Microstructure equilibrium cooling

ABC ternary system forms three binary eutectics and a ternary eutectic as shown below. Discuss equilibrium cooling paths for the overall compositions p, q and r indicated in the diagram. Discuss also the change in microstructure that should occur during cooling. 

Thus, during equilibrium cooling, the composition of the solid will mn down the solidus line, ri to S2 to S3, and so on, and the composition of the liquid in equilibrium with the solid runs down the liquidus from Zi to I2 to I3, and so on, as the liquid cools. The composition of the solid phase when all of the liquid has solidified will be equal to that of the original liquid phase. Not only does the composition of the solid and liquid phases change continuously as the temperature falls through the two-phase region, but the number of small crystals present also increases. When temperature T4 is reached, the microstructure of the solid consists of crystallites or grains... 

Fig.1 Typical microstructure of a 95Sn-3.5Ag solder resulting from non-equilibrium cooling showing primary dendrites of Sn-rich solid solution and interdendritic space filled with acicular AgsSn in a Sn-rich solid solution matrix.

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. 

Carbon steels as received "off the shelf" have been worked at high temperature (usually by rolling) and have then been cooled slowly to room temperature ("normalised"). The room-temperature microstructure should then be close to equilibrium and can be inferred from the Fe-C phase diagram (Fig. 11.1) which we have already come across in the Phase Diagrams course (p. 342). Table 11.1 lists the phases in the Fe-FejC system and Table 11.2 gives details of the composite eutectoid and eutectic structures that occur during slow cooling. 

Fig. 18. Continuous-cooling transformation diagram for a Type 4340 alloy steel, with superimposed cooling curves illustrating the manner in which transformation behavior during continuous cooling governs final microstructure (1). Ae3 is critical temperature at equilibrium. Ae1 is lower critical...

Alloys with compositions less than the point where the a transus meets the composition axis are termed a alloys. The CP alloy discussed in this chapter is an example of an a alloy. Those with compositions greater than the point where the 3 transus meets the axis are termed 3 alloys. Those with compositions in between have a microstructure of a and 3 phases at ambient temperature under equilibrium conditions. Two types of these alloys can be identified. One type has composition limits between the a transus and the Mj curve and can be described by the term a-P alloy. The T1-6A1-4V alloy discussed later in this chapter is a common a-p alloy. The second type is given the name metastable p alloy. Composition limits for metastable p alloys fall between the and the p transus. Metastable beta alloys can best be described as alpha-beta alloys that contain an appreciable level of beta stabilizers. The low difhisivity of the beta stabilizers promotes complete retention of beta phase to room temperature at moderate cooling rates. The Ti-15V-3Cr-3Al-3Sn and Beta 21-S alloys are common metastable beta alloys. 

In Fig. 5.6b, the microstructure appears in the form of two highly intertwined worm-like matrices of relatively uniform dimensions (widths on the micrograph). The compositional differences between adjacent worms grow with time until equilibrium compositions (those having the lowest free energy) are achieved, or until the phase separation is arrested by the cooling process. 

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. 

At this point it is instructive to examine the development of microstructure that occurs for isomorphous alloys during solidification. We first treat the situation in which the cooling occurs very slowly, in that phase equilibrium is continuously maintained. 

Figure 9.11 Schematic representations of the equilibrium microstructures for a lead-tin alloy of composition Q as it is cooled from the liquid-phase region.

Several of the various microstructures that may be produced in steel alloys and their relationships to the iron-iron carbon phase diagram are now discussed, and it is shown that the microstructure that develops depends on both the carbon content and heat treatment. This discussion is confined to very slow cooling of steel alloys, in which equilibrium is continuously maintained. A more detailed exploration of the influence of heat treatment on microstructure, and ultimately on the mechanical properties of steels, is contained in Chapter 10. 

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. 

For iron-carbon alloys (i.e., steels), an understanding of microstructures that develop during relatively slow rates of cooling (i.e., pearlite and a proeutectoid phase) is facilitated by the iron-iron carbide phase diagram. Other concepts in this chapter were presented as a prelude to the introduction of this diagram—the concepts of a phase, phase equilibrium, metastability, and the eutectoid reaction. In Chapter 10, we explore other microstructures that form when iron-carbon alloys are cooled from elevated temperatures at more rapid rates. These concepts are summarized in the following concept map ... 

---