Chapter 9. Phase Diagrams


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9.1 Introduction

    Definitions

    Component: pure metal or compound (e.g., Cu, Zn in Cu-Zn alloy, sugar, water, in a syrup.)

    Solvent: host or major component in solution.

    Solute: dissolved, minor component in solution.

    System: set of possible alloys from same component (e.g., iron-carbon system.)

    Solubility Limit: Maximum solute concentration that can be dissolved at a given temperature.

    Phase: part with homogeneous physical and chemical characteristics

9.2 Solubility Limit

    Effect of temperature on solubility limit. Maximum content: saturation. Exceeding maximum content (like when cooling) leads to precipitation.

9.3 Phases

    One-phase systems are homogeneous. Systems with two or more phases are heterogeneous, or mixtures. This is the case of most metallic alloys, but also happens in ceramics and polymers.

    A two-component alloy is called binary. One with three components, ternary.

9.4 Microstructure

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

  1. a single solid solution
  2. two separated, essentially pure components.
  3. two separated solid solutions.
  4. 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.

9.5 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.

9.6 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

    1. locate composition and temperature in diagram
    2. In two phase region draw tie line or isotherm
    3. note intersection with phase boundaries. Read compositions.

Fractions: lever rule

  1. construct tie line (isotherm)
  2. 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. 9.14) 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.)

9.7 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.

Development of microstructure in eutectic alloys

Case of lead-tin alloys, figures 9.9–9.14. A layered, eutectic structure develops when cooling below the eutectic temperature. Alloys which are to the left of the eutectic concentration (hipoeutectic) or to the right (hypereutectic) form a proeutectic phase before reaching the eutectic temperature, while in the solid + liquid region. The eutectic structure then adds when the remaining liquid is solidified when cooling further. The eutectic microstructure is lamellar (layered) due to the reduced diffusion distances in the solid state.

To obtain the concentration of the eutectic microstructure in the final solid solution, one draws a vertical line at the eutectic concentration and applies the lever rule treating the eutectic as a separate phase (Fig. 9.16).

9.8 Equilibrium Diagrams Having Intermediate Phases or Compounds

    A terminal phase or terminal solution is one that exists in the extremes of concentration (0 and 100%) of the phase diagram. One that exists in the middle, separated from the extremes, is called an intermediate phase or solid solution.

    An important phase is the intermetallic compound, that has a precise chemical compositions. When using the lever rules, intermetallic compounds are treated like any other phase, except they appear not as a wide region but as a vertical line.

9.9 Eutectoid and Peritectic Reactions

    The eutectoid (eutectic-like) reaction is similar to the eutectic reaction but occurs from one solid phase to two new solid phases. It also shows as V on top of a horizontal line in the phase diagram. There are associated eutectoid temperature (or temperature), eutectoid phase, eutectoid and proeutectoid microstructures.

    Solid Phase 1 Solid Phase 2 + Solid Phase 3

    The peritectic reaction also involves three solid in equilibrium, the transition is from a solid + liquid phase to a different solid phase when cooling. The inverse reaction occurs when heating.

    Solid Phase 1 + liquid Solid Phase 2

9.10 Congruent Phase Transformations

    Another classification scheme. Congruent transformation is one where there is no change in composition, like allotropic transformations (e.g., a-Fe to g-Fe) or melting transitions in pure solids.

9.11 Ceramic and Ternary Phase Diagrams

    Ternary phase diagrams are three-dimensional. Ceramic phase diagrams will be discussed in Ch. 13.

9.12 The Gibbs Phase Rule (not discussed)

    The Iron–Carbon Diagram

9.13 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:

Ferrite is soft and ductile

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.)

9.14 Development of Microstructures in Iron—Carbon Alloys

    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).

    Hypoeutectoid alloys contain proeutectoid ferrite plus the eutectoid perlite. Hypereutectoid alloys contain proeutectoid cementite plus perlite.

    Since reactions below the eutectoid temperature are in the solid phase, the equilibrium is not achieved by usual cooling from austenite. The new microstructures that form are discussed in Ch. 10.

9.15 The Influence of Other Alloying Elements

As mentioned in section 7.9, alloying strengthens metals by hindering the motion of dislocations. Thus, the strength of Fe–C alloys increase with C content and also with the addition of other elements.

 

Terms:

Austenite
Cementite
Component
Congruent transformation
Equilibrium
Eutectic phase
Eutectic reaction
Eutectic structure
Eutectoid reaction
Ferrite
Free energy
Hypereutectoid alloy
Hypoeutectoid alloy
Intermediate solid solution
Intermetallic compound
Invariant point
Isomorphous
Lever rule
Liquidus line
Metastable
Microconstituent
Pearlite
Peritectic reaction
Phase
Phase diagram
Phase equilibrium
Primary phase
Proeutectoid cementite
Proeutectoid ferrite
Solidus line
Solubility limit
Solvus line
System
Terminal solid solution
Tie line