The effects of thermo-mechanical processing on fatigue crack propagation in commercially pure titanium with a harmonic structure

Commercially pure (CP) titanium having a bimodal harmonic structure, in which coarse grains are enveloped by a network consisting of fine grains, was obtained by consolidating mechanically milled powders to enhance both strength and ductility. We have examined the fatigue properties of materials having harmonic structures to achieve sufficient performance for practical applications in the engineering fields. In this study, thermomechanical processing (TMP) was performed for the CP titanium having a bimodal harmonic structure to improve its fatigue properties. Four-point bending fatigue tests and fatigue crack propagation tests were conducted under ambient conditions to investigate the fatigue crack initiation and propagation in CP titanium having a harmonic structure following TMP. Applying TMP to CP titanium having a harmonic structure increased its fatigue life due to greater resistance to the initiation of fatigue crack as a result of grain refinement. In contrast, CP titanium having a harmonic structure shows lower threshold stress intensity range, Delta K-th, values following TMP as a result of reduced the degree of crack closure and lower effective threshold stress intensity range, Delta K-eff,K-th, values due to grain refinement.