Microstructural evolution during fatigue of ultrafine grained aluminum alloy

A comparative study of the surface microstructural changes during fatigue of a precipitation hardened (PH) aluminum alloy in ultrafine grained (UFG) and microcrystalline conditions was done using orientation imaging microscopy (OIM). With increase in applied stress amplitude, the fatigue life of fine grained PH alloy deteriorated. On the other hand, at lower stress amplitudes an improvement in fatigue life was noted. This trend was reversed for the coarse grained PH alloy. In UFG alloy, the fatigue life improved at all stress amplitudes. The variation in fatigue life response between coarse, fine and ultrafine grain PH alloy was due to the increased interaction between low angle and high angle boundaries with grain size decrease. Both, conservative and non-conservative dislocation movement during fatigue controls this process. In UFG alloy, additional interaction with the dispersed second phase causes an overall improvement in fatigue life. A direct evidence of room temperature recrystallization during fatigue in UFG alloy was also observed.