases formed in aging, the size of which varies from 5 – 10 to 100 – 150 nm; (4) secondary segregations of intermetallic transition metals, products of high-temperature decomposition of the solid solution of these metals in aluminum, which occurs in homogenization and other process heating stages, including heating for quenching and annealing; the size of such segregations can vary from 10 nm to tenths of a micron.