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The main goal of this investigation is to determine the influence of grain size on the mechanical properties and, specifically, the intrinsic ductility of nanocrystalline (nc) Fe. Ball-milled nc Fe was consolidated into compacts of near theoretical density by uniaxial warm pressing. Compaction parameters and annealing treatments resulted in a range of grain sizes for subsequent mechanical testing. The miniaturized disk bend test, hardness, and the automated ball indentation (ABI) method were used to test nanocrystal (nc) iron in compression and tension. The deformation and fracture morphologies of the tested samples were characterized by light and scanning electron microscopy. The hardness, as a function of the grain size, was described with a Hall-Petch slope, which was smaller than that in coarse-grained Fe. In tension, the material failed in a macroscopically brittle manner, while local ductility in very concentrated shear bands was observed. The compressive characteristics of the nc Fe were similar to those of a perfectly plastic material. The results are discussed in the context of the mechanical behavior of coarse-grained polycrystalline metals and alloys.

The low-cycle fatigue (LCF) behavior of a nickel-based single crystal superalloy with [001] orientation was studied at an intermediate temperature of T0 • C and a higher temperature of T0 + 250 • C under a constant low strain rate of 10 −3 s −1 in ambient atmosphere. The superalloy exhibited cyclic tension-compression asymmetry which is dependent on the temperature and applied strain amplitude. Analysis on the fracture surfaces showed that the surface and subsurface casting micropores were the major crack initiation sites. Interior Ta-rich carbides were frequently observed in all specimens. Two distinct types of fracture were suggested by fractogaphy. One type was characterized by Mode-I cracking with a microscopically rough surface at T0 + 250 • C. Whereas the other type at lower temperature T0 • C favored either one or several of the octahedral {111} planes, in contrast to the normal Mode-I growth mode typically observed at low loading frequencies (several Hz). The failure mechanisms for two cracking modes are shearing of γ precipitates together with the matrix at T0 • C and cracking confined in the matrix and the γ/γ interface at T0 + 250 • C.KEY WORDS low cycle fatigue, single crystal, nickel-based superalloy

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