Fatigue behaviour of two-phase titanium alloy in VHCF regime

Abstract

This paper is focused on fatigue crack initiation and early growth in two-phase titanium alloy VT3-1 (similar to Ti-6Al-4V) under VHCF loads. The material was produced by two different processes: forging and extrusion. Each kind of material was investigated under three different loading types (push-pull, pull-pull and fully reversed torsion). Fracture surfaces of the tested specimens were analyzed by scanning electron microscopy (SEM) for getting information on crack initiation sites and surrounded fracture surface zones. The results of such analysis were compared with microstructure of the titanium alloy for establishing a crack initiation and early crack growth mechanisms. It was found that crack initiation in this alloy is caused by single or an agglomeration (“cluster”) of alpha-platelets. Under fully reversed tension the fatigue life seems to depend on the geometry of alpha-platelets clusters whereas under tension tension loading such dependence was not observed. However, materials with larger alpha-platelets clusters (macro-zones) have a lower VHCF resistance. The comparison of tension and torsion VHCF test results show a higher slop of the S-N curve under torsion than under tension for both forged and extruded titanium alloys. Nonetheless, some similarities in crack initiation and propagation scenarios were outlined between tension and torsion loadings.