Contribution of ultrasonic surface rolling process to the fatigue properties of TB8 alloy with body-centered cubic structure

The effect of ultrasonic surface rolling process (USRP) as a severe plastic deformation technology was investigated on the evolution of microstructure, residual stress and surface morphology of TB8 alloys with body-centered cubic structure. Stress-controlled rotating-bending fatigue tests indicated increased fatigue strength in USRP samples prepared using different number of passes compared to the base material, which was attributed to the presence of gradient structure surface layers. Five subsequent USRP passes resulted in the highest fatigue strength, due to the optimal surface properties including higher extent of grain refinement, larger compressive residual stresses, smoother surface morphology and increased micro-hardness. However, the effect of USRP technology on improving fatigue strength of TB8 alloy was not significant in comparison with that of other titanium alloys (for example, Ti6Al4V), which was attributed to the notable surface residual stresses relaxation revealed from measurements on postfatigued USRP samples. Electron backscatter diffraction analysis confirmed that fatigue crack initiation occurred in the larger grains on the surface with high Schmid factor. Small cracks were found to propagate into the core material in a mixed transgranular and intergranular mode. Further analysis indicated that grain growth existed in post-fatigued USRP-treated TB8 samples and that the average geometrically necessary dislocations value reduced after fatigue loading. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

The ultrasonic surface rolling process (USRP) was found to significantly enhance the fatigue strength of TB8 alloys by creating gradient structure surface layers through multiple passes. However, the improvement in fatigue strength compared to other titanium alloys was limited due to notable surface residual stresses relaxation. Fatigue crack initiation was observed in larger grains on the surface, with subsequent propagation in a mixed transgranular and intergranular mode. Grain growth and reduction in geometrically necessary dislocations value were identified in post-fatigued USRP-treated TB8 samples.