HIGH. CYCLE FATIGUE PROPERTIES OF n-6AL-4V ALWYS

Cryogenic systems require highly efficient structural materials. Titanium alloys are blessed with many advantages like low permeability and low thermal conductivity as well as high strength and low specific gravity. However, very few cryogenic fatigue data are available probably because the test is extremely difficult. The present paper describes the high cycle fatigue property of Ti-6Al-4V alloys at 293 K, 77 Kand 4 K. The Ti-6Al-4V alloys with 0.054, 0.104, and 0.135 mass% oxygen were tested. The ingots were forged( forged material ) and part of them were further rolled ( rolled material ). Processing temperatures were in a+~ region and finally mill-annealing was done. The fatigue strength increased as the test temperature decreased, which can be accounted for by increased strength at lower temperature. The rolled materials had higher fatigue strength than the forged materials over the temperatures range tested and at given strength level, independently of the oxygen content. This is. believed to be due to the difference in the morphology of primary a grains, since the mean grain size itself was almost the same but the forged material had a "colony" structure in contrast with an equi-axed structure of the rolled one.