Initiation and Growth Behavior of Small Fatigue Cracks in Ultra-fine Grained P/M Aluminum Alloy at Elevated Temperatures

Abstract

Small fatigue crack initiation and growth behavior at elevated temperatures were investigated using ultra-fine grained P/M (Powder Metallurgy) aluminum alloy of which grain size was from 200 to 500nm. Reversed plane bending fatigue tests were conducted at three different temperatures of room temperature (R.T.), 200°C (473K) and 250°C (523K), and crack initiation and small crack growth were studied in detail by means of replication technique. The fatigue strength decreased as test temperature increased. The cracks initiated at the boundary between powders regardless of test temperatures, and total fatigue life was dominated by crack initiation life. The fatigue crack initiation and growth behavior was almost irrelevant to the test temperatures. Fatigue crack growth rates, da/dn, were slightly accelerated at a fixed Kmax with increasing test temperature, while da/dn evaluated in terms of Kmax/E (E : Elastic modulus) agreed well. The dependence of fatigue strength and crack growth rate on test temperature was much smaller than that of conventional aluminum alloys, indicating the high heat resistance of ultra-fine grained P/M aluminum alloy.