Effects of orientation on the fatigue crack growth behaviors of the ZK60 magnesium alloy in air and PBS

Strong anisotropic corrosion and mechanical properties caused by specimen orientations greatly limit the applications of wrought magnesium alloys. To investigate the influences of specimen orientation, the corrosion tests and (corrosion) fatigue crack growth tests were conducted. The rolled and transverse surfaces of the materials show distinct corrosion rate differences in the stable corrosion stage, but the truth is the opposite for the initial stage of corrosion. In air, specimen orientations have a significant influence on the plastic deformation mechanisms near the crack tip, which results in different fatigue fracture surfaces and cracking paths. Compared with R-T specimens, N-T specimens show a slower fatigue crack growth (FCG) rate in air, which can be attributed to crack closure effects and deformation twinning near the crack tip. The corrosion environment will not significantly change the main plastic deformation mechanisms for the same type of specimen. However, the FCG rate in phosphate buffer saline (PBS) is one order of magnitude higher than that in air, which is caused by the combined effects of hydrogen-induced cracking and anodic dissolution. Owing to the similar corrosion rates at crack tips, the specimens with different orientations display close FCG rates in PBS.