Effects of Microstructure and Loading Parameters on Fatigue Crack Propagation Rates in AF2-1 DA-6

Abstract

Fatigue crack growth is examined in AF2-lDA-6 over a wide spectrum of growth rates from lo-* to 10m6 m/cycle for two mean stress values (R = 0.7 and 0.05) and two frequencies (10 and 2 Hz) at 649 "C. The effects of one processing parameter, the cooling rate from the solution heat treatment, on Paris law growth rates is examined. In addition, the effects of changes in grain size on the same parameters are examined. No effect of cooling rate horn solution heat treatment or of grain size on Paris law crack growth is seen for high R ratio tests at 10 Hz. However, low R ratio tests at lower frequency, 2 Hz, indicate that crack growth rates can vary with microstructure. Namely, crack growth rates are higher for a slow cooling rate of 34 'C/mm for both the ASTM 10 % and 6 average grain sizes. The effect is more dramatic for the smaller grain size. Crack growth rates in that material, i.e. AF2-lDA-6 cooled at the slow rate with a grain size of ASTM 10 l/2, exhibited the highest growth rate of all when R=O.O5 and the sinusoidal load frequency was 2 Hz. Some implications of these results on forging of such alloys are discussed.