DEFECT STRUCTURE ANALYSIS IN CORROSION FATIGUED ALUMINUM-2024-T4 AND FATIGUED TITANIUM -6 ALUMINUM -4 VANADIUM ALLOYS.

摘要

X-ray double crystal diffractometer, transmission and scanning electron microscopy methods were employed to examine the cyclic deformation response of Al-2024-T4 in aqueous corrosive medium and of Ti-6Al-4V alloys in air. Special consideration was given to the cyclic response of surface and bulk properties; Cycling Al-2024-T4 in 3.5 pct NaCl solution reduced the fatigue resistance observed in air and the application of an anodic potential promoted further degradation of the fatigue resistance. The fatigue life in high cycle corrosion fatigue was controlled by the mean cyclic stress. The absence of an overall induced microplasticity at very low applied cyclic stresses, the cleavage fracture surface appearance, the frequency dependence as well as the observed cathodic polarization effects, suggested that hydrogen embrittlement may be the rate determining process in high cycle corrosion fatigue. In low cycle corrosion fatigue the maximum stress became the rate determining step for fatigue failure. Employing X-ray radiation of different penetration depth the measurement of induced excess dislocation density with depth from the specimen surface led to the capability for predicting the accrued damage and fatigue failure.; The fatigue resistance of Ti-6Al-4V was strongly dependent on the severity of precycling. The fatigue limit lost its significance if the alloy was subjected to a precycling treatment with a high stress amplitude. The interdependence of fatigue life and fatigue limit to precycling history was attributed to microcrack formation at the specimen surface. The fatigue damage could be either partially or totally eliminated by surface removal. In high cycle fatigue the alpha/beta interphase of the surface layer appeared to offer preferred sites for dislocation pile-ups and crack initiation. When subjected to low cycle fatigue, Ti-6Al-4V exhibited cyclic softening in the range of (epsilon)(,T) = 1.65 to 3.42 pct. Strain cycling induced in both the surface and bulk localized planar dislocation arrays in the slip bands of the alpha phase. The slip activities in the alpha phase became intense as cycling progressed and gave rise to crack initiation. The cyclic softening was attributed to the increase in mobile dislocation density in localized slip bands of the alpha phase.