Anomalous Fatigue Behavior and Fatigue-Induced Grain Growth in Nanocrystalline Nickel Alloys
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VOLUME 42A, JULY 2011—1799
Fig. 10—TEM micrographs of Ni coarsened grains; rotated to maintain the same X-Y coordinates as in Fig 9(a). (a) Bright-field imaging condition showing coarse grain aggregate embedded in the NC parent matrix. Moderate grain growth (indicated by arrows) is evident in the crack wake. (b) Detail of area marked in (a) showing dislocations in the grain interior (arrows), an irregular boundary (highlighted on the left), and several included grains (black circles), which are oriented within 3 deg to the parent grain.
Fig. 11—TEM cross sections of (a) NC Ni-20Fe tensile fatigue sample showing coarse grains at the crack initiation site and (b) annealed Ni-20Fe fatigue sample showing PSBs (black and white arrows) in microcrystalline grains. Red arrows note locations where dislocation ladders intersect the surface.
IV. grains near the crack wake. These collective dislocation structures occur over micron-scale dimensions and intersect the top surface of the sample at the locations shown by red arrows. At these locations, the top surface of the grain exhibits marked surface depressions, or socalled ‘‘intrusions,’’ coincident with the dislocation pileups. The micron length scale of the dislocation structures emphasizes the fact that truly NC grains cannot support persistent slip, because they are not large enough to contain such arrangements. 1800—VOLUME 42A, JULY 2011
DISCUSSION
The preceding results establish several important facts, the first of which is that these NC Ni alloys possess exceptional fatigue resistance, with S-N behavior far exceeding existing commercial alloys. The endurance limit of NC Ni and Ni-22Fe is quantitatively anomalous when scaled by the ultimate strength: whereas conventional Ni and its alloys typically have an endurance limit that is ~35 pct of the ultimate tensile strength, the Ni and Ni-22Fe alloys presented here exhibit endurance limits >60 pct of the ultimate tensile METALLURGICAL AND MATERIALS TRANSACTIONS A
strength. The average grain size of these alloys is well below 100 nm, and when stress levels are raised enough to cause failure, the fatigue process results in grain growth associated with crack initiation. TEM analysis of coarse grains suggests that the length scale of traditional dislocation-based mechanisms such as PSBs requires several hundreds of nanometers or even microns to operate. The fatigue behavior of these alloys represents an interruption of traditional fatigue failure processes analogous to the breakdown in Hall–Petch scaling associated with transitions in dislocation mechanisms.[18] The importance of small grain size in achieving high fatigue performance is demonstrated most clearly by comparing the performance of NC Ni and NC Ni-22Fe to the Ni-0.5Mn and annealed Ni alloys. While the maximum grain sizes of the former are approximately 150 and 75 nm, respectively, the Ni-0.5Mn and annealed Ni alloys contain grains several hundred nanometers in size. Grains this large located in regions of maximum stress would be more susceptible to persistent
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