Orientation dependence of cyclic deformation behavior and dislocation structure in Ti–5at.% Al single crystals

Abstract

Cyclic deformation behavior and substructure in single crystal Ti–5at.% Al alloy with different orientations fatigued at Δ ε t/2=0.4% were examined. The selected loading orientation includes: single prism slip, A; double prism slips [ 2 1 ̄ 1 ̄ 0 ], B; pyramidal slip, C; and twinning [0001], D. The testing results show that the crystallographic orientation has a strong effect on the cyclic stress response and the plastic deformation mode of Ti–5at.% Al single crystals. The crystals displayed an initial cyclic hardening followed by a striking softening period, and then a saturation stage was reached in specimens A and B. In contrast, an obvious cyclic saturation stage was obtained after first cyclic hardening until to fracture in specimens C and D. Trace analysis on the surface of specimens with an optical microscope shows that the ( 1 1 ̄ 00 ) single prism slip was operated in specimen A during cycling. The ( 10 1 ̄ 0 ) and ( 1 1 ̄ 00 ) double prism slips can be distinguished from the traces on the (0001) surface in specimen B. The ( 1 1 ̄ 01 ) pyramidal slip and the ( 1 1 ̄ 00 ) prism slip were activated simultaneously in specimen C. Twinning is the primary plastic deformation mode in specimen D. The twinning type includes: { 11 2 ̄ 1 }, { 10 1 ̄ 1 }, { 11 2 ̄ 2 } and { 10 1 ̄ 2 }. The substructure in the fatigued specimens was examined using TEM. Typical dislocation configuration is well developed saturation bundle structure (SBS) in specimen A, while it is the planar edge dislocations which are tangled on the primary ( 10 1 ̄ 0 ) plane and arranged parallel to the [0001] direction in specimen B. Fully developed loop patches were formed in specimen C. Typical deformed structure was the twin bundles and dislocations among twins in specimen D. The effects of plastic deformation mode on the cyclic stress response and the corresponding dislocation configuration of Ti–5at.% Al single crystals are then discussed.