A Critical Assessment of Cyclic Softening and Hardening Behavior in a Near-α Titanium Alloy During Thermomechanical Fatigue

Thermomechanical fatigue behavior of Ti-alloy Timetal 834 has been studied at two temperature intervals viz. 573 K to 723 K (300 A degrees C to 450 A degrees C) and 723 K to 873 K (450 A degrees C to 600 A degrees C) under mechanical strain-controlled cycling. Among the temperatures studied, the alloy exhibited initial cyclic softening followed by cyclic hardening at 723 K (450 A degrees C) in the temperature interval of 573 K to 723 K (300 A degrees C to 450 A degrees C). However, continuous cyclic hardening was observed at 723 K (450 A degrees C) in 723 K to 873 K (450 A degrees C to 600 A degrees C). At 573 K (300 A degrees C) and 873 K (600 A degrees C), cyclic softening was observed in the cyclic stress response curves in both the temperature intervals. The dislocation substructure was observed to be planar in both the modes of TMF loading. Based on TEM microstructures and few unconventional fatigue tests, the observed cyclic hardening is attributed to dynamic strain aging. The reduced fatigue life at 723 K to 873 K (450 A degrees C to 600 A degrees C) under OP-TMF loading was attributed to the combined effect of cyclic hardening (leading to early strain localization and crack initiation), oxidation, and development of tensile mean stresses.