Improvement in fatigue strength of notched Ti-6Al-4V alloy by short-time heat treatment

This study was conducted to improve the fatigue strength of notched Ti-6Al-4V alloy without a surface finish by short-time heat treatment. Specimens were heated at 1203 K and 1233 K for 60 s, and then quenched to create ST1203 material and ST1233 material, respectively. The microstructures of both heat-treated materials were composed of equiaxial α grains and the prior β phase. The fine acicular α′ phase was generated in the prior β phase by quenching. The results of electron diffraction suggested that the metastable β phase partially existed in the prior β phase. The volume fractions of the prior β phase and the α′ phase in this phase were higher in ST1233 material than in ST1203 material. The tensile strength was improved by the generation of the α′ phase; its improvement percentage was higher in ST1233 material than in ST1203 material. The fatigue strength was markedly improved by the heat treatment because crack propagation was strongly suppressed by the generation of the α′ phase and the strain-induced transformation of the metastable β phase. The improvement percentage of the fatigue strength was higher in ST1233 material (40%) than in ST1203 material (20%).