Influence of microstructure adjacent to grain boundary on intergranular cracking in Al–Mg–Si alloys

Abstract

The Al–Mg–Si alloys with excess Si without copper addition have recently been found to suffer from intergranular cracking. In the present study, we have investigated the effects of microstructure adjacent to grain boundaries on the intergranular cracking. It was confirmed that the alloy with excess Si without Cu addition had lower elongation to failure caused by higher fraction of intergranular fracture surface consisting of many dimples than balanced alloy and alloy with excess Si and Cu. Fractographic observations on the pair of fractured test pieces revealed that a grain boundary precipitate was present at the bottom of a dimple in either pieces. In the alloy with excess Si without Cu, difference in the number of grain boundary precipitates was found within the pair of the pieces, while equal number of precipitates were found in the pair in the other two alloys. Both the size of the precipitates and the amount of precipitation were larger in the alloy with excess Si without Cu than the other two alloys. The larger amount of precipitation meant the larger width of PFZ in the alloy. Thus, the larger fraction of intergranular fracture was closely correlated with the larger size of grain boundary precipitates and wider PFZ.