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KENZO FUKAURA, YOSHIHIKO YOKOYAMA, DAIEN YOKOI, NOBUHIRO TSUJII, and KANJI ONO The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M 7 C 3 carbides. These M 7 C 3 carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.

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The structure of an Al-Ni-Co decagonal ͑d-͒ quasicrystal has been investigated by scanning tunneling microscopy ͑STM͒. STM images with atomic-scale resolution have been obtained successfully for the surfaces of both tenfold and twofold planes. On the tenfold surface, large terraces and monoatomic-layer steps are formed. The symmetry of each layer is not decagonal but pentagonal and the two adjacent layers are related by the inversion symmetry. The step lines are rough, which can be attributed to the existence of many symmetrically equivalent low-energy steps. The atom adsorptions are often observed at locally symmetric sites. An analysis based on the high-dimensional description of the quasicrystalline structure has shown that the structure has nearly perfect quasiperiodic order for the decagonal quasicrystal. On the twofold surface, interlayer phason defects are observed, but the density of them is quite low. This fact indicates that the d-quasicrystal of the present sample is not in the random tiling state in which the configurational entropy related to the phason disorder stabilizes the quasicrystal.

Residual stresses in crystalline or glassy materials often play a key role in the performance of advanced devices and components. However, stresses in amorphous materials cannot easily be determined at the micron scale by diffraction, or by other conventional laboratory methods. In this article, a technique for mapping residual stress profiles in amorphous materials with high spatial definition is presented. By applying a focused ion beam (FIB)-based semidestructive mechanical relaxation method, the stresses are mapped in a peened and fatigued bulk metallic glass (BMG) (Zr 50 Cu 40 Al 10 at. pct). The residual stresses are inferred using finite element analysis (FEA) of the surface relaxations, as measured by digital image correlation (DIC), that occur when a microslot is micromachined by FIB. Further, we have shown that acceptable accuracy can in most cases be achieved using a simple analytical model of the slot. It was found that the fatigue cycling significantly changes the distribution of compressive residual stresses with depth in the plastically deformed surface layer. Our observations point to the scalability of this method to map residual stresses in volumes as small as 1 9 1 9 0.2 lm 3 or less.