The Connection between Recovery and Glide Type in Aluminum and Al-Mg

Abstract

The large size of Mg, and presumably also Li, substitutional atoms in aluminum leads to an abnormally high equilibrium vacancy concentration and results in planar glide in spite of high stacking fault energies and thus ease of cross slip. By contrast, apparently all other fcc planar glide alloys are associated with low stacking fault energies and correspondingly suppressed cross slip. As previously suggested, the reason for this peculiar behavior of Al-Mg (and by implication Al-Li) alloys is a concomitant reduction of `mushrooming . This is the interaction of glide dislocations with the supersaturated lattice vacancies that in wavy glide are generated in the course of plastic deformation. In the process of mushrooming the dislocations 'tangle' into three-dimensional arrangements, involving different Burgers vectors, namely through a mixture of glide, climb and nucleation of vacancy loops. Distinctly extended glide dislocations, inhibit both the formation of supersaturated vacancies and dislocation climb, and thus `mushrooming . In that case, therefore, dislocations are rather closely confined to their crystallographic slip planes, and 'planar glide' with underlying Taylor lattices results. In wavy glide materials, by contrast, glide generates in the order of a 10 - atomistic vacancy concentration per unit strain that causes the dislocations at the end of their paths to tangle and in the process also nucleate loops with all possible Burgers vectors. Besides 3-D dislocation mobility, the latter are needed for cell formation in stage III. In Al-Mg (and presumably Al-Li) alloys, easy cross slip conveys the requisite 3-D mobility, but the high equilibrium vacancy concentrations inhibit loop nucleation and thus the availability or the requisite extra Burgers. In this paper the recovery expected from these two types of dislocation structures, i.e. mosaic block structures and Taylor lattices, is theoretically investigated and shown to be associated with different recovery behaviors that conform to observation in pure aluminum and Al-Mg alloys, respectively.