Fatigue damage accumulation in titanium alloy IMI 834.

As current aerospace materials are subjected in service to increasingly onerous conditions of stress and temperature, the hazard of fatigue failure becomes more acute. Engineers utilise the methodology of fracture mechanics to estimate fatigue crack growth rates but fatigue crack initiation, which involves the interplay of many microprocesses, is only investigated empirically. The aim of this study was to investigate the fatigue damage accumulation mechanisms in the titanium alloy IMI 834 in order to develop a fundamental understanding of the controlling physical processes and the micromechanisms which occur at the dislocation level. Load controlled four point bend test specimens of IMI 834 were cyclically fatigued to failure with an R ratio of 0.1 over a range of maximum stress levels and the fatigue and fracture surfaces were examined by optical and scanning electron microscopy. The examination of cross-sectional foils prepared from the fatigue surface enabled the fatigue damage to be examined in the T.E.N. as a function of orientation and depth below the specimen surface. The distribution, orientation and type of slip bands were identified in the primary-a and the transformed-fJ grains, and their interaction with secondary phases, precipitates and grain boundaries was determined. The results show that fatigue damage accumulation in INI 834 occurs primarily on basal slip bands in the primary-a phase and on basal and prismatic slip bands in the transformed-fJ phase. The segregation of a-stabilising elements to the primary-a phase during alloy processing allows the formation of an ordered phase which increases the propensity for planar slip on the basal plane. A mechanism for fatigue crack initiation along this plane is proposed. In addition, the occurrence and identification of an interface phase is discussed in the light of current theories regarding this phase.