Comparison of the fatigue and fracture of α Plus β and β titanium alloys

The present study compares the fatigue and fracture properties of the high-strength beta titanium alloy, beta -Cez with the conventional alpha + beta titanium alloy Ti-6Al-4V, because of increasing interest in replacing alpha + beta titanium alloys with beta titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy in an alpha + beta -processed condition (for a typical turbine blade application) and the beta -Cez alloy in two distinctly different alpha + beta -processed and fl-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both beta -Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance against crack initiation) of 375 MPa (R = - 1) as compared to the beta -Cez alloy (similar to 600 MPa, R = - 1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of the Ti-6Al-4V material is superior when compared to both beta -Cez conditions., However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance against fatigue crack growth. For the beta -Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical processing, maximum K-Ic-values of 68 MPa rootm of the beta -processed beta -Cez condition (tested in the longitudinal direction) decreased by similar to 50 pct in the presence of small cracks (1 min). A similar decrease in fracture toughness was obtained by loading the beta -processed beta -Cez condition perpendicular to the flat surfaces of the pancake-shaped beta grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the alpha + beta -processed beta -Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue crack initiation and propagation resistance (especially against small fatigue cracks).