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

The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional a + β titanium alloy Ti-6Al-4V, because of increasing interest in replacing a + β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy in an a + β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different a + β-processed and β-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 β-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 β-Cez alloy (∼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 β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness conventional large crack sizes) by thermomechanical processing, maximum Klc-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β 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 a + β-processed β-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).