The Production of nanocrystal materials with high fatigue damage resistance based on clarification of non-scale fatigue damage mechanism

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Fatigue crack growth tests were carried out using CT specimens of ultra-fine grained P/M (Powder Metallurgy) aluminum alloy of which grain size was from 200 to 500 nm. Fatigue crack growth behavior was investigated under constant amplitude and repeated two-step variable amplitude load sequences. It was found that roughness induced crack closure played an important role in closure behavior of this material. Although the roughness of fracture surface was smaller than the other conventional aluminum alloys, the roughness induced crack closure was dominant because of the small crack tip opening displacement (CTOD) with respect to the roughness level. Under repeated two-step loading, crack opening load was lower than that under constant amplitude loading with the same K value as the high level load, and the crack growth retardation caused by load history was not prominent comparing with other aluminum alloys. It is considered that the crack opening load under variable amplitude loading was … More dominated by the roughness induced crack closure. The small contribution of the plasticity induced crack closure was due to the constraint of plastic strain by the grain boundaries of ultra-fine grains.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 500 nm. Reversed plane bending fatigue tests were conducted at three different temperatures of room temperature (R.T.), 200℃ (473K) and 250℃ (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. Less