Fatigue crack growth behavior in semi-liquid die-cast Al-7%Si-0.4%Mg alloys with fine effective grain structure

Structural control was performed on Al-7%Si-0.4%Mg alloy castings by three different fabrication routes: sand mold cast and HIP, permanent mold cast a nd semi-liquid die-cast. In contrast to the ordinary dendrite structure in the sand and permanent mold cast alloys, the microstructure of the semi-liq uid die-cast alloy was characterized by colonies consisting of single or se veral dendrite cells. Crystallographic misorientation among these colonies is large and they can be regarded as 'effective grains'. Fatigue crack grow th tests were performed using CT specimens at a stress ratio of 0.1 and eff ects of microstructure were examined. Difference in the overall growth rate s was not evident in three samples. However, crack growth path and fatigue fracture surface of the semi-liquid die-cast alloys was different from thos e in the others. The fatigue crack grew straight even at small DeltaK level s. Large area of the fatigue fracture surface was covered with striations. These characteristic features are correlated with the relationship between the plastic zone size at the crack tip and the effective grain size in the semi-liquid die-cast alloys. (C) 2001 Elsevier Science B.V. All rights rese rved.