Effect of Microstructure on High-Cycle Fatigue Properties of Ti-6Al-4V Alloy Forging at Cryogenic Temperatures

The effect of microstructure on the high-cycle fatigue properties of Ti-6Al-4V alloy forging at cryogenic temperature was investigated using four materials with different microstructure. One of them was alpha+beta annealed and has bimodal microstructure which consists of equiaxed-alpha and alpha+beta lamellar microstructure. Three other materials were water quenched (WQ), air cooled (AC) or furnace cooled (FC) after beta-annealing. The beta-annealed materials have lamellar microstructure and alpha lamellar width increase in the order of WQ, AC and FC materials. The fatigue test was done with uni-axial load-controlled and a stress ratio of R = sigma(min)/sigma(max) = 0.01 at 20 K (in gaseous helium), 77 K (in liquid nitrogen) and 293 K (in laboratory air). The 10(7)-cycles fatigue strengths at 20 K decrease in the order of WQ, AC, FC and the alpha+beta annealed material. Namely, the beta-annealed Ti-6Al-4V alloys, which has lamellar alpha microstructures shows higher fatigue strengths than bimodal microstructure.