EFFECT OF GRAIN-SIZE ON THE FATIGUE CRACK-PROPAGATION BEHAVIOR OF AGE-HARDENED ALLOYS IN INERT AND CORROSIVE ENVIRONMENT

The influence of grain size on the fatigue crack propagation behavior of age-hardened Al-Zn-Mg-Cu and Ti-Al alloys has been investigated in vacuum and 3.5% NaCl solution. It was found that an increase in grain size reduced the crack propagation rates in vacuum considerably for both alloys. Crack propagation in vacuum occurred along slip bands in both alloys. In NaCl solution crack propagation rates increased by at least one order of magnitude for both alloys as compared to the results in vacuum. In NaCl solution the grain size dependence of crack propagation rates almost disappeared for the Al-alloy whereas the differences in propagation rates between different grain sizes still existed for the Ti-alloy. Crack propagation in NaCl solution occurred along slip bands for the Ti-alloy similar as in vacuum, while the Al-alloy showed fracture along grain boundaries at low ΔK-values with a transition to slip band fracture at higher ΔK-values. This transition in the Al-alloy occurred at ΔK-values which coincided approximately with the observed threshold values for the corresponding grain size found in vacuum. © 1979.