The isolation of the excess phase from the supersaturated solid solution (Part 2).

With the appearance of β-phase crystals, the α-solution surrounding them is depleted by component B. Near the β phase, the solid α-solution has the Сг composition, and in the distance the composition remains the same. This heterogeneity of the solution is weakened with diffusion in the α-phase and the β-phase release. If the exposure at Тв is prolonged, the separation of the β phase will cease and the stable equilibrium between the α crystals of the Сг and β composition will be established by crystals of the composition Сд. The quantitative ratio of the phases in this case is β/α = гв/вд. To further isolate the phase β, it is necessary to cool the alloy. When it is super cooled, for example, to the Те temperature, the α phase of the composition Сг is supersaturated by the component B (the degree of supersaturation of С3Си) and the phase β continues to be released due to the growth of the existing β crystals and the formation of new ones, which is possible with a large supersaturation of the solution. At the small supersaturations, there are few embryos, the β-phase crystals are large and equiaxial (Fig. 1 b).

With the acceleration of cooling, the separation of the β-phase begins at the high supersaturations (supercoolings) and embryos arise more. If they appear at the grain boundaries of the α phase, accelerated cooling leads to the formation of a boundary shell of the β phase (Fig. 1c).

The excess phase can be both liquid and gaseous. The appearance of the liquid at the boundaries of the crystals of the initial solution leads to the catastrophic drop in the strength of metal products. The strength is also reduced if the excess phase is gaseous. Especially dangerous hydrogen, the diffusion rate of which in metals is large and at the room temperature. The hydrogen dissolves in metals in the form of protons and atoms. With decreasing temperature, its solubility decreases. If the cooling is slow, hydrogen has time to diffuse to the surface of the products and escape into the atmosphere. With accelerated cooling, the solution is strongly supersaturated with hydrogen. Standing out in the defective parts of the crystals, the atomic hydrogen is converted into a molecular gas (it is milled), which leads to an increase in volume. At the points of hydrogen evolution, the pressure is greatly increased, and tears (flockenes) are formed in the crystals. Many metals are susceptible to hydrogen brittleness. To prevent it, the products are kept in a warm state for a long time and slowly cooled, which helps to remove hydrogen.

The rate of growth of the excess phase depends on the diffusion mobility of the B atoms in the initial solution. With the slow cooling, the atoms of component B have time to diffuse to the boundaries of the grains of the solid solution, where β crystals are usually formed. If the cooling is accelerated, the sections of the α-solid solution removed from the intergranular surface are so strongly supersaturated that it becomes possible to form β phase nuclei and in the bulk of the crystals of the initial solution on the defects present here.

The crystals of some intermediate phases can grow and are ordered, as a result of displacement of dislocations. The rate of growth is controlled by diffusion, and in this case, since the β phase increases with the influx of the atoms of component B to the interphase surface. As a result, the plate and needle (5-crystals coherent with the parent solution grow (Fig. 1 d).

With very rapid cooling (quenching), the separation of the β phase is prevented. The quenching solution formed during quenching is, however, unstable, and dispersion hardening becomes possible.

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