The polymorphic transformations in alloys (Part 4).

The polymorphic transformations in alloys lying outside the eutectoid (KML) and peritectoid (MKL) lines do not differ from the one considered earlier (see Fig. 1).

The transformations begin with the appearance of embryos of the new modification and end with a complete transition of the high-temperature modification β into one of the low-temperature α1 or α2. The polymorphic transformation also begins in alloys within the limits of concentrations. СкСл (Fig. 2а) and СмСл (Fig. 2б), however, in the future it is complicated by eutectoid (β → α1 + α2) or peritectoid (β + α2 → α1) transformations.

All phases involved in the eutectoid or peritectoid transformation are solid. During the peritectoid transformation that occurs when the alloy is cooled, a third solid phase forms from the two solid phases. Two solid phases can transform into one and in eutectoid alloys, but only with heating (α1 + α2 → β). If in peritectoid alloys one solid phase passes into the other two under heating, then in eutectoid alloys this occurs upon cooling.

The restructuring of the lattice with the change in pressure or temperature is also possible in intermediate phases. Two modifications have, for example, ZnS, CsCl, three TiO2, SiO2. These transformations differ little from the transformations of solid solutions: the restructuring is possible in an unordered and ordered way, with redistribution of components between phases and without it. When changing the packing of atoms in intermediate phases, eutectoid and peritectoid transformations are also possible.

Polymorphic transformations in pure metals and alloys greatly complicate the phase diagrams. This is particularly true for systems with three or more components, all of which can be polymorphic. Double charts also become more complicated. As an example, let us give a diagram of the state of alloys, one of whose components (A) has three modifications, and the other (B) is not polymorphic (Fig. 2c).

Below the solidus line there are seven regions of single-phase states: in three – solid solutions of component B are stable in different modifications of component A – α1, α2 and α3; in one – solid solution A in B (β) and in three – intermediate phases δ, γ and ε. In the system, eight three-phase equilibrium are possible: two peritectoid (α3 + α2 + Ж and α2 + δ + Ж), two eutectic (δ-Ж + γ and γ + Ж + β), two peritectoid (α2 + α1 + γ and α1 + Ε + γ) and two eutectoid (α2 + δ + γ and ε + + γ + β). The presence of polymorphic transformations in metals and alloys expands the ways of influencing the phase composition, structure and properties of metal products. Changing the conditions of heating and cooling, decoupling some processes and delaying others, it is possible to obtain metals and alloys with different structure and properties. Many methods of heat treatment are based on recrystallization and are widely used in engineering.

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