The Effects of Thin Anodic Films on Cyclic Deformation of Tungsten (Fatigue, Microstrain).

Abstract

The dynamics of edge and screw dislocation motion as a function of temperature play a decisive role in the mechanical response of bcc metals. Cyclic deformation over a range of plastic strain levels has been used to investigate many aspects of the cyclic stress-strain response of tungsten single crystals at room temperature. The results of this investigation fully support a two-dislocation model of deformation for bcc metals at low homologous temperatures (T (LESSTHEQ) 0.15T(,m)). At cyclic microstrains ((epsilon)(,pl) (LESSTHEQ) 5 x 10('-4)), cyclic saturation is reached at low stresses through motion of primarily edge (non-screw) dislocations, whereas at cyclic macrostrains ((epsilon)(,pl) > 5 x 10('-4)), high saturation stresses are reached due to the more difficult motion of screw dislocations. The occurrence of stress asymmetry and cyclic stress-strain behavior which fits a two-dislocation model are demonstrated in crystals of several orientations. The results indicate that the nearly isotropic elastic response of tungsten produces the nearly ideal stress asymmetry expected for bcc crystals. Thin anodic films ((TURN)80 nm) have been utilized to determine the effects of surface modification on the cyclic stress-strain behavior of tungsten. Coincident with a difference in the operant dislocation mechanisms, the effects of surface modification at low and high plastic strain amplitudes are different. At cyclic microstrains, the anodization of tungsten produces a large reversible hardening effect. At cyclic macrostrains the anodization of tungsten produces a smaller, less reversible softening effect. These two effects are explained in terms of dislocation dynamics and dislocation multiplication and annihilation processes. Additionally, these effects are described with regard to strong softening effects observed in monotonic deformation of modified bcc metals.