Deformation induced vacancies and work hardening of metals

The paper follows a previous one on effects of non-equilibrium vacancies on strengthening tin by focussing on the formation mechanisms of vacancies an d/or vacancy agglomerates through plastic deformation, and on their influen ces to both work hardening after deformation ('static effects'), and during it ('dynamic effects'). One has also to distinguish between (i) direct lan d (ii) indirect effects of vacancies/agglomerates to hardening characterist ics. While at small strain the direct interaction of vacancies/vacancy aggl omerates with dislocations governs the macroscopic hardening characteristic s, at large strains the indirect effect of these vacancies/agglomerates pre vail; here, the growing number of dislocations acts as sinks for the vacanc ies/agglomerates which, however, induce enhanced climb and annihilation of edge dislocations leading to marked softening effects even at low deformati on temperature T approximate to 0.2 T-m (T-m is the melting temperature in K). Examples are given for effect (i) where both the critical resolved shea r stress tau (c) and the hardening coefficient d tau /dy were found to chan ge up to 20% in hcp - metals and alloys, but still markedly in fee ones. Ef fects of type (ii) are shown to typically rule the so-called stage V of def ormation at low deformation temperatures, for dynamic but also static cases . With the latter, additional annihilation of edge dislocations will occur during unloading which can lead to an absolute decrease of dislocation dens ity and thus, of macroscopic strength. Consequently, iterative modes of def ormation such as rolling, extrusion and wire drawing are predestined to sho w this effect. Both the direct as well as the indirect effect of deformatio n induced vacancies deserve particular interest in cases where conventional large strain deformation modes are combined with hydrostatic pressure in o rder to produce ultrafine grained or nanocrystalline metals. These material s exhibit outstanding physical properties, e.g. a strength being up to a fa ctor 3 higher than the same material with coarse grains. The hydrostatic pr essure is thought to restrict diffusion via vacancies which should allow fo r higher accumulation of both deformation induced vacancies and/or dislocat ions. First experimental results are presented which suggest the extra hard ening to arise mainly from vacancies or vacancy agglomerates, in the same o f a direct interaction with dislocations as defined above.