Fatigue fracture mechanisms for caliber rolled AZ91D magnesium alloys

In this, paper the fatigue fracture mechanism for caliber rolled AZ91D and 1.5 mass% Ca added AZ91D magnesium alloys was investigated in relation to the mechanical properties and the microstructures. Fatigue test was carried out under axial loading condition controlled stress amplitude at room temperature, load ratio of R=0.1, 0.5 and cyclic frequency from 0.5 Hz to 10 Hz. As results, both of the grain sizes of caliber rolled AZ91D and 1.5 mass% Ca added AZ91D magnesium alloys were refined to 20 μm. Their ultimate tensile strength and 0.2% proof stress were 329 MPa and 268 MPa, respectively, and the elongation was 10.7%. The fatigue limits were clearly observed on their S-N curves. The alternating stress range of fatigue limit was about 164 MPa. The EPMA analysis on the fracture surface revealed that the fatigue cracks would initiate from network of Mg₁₇Al₁₂(AZ91D) or Al₂Ca (1.5 mass% Ca added AZ91D) phase in the aluminum rich zone. The shapes of the Mg₁₇Al₁₂ and Al₂Ca phases were provided as a half ellipse cracking at a specimen surface. The crack initiation stress estimated on the basis of the threshold of the fatigue crack propagation properties, ΔK_th=1.5 MPa· m ^1/2 reported by D. L. Goodenberger [1], was about 160MPa. As the estimated value was nearly equal to the fatigue limit, the fatigue strength would be improved due to the decreasing the size of the intermetallic compound.