INFLUENCES OF MICROSTRUCTURE ON FATIGUE CRACK PROPAGATING PATH AND CRACK GROWTH RATES IN TC4ELI ALLOY

Long fatigue crack propagating path in TC4ELI alloy with lamellar microstructure was studied by SEM. Statistics of angles between cracking path and α lamella exhibits that fatigue crack is apt to parallel α lamella or cross α lamella vertically, which will induce the ladder shape of crack propagating path. The parallel and vertical propagating patterns were interpreted with the weak α/β interface and the orientation of hcp α-Ti unit cell. Analysis of crack propagating path helps to understand the effect of microstructure on fatigue crack growth (FCG) rates. FCG rates tests with different microstructures reveal that in near fatigue crack threshold (ΔKth) region the thickness of α lamella is the main influential factor on fatigue crack growth rate da/dN ,a nd the da/dN seems impervious to β grain size. However, in Paris region the FCG rates are both influenced by grain size and lamellar thickness.