Effect of twins and plastic-deformation anisotropy of extruded magnesium alloy on fatigue crack growth and crack closure behavior

To study the effect of twins and plastic deformation anisotropy on the fatigue crack growth (FCG) behavior and crack closure behavior of extruded AZ31B magnesium alloys, two types of single-edge notched tensile specimens were prepared, with loading directions parallel (specimen E) and perpendicular (specimen T) to the extrusion direction (ED). FCG experiments using these specimens were performed at stress ratios R = 0.1 and −1, and cyclic speed of 10 Hz. In extruded magnesium alloy materials, the c-axis of the crystal is radially oriented on a plane perpendicular to the ED, indicating the generation of a strong texture. In the FCG experiment at R = −1 for specimen E, elongation in the c-axis direction occurred under compressive load, which generated {101‾2} <101‾1> tensile twins. The tensile twins increased the material deformation and fatigue crack opening distance. As a result, the effective stress intensity factor range ΔKeff increased, leading to the acceleration of the FCG velocity. At R = 0.1, tensile twins were not generated because the load waveform had no compressive load component; there was no change in ΔKeff and no acceleration of FCG. For specimen T at R = 0.1 and −1, tensile twin generation was expected because elongation in the c-axis direction occurred under tensile load. However, tensile twins did not occur, which is attributed to the effect of plastic deformation anisotropy. Therefore, for specimen T, the increase in ΔKeff and FCG acceleration did not occur, and no difference due to the R value was observed.