Monotectic reaction

Al-Pb. Both lead [7439-92-17, Pb, and bismuth [7440-69-9] Bi, which form similar systems (Fig. 17), are added to aluminum ahoys to promote machinahility by providing particles to act as chip breakers. The Al—Pb system has a monotectic reaction in which Al-rich Hquid free2es partiahy to soHd aluminum plus a Pb-rich Hquid. This Pb-rich Hquid does not free2e until the temperature has fahen to the eutectic temperature of 327°C. SoHd solubiHty of lead in aluminum is negligible the products contain small spherical particles of lead which melt if they are heated above 327°C. 

For the niobium-copper system different phase diagrams of the simple eutectic type (with the eutectic point very close to Cu) have been proposed, either with an S-shaped near horizontal liquidus line or with a monotectic equilibrium. It was stated that the presence of about 0.3 at.% O can induce the monotectic reaction to occur, whereas if a lesser amount of oxygen is present no immiscibility gap is observed in the liquid. 

This kind of reaction is called a monotectic reaction and the point m is referred to as die monotectic point. 

In contrast to eutectic systems, in which both components solidify below eutectic temperature, a monotectic reaction is characterized by the breakdown of a liquid into one solid and one liquid phase (Singh et al, 1985), i.e. one liquid phase decomposes into a solid phase and a liquid phase when the temperature is below the monotectic temperature. Figure 1(C) shows the phase diagram of a typical monotectic system. The monotectic composition is determined by the intersection of a liquidus line and a liquid miscibility gap (Singh et al., 1985). Generally, monotectic systems are less studied than eutectic systems. 

Monotectic reactions, in which a liquid decomposes into a solid and a second liquid, can occur in systems that have excess heats of mixing in both the solid and liquid phase. Monotectic systems also have a region of liquid phase immiscibility. Unless done very rapidly, attempts to solidify through this two-phase liquid region will result in almost complete phase separation because of interfacial effects. Monotectoid reactions are also possible in which an AB solid solution separates into an A-rich solid solution and a B-rich (3 phase. 

Monotectic reactions in which a liquid phase decomposes to form a solid and another liquid phase (Fig. 5.15) have also received little attention. The experimental work of Delves and the theoretical treatment of Chadwick showed that either regular rod-like structures or macroscopic phase separation should occur, depending on the various interphase surface energies. Subsequently Livingston and Cline,working with copper-lead alloys, established that the growth conditions also had a marked influence on the nature of the transformation structure. Their results are summarised in Fig. 5.16. It would be valuable if these studies could be extended to other suitable systems. 

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