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.