The Fatigue Behavior of γ/α₂ Titanium Aluminides

Abstract

Axial load controlled high cycle fatigue experiments were conducted on the γ/α2 alloy, Ti-48A1-1V-0.2C (at%), at 23 and 815°C. Four different microstructures, produced through thermomechanical processing, were evaluated to examine the influence of grain size and α2 content on fatigue behavior. The load controlled fatigue life was significantly reduced by increasing grain size and unaffected by α2 content at both 23 and 815°C. Although, α2 content did not greatly influence high cycle fatigue life, the room temperature crack initiation and fast fracture was changed from transgranular to partially intergranular as the volume fraction of α2 was reduced in the fine grain size material. The fatigue strength at 107 cycles (FS) to ultimate tensile strength (UTS) ratio was 0.8 to 0.9 at 23°C and 0.5 to 0.6 at 815°C for all microstructures examined. Low tensile ductility, high work hardening rate and the difficulty in forming strain local-izations all aided the high FS/UTS ratio. The dislocation microstructures produced by fatigue at room temperature were examined in the fine grained high α2 (ductile) microstructure. They consisted of loop patches of all <110] regular dislocations without any <101] or <011] super dislocations because of the large difference in CRSS for these dislocation. The inability to nucleate and move superdislocations inhibited the formation of persistent slip bands as is often found in high and intermediate stacking fault FCC metals.