展伸マグネシウム合金AZ61の高サイクル疲労強度特性に及ぼす応力比の影響

抄録

To discuss an effect of stress ratio on high cycle fatigue properties of the extruded magnesium alloy, axial loading fatigue tests have been performed under three conditions of stress ratio, R, of 0, -1 and -1.5 in laboratory air at ambient temperature using hourglass shaped specimens of extruded AZ61 magnesium alloy. Clear fatigue limit existed on the S-N curve obtained from the testing condition of R = 0, while under the tests of R = -1 and -1.5, specimens showed a step-wise S-N curve on which two knees appear. From the detailed observation with a SEM, fracture surface was divided characteristic five regions depending on the applied stress amplitude level. Facet-like smooth area was observed at crack origin only under low stress amplitude level and long fatigue life regime of R = -1 and -1.5, and whole stress amplitude level of R = 0. Fatigue crack initiation mechanism changed from the twin-induced failure mode at high stress amplitude level to the slip-induced one at low stress amplitude level. This transition was occurred by the relation between the minimum stress during a cycle and the offset 0.2% compressive yield stress at which deformation twin occurred. The deformation twin is formed during fatigue process when the minimum stress exceeds the yield stress in compression that is smaller than that in tension. It was pointed out that the high cycle fatigue properties were strongly affected by the formation of twin due to anisotropy of the materials processed with extrusion and crystallographic nature.