Grain boundary eutectic

Under equiUbrium conditions, magnesium can contain as much as 12.7% aluminum in soHd solution at the eutectic temperature. However, the slow diffusion of aluminum to the grain boundary leads to a coring effect in primary crystals and a hard-phase magnesium—aluminum compound(17 12)... 

An important example of an MMC in situ composite is one made by directional solidification of a eutectic alloy. The strength, (, of such an in situ metal-matrix composite is given by a relationship similar to the HaH-Petch relationship used for grain boundary strengthening of metals ... 

Sn + Pb is a two-phase eutectic system in which fine crystals of Pb with a linear parameter of 0.01 to 0.02 fim are localized along the grain boundaries of large Sn crystals (3 to4//m). A comparison of experimental... 

In conventional aluminum-deoxidized steels, the extra-low dissolved oxygen content maintains the sulfur in solution in the liquid stoel until the end of the solidification process. Since sulfur exhibits virtually no solubility in the solid steel — less than 0,001% in the bcc structure — a > iS-Fe eutectic suddenly precipitates at the as cast grain boundaries, the so called "Type II" structure described by Sims ( ). This weakens the as cast structure and also results in elongated MnS inclusions in the hot rolled steel plates, coils, sheets, bars, wires etc, as shown in Figure 5,... 

Figure 21. Sketches of the grain boundary eutectic, RE-Fe at various La and Ce concentrations...

Fig. 1.2. Optical micrographs of the transition zone between commercial purity iron or steel and aluminium. The Fe2Al5 intermetallic compound layer becomes progressively flatter and thinner with increasing carbon content of steel. The A1 + an intermetallic compound eutectic is distributed at grain boundaries of aluminium solid solutions.

Figure5.4.1) Contact angle versus time for a eutectic (Ag-Cu) dropon polycrystalline W at 900°C in a high vacuum. Before the experiment, the W substrate was heat-treated in high vacuum at 1100°C for 2 h. Despite this treatment, the surface remained oxidised and a slow spreading, controlled by W deoxidation, was observed. 2) The same without prior heat treatment of W. In this case segregation of O at the W surface, by fast grain-boundary diffusion, prevents deoxidation of the substrate, resulting in non-wetting behaviour. From Lorrain (1996).

Translucent A1203 from the People s Republic of China contains La203 in addition to MgO and Y203. A further depression in the eutectic temperature is expected, which probably lowers the sintering temperature even further. The grain size distribution is approximately the same as that shown in Fig. 19, but a higher concentration of grain-boundary phases is present. 

The formation of a liquid scale would be expected to give rise to the rapid attack that can be associated with rapid diffusion through the liquid. If the liquid is also a eutectic between the sulphide and the metal, then the reaction at the scale-metal interface involves a dissolution step. Such dissolution is capable of occurring more rapidly from some surface orientations than from others. In addition the dissolution is likely to be very rapid at the disordered areas of the grain boundaries. It is to... 

Above 600 °C, the formation of a eutectic liquid in the Ni-S-0 system at the metal-scale interface gives rise to grain-boundary penetration of the metal, as shown... 

Figure 7.14 Optical micrograph showing an example of grain-boundary penetration by eutectic liquid in the Ni-S-O system following penetration of a preformed oxide layer by SO2 at 1000 °C.

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