Effect of microstructure on fatigue behavior of AZ31 magnesium alloy

Fatigue experiments were carried out in laboratory air using an extruded magnesium alloy, AZ31. In the alloy (designated as material E), a lamellar structure which is parallel to the extruded direction exists. This type of lamellar structure of the extruded material is not common for the AZ31 magnesium alloy. In the present study, two kinds of specimens, with axial directions parallel (designated as EP specimen) or vertical (designated as EV specimen) to the lamellar structure, were used for comparison. By comparing the S-Ncurves and crack generation and propagation characteristics of both specimens, effects of the lamellar-structure of the AZ31 magnesium alloy on the fatigue characteristics were studied. In the EP specimens, fatigue cracks initiated at a very early stage of the fatigue process, so the greater part of the fatigue life was occupied with fatigue propagation. Crack retardations or arrests due to the lamellar structure were observed. Accordingly, a sharp bend in the S-N curve was observed. In the EV specimens, fatigue cracks also initiated at an early stage of the fatigue process. However, in the EV specimens, many cracks were generated and propagated as compared to the EV specimens. Accordingly, the effect of crack coalescence is reflected in the propagation behavior of the EV specimens. The rate of fatigue crack growth in the ENT specimens was more rapid than in the EP specimens, and this led to a reduction in fatigue lives for the former as compared with the latter. In addition, a rolled magnesium alloy without the lamellar structure (designated as material R) was also tested. This type of microstructure is common for the AZ31 magnesium alloy. Two specimens with two different axial directions, parallel (RP specimen) or vertical (RV specimen) to the rolling direction, were prepared for testing. However, no differences in the fatigue behavior between the R-P and RV specimens were observed. This result differs from that of the extruded materials, EP and EV. The rate of fatigue crack growth in the rolled specimen was more rapid than that of the EP specimens and led to a reduction in the fatigue lives as compared with the EP specimens. (C) 2007 Elsevier B.V. All rights reserved.