粗大粒を有するマグネシウム合金AZ31の応力比R = -0.6における粒界疲労き裂発生挙動

抄録

Plane bending fatigue tests were conducted using coarse-grained magnesium (Mg) alloy, AZ31, at stress ratio R = -0.6. Prior to the fatigue tests, grain orientations were analyzed by EBSD to investigate fatigue crack initiation mechanism and the effect of grain orientations. Since annealing was performed for grain coarsening process, there was no texture as seen in the as-received Mg alloy. Transgranular crack initiation was observed in the fatigue tests at R = -1 and 0.1 in the previous studies. But fatigue cracks predominantly initiated along grain boundaries at R = -0.6. Crystallographic effects on intergranular crack initiation were summarized as follows. Fatigue cracks initiated along grain boundaries where Schmid factors of basal slips of two grains across the grain boundary were very high. In addition, the misorientation angles of c-axes of both grains were large. Grain boundary with those characteristics tends to induce accumulation of dislocations. Subsequently, fatigue crack initiated along that grain boundary. In the present study, fatigue test results at different stress ratios of R = -1，0.1 and -0.5 were evaluated using equivalent stress amplitudes based on SWT (SmithWatson-Topper) method. However, there was a large scatter in the fatigue lives depending on the stress ratios, which could be attributed to the different crack initiation mechanism at different stress ratios.