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In the super-long fatigue life regime the fatigue properties of four high strength 42CrMo steels, two of commercial and two of zero-inclusion grade, respectively, are studied. SEM analysis shows the fatigue crack initiates mostly from non-metallic inclusion for commercial 42CrMo steels, and from the surface matrix for two zero-inclusion steels. The fatigue testing results indicate that, at 2 x 10(6) to 10(9) cycles, it is very difficult to cracking for zero-inclusion steels. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The cyclic hardening and saturation behaviors of copper tricrystals and bicrystals were investigated in strain-controlled multiple step tests. The results show that, for the inclined grain boundary (GB) bicrystal with single slip components, the cyclic stress strain (CSS) curve exhibits a plateau or quasi-plateau region, while the CSS curve of tricrystals shows no plateau. Observations of surface morphologies indicated that owing to the strain incompatibility of three grains, at lower strain amplitude the triple junction (TJ) retards obviously the primary slip in grains and makes deformation near it smaller than that near the bicrystal GB, while at higher strain amplitude slip can be distributed near the TJ homogeneously. The probability of crack initiation at the same TJ is closely related to the loading direction. The saturation dislocation structures of tricrystal specimens under the strain amplitude of the last step were explored by the electron channeling contrast technique in SEM (ECC-SEM). Loop patches with persistent slip band (PSB) ladders embedded were found even in the TJ vicinity for all grains. Dislocation-free zones (DFZ) occurred in the vicinity of TJ and GB, and the difference in shapes between them is due to the difference in internal stress field. Misoriented cell structure and dislocation wall structure were found near the crack tip, and the formation of them is associated with the cooperative action of crack tip, GB, grain orientation and the applied strain amplitude.

Cyclic deformation behavior and surface deformation features of [011] multiple-slip-oriented single crystals were investigated at constant plastic shear strain amplitude (gamma(pl)) in the range of 1.1x10(-4)- 7.2 x 10(-3) at room temperature in air. It was revealed that the cyclic deformation characteristic of [011] copper single crystal is quite different from that of [001] and [(1) over bar 11] multiple-slip-oriented copper single crystals. The [011] crystal exhibits a rather low initial hardening rate, which does not increase notably even under higher plastic strain amplitudes. The cyclic stress-strain (CSS) curve of the [011] crystal exhibit a clear plateau region over the range of plastic strain amplitude investigated. Surface observations indicated that the primary persistent slip bands (PSBs) already occur under a lower strain amplitude of 1.1x10(-4), but the operation of secondary slip was strongly suppressed by the corresponding dislocation interactions even at high strain amplitudes. This slip characteristic was suggested to be associated with the occurrence of the plateau region. When gamma(pl)greater than or equal to 2.5 x 10(-3), two types of deformation bands (DBI and DBII) formed on the specimen surface and their habit planes are perpendicular to each other strictly. An analysis based on the classical crystallographic deformation geometry was proposed to interpret the existence of an irreversible rotation of crystal in single crystal subjected to symmetrical push-pull loading. This phenomenon is assumed to be an essential reason for the formation of DBI and DBII. When gamma(pl)greater than or equal to 5.0 x 10(-3) another type of deformation band (DBIII) was observed on the specimen surface and its habit plane is exactly (001) with the maximum shear stress acting on it. The favorable macroscopic state of stress may be responsible for the formation of DBIII, giving rise to the cyclic softening in the cyclically deformed [011] copper single crystals at high strain amplitudes (y(pl) greater than or equal to 5.0 x 10(-3)). (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ali rights reserved.

Fatigue crack propagation behavior of compact tension specimens of Ni3Al(CrB) single crystals has been examined at room temperature (RT) and elevated temperature. The results indicate that crystallographic cracking occurs on two or more {111} planes simultaneously at RT. With increasing temperature, the cube slip plane becomes active and plays an important role in determining the cracking path. Orientation and temperature dependence of the fatigue crack initiation threshold from the notch root exists. Similar to the flow stress, the crack initiation threshold increases with temperature, and the threshold for ((1) over bar 00)[010] orientation is higher than that for (110)[1(1) over bar0$]. Both orientation and temperature affect the Fatigue crack growth rates (FCGRs). The FCGRs of ((1) over bar 00)[010] are higher than that of (110)[1(1) over bar0$] in the Paris regime. A crystallographic relationship between the primary crack plane and the secondary slip plane ahead of the crack rip is suggested to explain the dependence of FCGRs on orientation. The oxidation embrittlement at the crack tip and the activated cube slip should be responsible for the acceleration of FCGRs in the Paris regime and the oxide-induced crack closure has a significant effect on the deceleration of FCGRs in the near-threshold regime at elevated temperatures. Both the relationship of orientations between octahedral slip planes and the applied load axis and the increasing activity of the cube slip plane with temperature determine the significant variation of fracture path with temperature. (C) 1997 Acta Metallurgica Inc.