Thermomechanical surface treatment of titanium alloys

Thermomechanical treatments are widely used to optimize the properties of high-strength titanium alloys for a given application. Mechanical surface treatments such as shot peening generally - but not always - result in improved fatigue behavior of titanium parts. Shot peening changes the surface properties in three respects: surface roughness, residual stresses, and degree of work-hardening are altered. Because these three parameters can independently influence crack nucleation and crack propagation, the overall fatigue life is a complex function of the surface condition. By separating the individual contributions of these parameters to the total fatigue life, it can be determined that an increase in surface roughness leads to early crack nucleation, while an increase in surface strength due to work-hardening delays the crack initiation phase. Furthermore, once surface cracks are present, these propagate faster in work-hardened material, but their growth is retarded by compressive residual stresses. Based on this analysis, recommendations for surface treatments for service at high temperatures (where the beneficial compressive stresses may anneal out) will be presented. Novel methods for improving fatigue performance by combining mechanical surface treatments and thermal treatments will be described.