Cyclic plastic deformation of tantalum and niobium at very high numbers of cycles

Niobium and tantalum in the annealed and cold-worked condition have been cycled at ultrasonic frequency at 90% of the fatigue limit at 2×108 cycles. Tantalum showed first cyclic softening followed by cyclic hardening after 105 cycles. Niobium showed cyclic softening only. Cyclic stress–strain behaviour of cold worked and annealed material showed similar trends.