EFFECT OF PRECIPITATE FREE ZONES ON LOW-CYCLE FATIGUE LIFE OF AL-ZN-MG ALLOY

The effect of precipitate free zones (PFZs) on the low cycle fatigue (LCF) life of an Al-4.70 wt-%Zn-2.80 wt-%Mg alloy at room temperature was investigated. The alloy used in the present study was uniquely prepared to have different widths of PFZ, while keeping the distribution and size of predominant precipitates in the matrix or on grain boundaries almost constant. As the width of the PFZ increased from 0.073 to 0.13 mu m, the LCF life was observed to increase to some extent. However, the tensile properties such as yield strength, tensile strength, and elongation remained approximately unchanged. Studies using transmission electron microscopy show that the extent of planar slip decreases with increasing PFZ width throughout cyclic deformation. This result suggests that the increase in LCF life is probably due to relaxation of the local stress in the soft PFZ regions. (C) 1995 The Institute of Materials.