Phase diagram, what

The other place where the constitution is not fully defined is where there is a horizontal line on the phase diagram. The lead-tin diagram has one line like this - it runs across the diagram at 183°C and connects (Sn) of 2.5 wt% lead, L of 38.1% lead and (Pb) of 81% lead. Just above 183°C an alloy of tin -i- 38.1% lead is single-phase liquid (Fig. 3.5). Just below 183°C it is two-phase, (Sn) -i- (Pb). At 183°C we have a three-phase mixture of L -I- (Sn) -I- (Pb) but we can t of course say from the phase diagram what the relative weights of the three phases are. 

Consider the following phase diagram. What phases are present at points A through H Identify the triple point, normal boiling point, normal freezing point, and critical point. Which phase is denser, solid or liquid ... 

What is a phase diagram What useful information can be obtained from study of a phase diagram ... 

Find the existence area of Ice I in the bottom phase diagram of Fig. 6.6 and draw a temperature-pressure projection, similar to curve (c) above the phase diagram. What is the difference ... 

Whenever you have to report on the structure of an alloy - because it is a possible design choice, or because it has mysteriously failed in service - the first thing you should do is reach for its phase diagram. It tells you what, at equilibrium, the constitution of the alloy should be. The real constitution may not be the equilibrium one, but the equilibrium constitution gives a base line from which other non-equilibrium constitutions can be inferred. 

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

What defines the constitution of an alloy If you can t remember, refer back to the definition on p. 311 and revise. The phase diagram gives all three pieces of information. The first you know already. This section explains how to get the other two. 

Eutectics and eutectoids are important. They are common in engineering alloys, and allow the production of special, strong, microstructures. Peritectics are less important. But you should know what they are and what they look like, to avoid confusing them with other features of phase diagrams. 

The equilibrium phase diagram or solubility-supersolubility plot (Miers and Isaac, 1907), shown in Figure 3.1, provides a useful starting point for considering why crystallization occurs and what type of process might be most suitable for production of a particular substance. It can be divided into three zones (Ostwald, 1897)... 

We must start with fluid behavior to understand the basic concepts of unified chromatography. We must forget most of what we know from common experience about liquid and gas behavior since this experience is tied with ambient conditions. Instead, we must embrace the new possibilities afforded by temperatures and pressures that are different from ambient. This new view requires phase diagrams (17, 18). 

To understand what a phase diagram implies, consider first the curves AB and AC, and the line AD in Figure 9.5. Each of these lines shows the pressures and temperatures at which two adjacent phases are at equilibrium. 

In the three areas of the phase diagram labeled solid, liquid, and vapor, only one phase is present. To understand this, consider what happens to an equilibrium mixture of two phases when the pressure or temperature is changed. Suppose we start at the point on AB... 

A feature of the phase diagram in Fig. 8.12 is that the liquid-vapor boundary comes to an end at point C. To see what happens at that point, suppose that a vessel like the one shown in Fig. 8.13 contains liquid water and water vapor at 25°C and 24 Torr (the vapor pressure of water at 25°C). The two phases are in equilibrium, and the system lies at point A on the liquid-vapor curve in Fig. 8.12. Now let s raise the temperature, which moves the system from left to right along the phase boundary. At 100.°C, the vapor pressure is 760. Torr and, at 200.°C, it has reached 11.7 kTorr (15.4 atm, point B). The liquid and vapor are still in dynamic equilibrium, but now the vapor is very dense because it is at such a high pressure. 

The phase diagram for helium is shown here, (a) What is the maximum temperature at which superfluid helium-II can exist (b) What is the minimum pressure at which solid helium can exist (c) What is the normal boiling point of helium-I ... 

The phase diagram for carbon, shown here, indicates the extreme conditions that are needed to form diamonds front graphite, (a) At 2000 K, what is the minimum pressure needed before graphite changes into diamond (b) What is the minimum temperature at which liquid carhon can exist... 

Use the phase diagram for carbon dioxide (Fig. 8.7) to predict what would happen to a sample of carbon dioxide gas at —50°C and 1 atm if its pressure were suddenly increased to 73 atm at constant temperature. What would be the final physical state of the carbon dioxide ... 

Use the phase diagram for compound X below to answer these questions (a) Is X a solid, liquid, or gas at normal room temperatures (b) What is the normal melting point ol X ... 

In each of the composition diagrams in Fig. 14.2, the numbers represent a series of reactions run at a defined composition and temperature. These are isometric sulfur slices through three-dimensional K/P/RE/S quaternary phase diagrams. As just one example of what we have studied. Table 14.1 identifies the compositions at each point and the resulting phase(s). We have rigorously studied how phase formation is dependent upon the compositions of reactions for the rare-earth elements Y, Eu, and La and we have also discovered key structural relationships between the rare-earth elements, indicating a significant dependence on rare-earth and alkali-metal size for sulfides and selenides. 

What happens to a substance as temperature changes at constant pressure can be determined by drawing a horizontal line at the appropriate pressure on the phase diagram. 

Phase diagrams can be used to determine what phase of a substance is stable at any particular pressure and temperature. They also summarize how phase changes occur as either condition is varied. Example provides an illustration. 

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