Investigation on Mode I Propagation Behavior of Fatigue Crack in Precipitation-Hardened Aluminum Alloy with Different Mg Content

The influence of excess Mg on the Mode I propagation of fatigue crack was examined in newly developed precipitation-hardened Al alloy containing Zr and excess Mg. The aim of this study was to evaluate the underlying factor affecting fatigue crack growth rate in the stage II region. For this purpose, the rotating bending fatigue tests were performed in constant amplitude loading, and replication technique with an optical microscope was used to measure the crack growth in the Al alloys. Through analyses of the crack propagation on the specimen surface and striation formation of the fracture surface, the effects of excess Mg in the Al alloys were clarified to promote the occurrence of mode I fatigue crack, and decelerate the fatigue crack propagation. These facts suggest that the dynamic strain aging of Mg induces the formation of fatigue striation and reduce the driving force of the crack propagation. The findings were supported by the fractographic observations in the fatigue crack propagation region.

Modification Behavior and Microstructure Evolution during Solution Heat Treatment of A356 Alloys with Al2Ca and Excess Mg

Si modification behavior by twin probability approach with X-ray diffractometry (XRD) technique and microstructural evolution of the A356 alloys with Al2Ca and Mg exceeded up to 1.5 mass% during the solution heat treatment were investigated. Al2Ca added alloys showed the modified Si particles in the microstructure and higher twin density than those of Al2Ca-free alloy. During the solution heat treatment, the Si particles of Al2Ca-free alloy became coarser with the time at 540 °C, while there was no coarsening in the Al2Ca added alloys throughout all the conditions.