Metal microcrack

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

In many materials, the inherent flaws are easily recognized. Brittle polycrystalline materials, for example, contain microcracks, voids, and other imperfections that can be identified in micrographs, and are expected to provide sites for internal fracture activation. Artificial flaws introduced into a hollow metal shell by uniform scoring can be expected, under rapid expansion, to fracture the shell along the paths of scoring. 

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

Fig. 13. Schematic representation of oxygen dissolving froin the oxide into the titanium metal at high temperatures. The interface is weakened with the formation of voids, porosity, and microcracks and with the embrittlement of the interfacial metal region [52).

This concept may be invoked to account for electrolyte formation in microcracks in a metal surface or in the re-entrant angle formed by a dust particle and the metal surface. More importantly, it can also explain electrolyte formation in the pores of corrosion product and hence the secondary critical humidity discussed earlier. Ferric oxide gel is known to exhibit capillary condensation characteristic and pore sizes deduced from measurements of its adsorptive capacity are of the right order of magnitude to explain a secondary critical relative humidity as70 7o for rusted steel . 

The solid electrolyte is always visible to the XPS through microcracks of the metal films. As already discussed, some porosity of the metal film is necessary to guarantee enough tpb and thus the ability to induce electrochemical promotion. In order, however, to have sufficient signal from species adsorbed on the metal it is recommended to use films with relatively small porosity (crack surface area 10-25% of the superficial film surface area). 

Solid metals obtained upon solidification of the molten metal exhibit grain structure. They consist of fine crystallites randomly oriented in space. The size of the individual crystallites (grains) is between 10 m (fine-grained structure) and 10 m (coarse-grained structure). The crystal stracture of the individual grains as a rule is not ideal. It contains various types of defects vacant sites, interstitial atoms or ions, and dislocations (lattice shearing or bending). Microcracks sometimes evolve in the zones between crystallites. 

The metal surface is polycrystalline and has a rather complex profile. Because of different crystallite orientations at the surface, different crystaf faces are exposed, such as smooth fow-index faces and stepped high-index faces. Surface texture where a particufar kind of face is predominant can devefop in individual cases. Microcracks and various lattice defects (dislocations, etc.) will also emerge at the surface. 

All RNiPb plumbides are metallic conductors which show a weak curvilinear temperature dependence above 100 K. Due to the microcracks in the samples and the difficulty in cutting regularly-shaped bars, only normalized resistivity data could be measured. For YNiPb the resistivity curve could be fitted with the Bloch-... 

The new phases are characterized by considerable volume changes in relation to the basic material. The stresses occur on the interphase boundaries, and microcracks are formed. The example of such damage is metal embrittlement when forming hydride phases. The internal stresses also have an effect on kinetics of a new phase growth. Let us consider the residual stresses in a hollow cylinder. Maximal concentration of the impurity atoms occurs on the area boundary, where the new phase formation takes place. Its further growth is realized at the expense of impurity atoms diffusion. The task of defining kinetics of the new phase growth in the hollow cylinder is mathematically formulated as follows... 

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