Fatigue Crack Propagation Behavior of Textured Polycrystalline Magnesium Alloys

Abstract

This paper describes the fatigue crack propagation behavior of rolled and extruded AZ31B magnesium alloys (grain size: approximately 20 and 15 μm, respectively). Fatigue crack propagation tests were performed on center cracked plate tension specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axes were parallel to the rolling and extrusion directions; fatigue cracks propagated parallel to the transverse direction (L-T specimen of rolled AZ31B), parallel to the short transverse direction (L-S specimen of rolled AZ31B), and perpendicular to the extrusion direction (E-R specimen of extruded AZ31B). The crack growth rate (da⁄dN) of the L-S specimen was approximately 10 times lower than that of the L-T specimen in the examined stress intensity factor (ΔK) range. Fracture surfaces of the L-T and L-S specimens showed many steps parallel and perpendicular, respectively, to the macroscopic crack growth direction. The plot of the da⁄dN versus the ΔK range for the E-R specimen shows two regimes with different slopes in the examined ΔK range. The fracture surface was covered by various directional steps independent of macroscopic crack growth direction, and the fracture surface roughness at low ΔK was larger than that at high ΔK. SEM-EBSD observations revealed that the c-axis direction is unfavorable for the fatigue crack propagation in rolled AZ31B magnesium alloy. Free deformation twins were observed around the fatigue crack path in the L-T, L-S, and E-R specimens.