TEM observations of fatigue damage accumulation at the surface of the near-α titanium alloy IMI 834

Abstract

A high performance near-α titanium alloy (IMI 834) was cyclically fatigued to failure under load controlled four point bend testing with an R ratio of 0.1 to a maximum stress of 80–100% of the 0.2% proof stress. Detailed transmission electron microscope (TEM) characterisation of the damage accumulation was performed as a function of depth below the surface on samples from two orthogonal orientations. The slip band type and density was determined as a function of the resolved shear stress acting on the slip planes and their proximity to the surface. In the primary-α, fatigue damage accumulation occurred primarily by 〈a〉 slip on basal planes. Evidence of 〈c + a〉 and 〈a〉 prism slip was also found, but only where the resolved shear stress on the basal planes was low. Significantly, basal slip was identified on planes on which the resolved shear stress was low. In the transformed-β phase strain accumulation was substantially greater and was accommodated approximately equally on both basal and prism planes. In all cases, strain was a maximum in the surface grains and decreased with depth at a greater rate than the applied stress. Direct TEM evidence of crack nuclei was found. Cracks formed subsurface at stress concentrations on basal planes and grew along the basal plane towards the surface. The crack initiation mechanism is discussed in relation to strain discontinuities, resolved shear and tensile stresses, chemical composition and slip band length.