| Previous research suggested that the resistance to fatigue crack initiation and microcrack propagation would be very high if the inclusion and grain size were very small. Both are achieved in alloys made from atomized powders such as the recently developed Al base Zn-Mg-Cu-Co alloys (P/M 7091). The size of the constituent particles dispersoid or inclusions are less than 0.5 (mu)m and the grain size is about 7 (mu)m in the extrusion direction. Fatigue crack initiation and microcrack propagation were therefore studied in P/M 7091 type alloys by direct observation with a metallograph and by examination of surface replicas.;In P/M 7091 recrystallized by thermomechanical treatment, fatigue cracks initiate and propagate along slip bands. While such samples are more resistant to fatigue crack initiation, propagation along slip bands is much more rapid than along grain boundaries in P/M 7091 type alloys by a factor of 7. Compared to conventional ingot metallurgy Al base alloys, for example 2024-T4 aluminum alloys, the main advantage of quenched and aged P/M 7091 is its great resistance against microcrack propagation by a factor of 4 by fatigue loading.;Occasional pits were produced on the sample surface during preparation. They were formed probably around a few large inclusions and these aided fatigue crack initiation.;Stress concentration factors at notch roots causing the fatigue crack initiation were obtained using various notched samples by comparing applied nominal stress for un-notched samples to notched samples with the same fatigue crack initiation life. Stresses at notch roots calculated by those stress concentration factors differ from those obtained from appropriately calculated elastic stress concentration factors by 10 to 15%.;In contrast to most ingot metallurgy alloys where the fatigue cracks initiate and initially propagate along slip lines, in quenched and aged P/M 7091 the fatigue cracks initiate and initially propagate along grain boundaries. The grain boundary embrittlement is not due to grain boundary precipitates (MgZn(,2)), a precipitate free zone near grain boundaries, Co(,2)Al(,9) dispersoids, nor the very small grain size. By the process of elimination, the failure mode was attributed to Al(,2)O(,3) particles at grain boundaries originating from the atomization process. The high elastic modulus of Al(,2)O(,3) is an important factor. |
