Monotectic and Monotectoid Systems

A schematic showing the construction of a monotectic phase diagram from free energy curves. 

As a system with monotectic composition is solidified from the melt, the liquid transforms to a plus B-rich melt (L2) at the monotectic temperature. Again the relative amoimts are determined by the lever rule. This second phase liquid eventually transforms to a eutectic microconstituent at T. Since the B-rich phase is incorporated into the a phase as a liquid, it tries to reduce its interfadal energy by spherodizing. Therefore the final eutectic microconstiuents are generally foimd in the form of spherical particles. 

Attempts to form alloys with compositions under the two-liquid dome will result in an almost completely segregated s) tem because of the density differences between the two immiscible liquid phases. Even attempts to form such alloys under microgravity conditions led to similar results, but for a different reason. Instead of buoyancy effects, the phases became separated because of their relative interfadal energy differences with the container walls (see Cahn, J.W., /. Chem. Phys.). 

A number of binary systems will form an AB solid solution at higher temperatures because of the entropy of mixing term, but will become immisdble at lower temperatures, segregating into ai, an A-rich AB solid solution and ol, a B-rich solid solution. In some cases a monotectoid reaction, U2 ai + p, will also occur. Examples include Al-Zn, Hf-Ta, Nb-Zr, and Nb-U. The monotectoid reaction in the Al-Zn system is shown in Eigure 12.26. 

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