The effect of cooling rate and calcium content on mechanical properties and the solidification characteristics of ZX magnesium alloys using cooling curve thermal analysis

The fatigue behavior and failure mode of SAE4320 bearing steel have been tuned through austenite content and grain size control via carburization and heat treatments. It is interesting that the interior crack sources change from inclusions in the 830 steels, to matrix + inclusions in the AP steel and to the major matrix in the 1150 steel. This transition is directly related to the residual austenite(RA) content, the prior austenite grain sizes(PAGS) and the RA segregates. Larger PAGS will change the phase relationship between the inclusions and the original austenite grain boundaries, and introduce more matrix crack sources near the austenite grain boundaries through the martensitic transformation of austenite. The unexpected ring-shaped granular bright facet (GBF) zones, appears around all inner inclusion and matrix crack sources on the RBF fracture surfaces in the low and high cycle fatigue regimes beyond 3 × 10 4 cycles for all three steels. The reason is that the local stress intensity factors for most of the subsurface inclusion/matrix sources have lower values than the threshold stress intensity factor, K th , ∼ 5 MPa•m 0.5 . This research may inspire more interesting and promising research and applications of advanced steels through microstructure design .