Phase Diagrams

Phase Diagrams:

The properties of an alloy do not depend only on concentration of the phases but how they are arranged structurally at the microscopy level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space. 

A binary alloy may be a single solid solution two separated, essentially pure components. 

two separated solid solutions. a chemical compound, together with a solid solution. 

The way to tell is to cut the material, polish it to a mirror finish, etch it a weak acid (components etch at a different rate) and observe the surface under a microscope. 

Phase Equilibria: Equilibrium is the state of minimum energy. It is achieved given sufficient time. But the time to achieve equilibrium may be so long (the kinetics is so slow) that a state that is not at an energy minimum may have a long life and appear to be stable. This is called a metastable state. A less strict, operational, definition of equilibrium is that of a system that does not change with time during observation. 

Equilibrium Phase Diagrams: Give the relationship of composition of a solution as a function of temperatures and the quantities of phases in equilibrium. These diagrams do not indicate the dynamics when one phase transforms into another. Sometimes diagrams are given with pressure as one of the variables. In the phase diagrams we will discuss, pressure is assumed to be constant at one atmosphere. 

Binary Isomorphous Systems: This very simple case is one complete liquid and solid solubility, an isomorphous system. The example is the Cu-Ni alloy of Fig. 9.2a. The complete solubility occurs because both Cu and Ni have the same crystal structure (FCC), near the same radii, electronegativity and valence. The liquidus line separates the liquid phase from solid or solid + liquid phases. That is, the solution is liquid above the liquidus line. The solidus line is that below which the solution is completely solid (does not contain a liquid phase.) 

Interpretation of phase diagrams

Concentrations: Tie-line method locate composition and temperature in diagram. In two phase region draw tie line or isotherm 

note intersection with phase boundaries. Read compositions. 

Fractions: lever rule construct tie line (isotherm) obtain ratios of line segments lengths. 

Note: the fractions are inversely proportional to the length to the boundary for the particular phase. If the point in the diagram is close to the phase line, the fraction of that phase is large. 

Development of microstructure in isomorphous alloys 

a) Equilibrium cooling: Solidification in the solid + liquid phase occurs gradually upon cooling from the liquidus line. The composition of the solid and the liquid change gradually during cooling (as can be determined by the tie-line method.) Nuclei of the solid phase form and they grow to consume all the liquid at the solidus line. 

b) Non-equilibrium cooling: Solidification in the solid + liquid phase also occurs gradually. The composition of the liquid phase evolves by diffusion, following the equilibrium values that can be derived from the tie-line method. However, diffusion in the solid state is very slow. Hence, the new layers that solidify on top of the grains have the equilibrium composition at that temperature but once they are solid their composition does not change. This lead to the formation of layered (cored) grains (Fig) and to the invalidity of the tie-line method to determine the composition of the solid phase (it still works for the liquid phase, where diffusion is fast.) 

Binary Eutectic Systems Interpretation: Obtain phases present, concentration of phases and their fraction (%).

Solvus line: limit of solubility Eutectic or invariant point. Liquid and two solid phases exist in equilibrium at the eutectic composition and the eutectic temperature.

Note:  the melting point of the eutectic alloy is lower than that of the components (eutectic = easy to melt in Greek). At most two phases can be in equilibrium within a phase field. Single-phase regions are separated by 2-phase regions. 

The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram:

This is one of the most important alloys for structural applications. The diagram Fe—C is simplified at low carbon concentrations by assuming it is the Fe—Fe3C diagram. Concentrations are usually given in weight percent. The possible phases are: 

a-ferrite (BCC) Fe-C solution 

g-austenite (FCC) Fe-C solution 

d-ferrite (BCC) Fe-C solution 

liquid Fe-C solution 

Fe3C (iron carbide) or cementite. An intermetallic compound. 

The maximum solubility of C in a- ferrite is 0.022 wt%. d-ferrite is only stable at high temperatures. It is not important in practice. Austenite has a maximum C concentration of 2.14 wt %. It is not stable below the eutectic temperature (727 C) unless cooled rapidly (Chapter 10). Cementite is in reality metastable, decomposing into a-Fe and C when heated for several years between 650 and 770 C. 

For their role in mechanical properties of the alloy, it is important to note that: 

1. Ferrite is soft and ductile

2. Cementite is hard and brittle

Thus, combining these two phases in solution an alloy can be obtained with intermediate properties. (Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.) 

Development of Microstructures in Iron—Carbon Alloys

1. The eutectoid composition of austenite is 0.76 wt %. When it cools slowly it forms perlite, a lamellar or layered structure of two phases: a-ferrite and cementite (Fe3C). 

2. Hypoeutectoid alloys contain proeutectoid ferrite plus the eutectoid perlite. 

3. Hypereutectoid alloys contain proeutectoid cementite plus perlite. 

4. Since reactions below the eutectoid temperature are in the solid phase, the equilibrium is not achieved by usual cooling from austenite.