Fatigue Crack Initiation and Propagation in High Strength Aluminum P/M Alloys.
Final rept. 30 Sep 78-29 Sep 81,
NORTHWESTERN UNIV EVANSTON IL TECHNOLOGICAL INST
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Fatigue crack initiation in extruded X7091 RSP-PM aluminum type alloys occurs at grain boundaries at low and at high stresses and thus the expected increase in crack initiation resistance from the smaller constituent particle size and grain size than in ingot alloys is not achieved. By a process of elimination the grain boundary embrittlement was attributed to Al2O3 particles formed during atomization and segregated to some grain boundaries. It is not due to the small grain size, to Co2Al9, to precipitates at grain boundaries nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091 done by Alcoa with 0.8 Co to produce large recrystallized grains resulted in initiation of fatigue cracks at slip bands and a much greater resistance to initiation of fatigue cracks at low stresses. Microcrack growth is faster in the thermomechanically treated samples as well as in ingot alloys than in the extruded and aged X7091. However, the threshold stress intensity range in the former is larger, 1.6 MNcu m per sq m, compared to 0.9 MNcu m per sq m for extruded and heat treated samples. It would appear desirable to investigate fatigue crack initiation in much finer grained thermomechanically treated X7091-RSP-PM alloys. These might have the higher resistance to fatigue crack initiation. Author
- Metallurgy and Metallography