Much attention has been paid recently to the kinetics and micromechanisms of fatigue crack growth in various materials. Of particular interest are magnesium alloys, which find expanding applications in structures requiring high specific strength. This paper reports experimental results on Mg-Nd-Zr, Mg-Y-Cd, and Mg-Y-Cd-Zn alloys in different structural states. We measured the fatigue crack propagation rates and the values of the threshold point Kth, the transition points K1−2, K*, and K2−3, and the critical point Kfc in air at 293 K and in a vacuum of 10−4 Pa at 293 and 140 K. The plastic zone size around a fatigue crack was studied by X-ray diffraction and the micromechanisms of its growth by electron fractography.
Vacuum is shown to produce a decrease in the rate of fatigue crack growth in the whole range of Kmax values, while Kth and Kfc remain undergoing. The effect of vacuum was most prominent in thermally hardened alloys undergoing cyclic softening. A reduction in temperature from 293 to 140 K produced an increase in Kth and a decrease in the rate of fatigue crack growth in Region I and at the beginning of Region II for all alloys. In Region III, however, the rate of fatigue crack growth and Kfc changed ambiguously.
The plastic zone size was found to be larger in vacuum than in air; a reduction in temperature produced a decrease in its value regardless of the alloy structural state. The micromechanisms of fatigue crack growth were found to be dependent on composition and initial structural state of alloys, and variations in these parameters with low temperature correlated with the plastic zone size and the rate of fatigue crack growth. Our studies show the existence of certain structural and substructural criteria for the transition points K1−2, K*, and K2−3.