Binary phase diagrams building blocks

Figure 2.12a. Building blocks of binary phase diagrams examples of single-phase (two-variant) and two-phase (mono-variant) fields. In the figure the indication is given of the phases existing in the various fields and respectively of their number. The phase equilibrium composition in the two-phase fields is defined by the boundary (saturation) lines of the single-phase regions. (Pt), (Ag),...

Figure 2.12b. Building blocks of binary phase diagrams examples of two-phase (mono-variant) reactions. On the left the transformation, on cooling, from the a phase to (3 is shown. For the composition in equilibrium conditions the transformation starts at temperature 7) and ends at Tf. Notice that at an intermediate temperature as Tm only a partial transformation has been carried out a certain quantity of a (having the composition corresponding to a ) will coexist with [3 (of composition [3 ). On the right, the a phase undergoes, on cooling, a de-mixing in the two a and a" phases having at temperature 7) the compositions x and x", at temperature T2 the compositions x2, xt2, etc.

Figure 2.13. Building blocks of binary phase diagrams examples of three-phase (invariant) reactions. In the upper part the general appearance, inside a phase diagram, of the two types of invariant equilibria is presented, that is, the so-called 1 st class (or eutectic type) and the 2nd class (or peritectic type) equilibria. In the lower part the various invariant equilibria formed by selected binary alloys for well-defined values of temperature and composition are listed. In the Hf-Ru diagram, for instance, three 1 st class equilibria may be observed, 1 (pHf) — (aHf) + HfRu (eutectoid, three solid phases involved), 2 L — (3Hf + HfRu (eutectic), 3 L —> HfRu + (Ru) (eutectic).

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