Effect of Microstructure on the Fatigue Crack Growth Behavior in Al�Zn�Mg�Cu Alloy

Abstract

High-strength Al�Zn�Mg�Cu alloys are used in airframe structures, such as bulk heads, wing spars, and lug joints. In this investigation, the effect of RRA microstructure on the fatigue crack growth rate (FCGR) behavior is studied. The 7010 aluminum alloy was heat treated to two different conditions, i.e., T6 and RRA. The microstructure of the heat-treated alloy is characterized by using transmission electron microscope (TEM). The FCGR tests were performed as per ASTM E647 standard by using a 100�kN servo-hydraulic test machine. The tests were performed using standard compact tension (CT) specimens with a stress ratio, R = 0.7 using a sine wave form at 10�Hz in a standard laboratory air environment. The matrix microstructure of the RRA-treated alloy consists of fine scale ?� (MgZn2) precipitates with increased interparticle spacing when compared to closely packed ?� precipitates in the standard T6-treated alloy. The grain boundary precipitates are coarsened and discrete in the RRA-treated alloy, while it is continuous in T6 condition. An improvement in the threshold stress intensity factor range (?Kth) by about 0.65�MPa?m is observed in RRA-treated alloy compared to the T6-treated alloy. The FCGR was observed to be lower by 2 times in RRA-treated alloy compared to T6-treated alloy over the major portion of FCGR curve. The increased free slipping distance between the matrix precipitates in RRA-treated alloy is correlated to the improved fatigue crack growth resistance of the RRA-treated aluminum alloy. � 2020, Springer Nature Singapore Pte Ltd.