Effect of twinning and aging treatment on fatigue process of ZK60 magnesium alloys

For magnesium alloy ZK60 with different heat treatments, controlled plastic strain amplitude fatigue tests were conducted. Effects of twinning and aging treatments on fatigue process were investigated by analyzing the second derivative of hysteresis half-loop. It is found that during compression the convex peak appears at the strain where twinning begins. It is also found that during tension the convex peak appears at the strain where detwinning begins and the concave peak appears at the strain where slip begins to be predominant. Analyzing the strain at the convex peak in compression at the plastic strain amplitude of 6 × 10^-3, for solution treated material and over-aged material twinning hardens with increasing cycle. On the other hand, for T6 material twinning softens from 5 cycle to N_f/5 cycle and then hardens from N_f/5 cycle to N_f/2 cycle. It was suggested that residual twins cause twinning hardening. It was also suggested that cut and dissolution of rod-like precipitates should cause twinning softening. Due to different states of precipitates of materials, the twinning softening appeared for only T6 material at the plastic strain amplitude of 6 × 10 ^-3. Moreover, the softening were not found for T6 material under the plastic strain amplitude of 4 × 10^-3.