Microstructure nonequilibrium cooling

Binary phase diagrams are maps that represent the relationships between temperature and the compositions and quantities of phases at equilibrium, which influence the microstructure of an alloy. Many microstructures develop from phase transformations, the changes that occur when the temperature is altered (typically upon cooling). This may involve the transition from one phase to another or the appearance or disappearance of a phase. Binary phase diagrams are helpful in predicting phase transformations and the resulting microstructures, which may have equilibrium or nonequilibrium character. 

Some of the consequences of nonequilibrium solidification for isomorphous alloys will now be discussed by considering a 35 wt% Ni-65 wt% Cn alloy, the same composition that was used for equilibrium cooling in the previons section. The portion of the phase diagram near this composition is shown in Figme 9.5 in addition, microstructures and associated phase compositions at varions temperatures upon cooling are noted in the circular insets. To simplify this discussion, it will be assumed that diffusion rates in the liquid phase are sufficiently rapid snch that equilibrium is maintained in the liquid. 

In this discussion of the microstructural development of iron-carbon alloys, it has been assumed that, upon cooling, conditions of metastable equilibrium have been continuously maintained that is, sufficient time has been allowed at each new temperature for any necessary adjustment in phase compositions and relative amounts as predicted from the Fe-FejC phase diagram. In most situations these cooling rates are impracti-cally slow and unnecessary in fact, on many occasions nonequilibrium conditions are desirable. Two nonequilibrium effects of practical importance are (1) the occurrence of phase changes or transformations at temperatures other than those predicted by phase boundary lines on the phase diagram, and (2) the existence at room temperature of nonequilibrium phases that do not appear on the phase diagram. Both are discussed in Chapter 10. 

The characteristics of the nonequilibrium, Pb-Sn solder microstructure are threefold. First, the compositions of the phases are not accurately represented by the phase diagram boundary lines. Second, the relative quantities of the phases are not accurately described by the lever rule. Third, the spatial distribution of phases in the microstructure are sensitive to the cooling rate. For example, in terms of individual phase compositions, excessive Sn may be retained in the Pb-rich a phase, causing a supersaturation condition. In effect, the phase boundary line between the a single-phase field and the (a -I- p) two-phase field is shifted to the right. (A more detailed discussion of approximating a nonequilibrium microstructure from the equilibrium phase diagram can be found in Refs. 5 and 7.) A consequence to the solder microstructure caused by a supersaturated Pb-rich phase is the precipitation of Sn particles in the Pb-rich phase. 

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