Understanding Phases & Transitions in Iron-Carbon System: Equilibrium Phase Diagrams, Lecture notes of Materials Physics

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PHASES

• (^) PHASE is simply a form of material possessing a characteristic structure and characteristic properties.
• (^) PHASE has a definable structure, a uniform and identifiable chemistry (also known as composition), and distinct boundaries or interfaces that separate it from other different phases.
• (^) PHASE can be continuous (like the air in a room) or discontinuous (like grains of salt in a shaker).
• (^) PHASE can be solid, liquid, or gas.
• (^) PHASE can be a pure substance or a solution, provided that the structure and composition are uniform throughout. EXAMPLE: Alcohol and water - form a single phase when combined; no boundaries across which structure and/or chemistry changes. Oil and water - form isolated regions with distinct boundaries and must be regarded as two distinct phases. Ice cubes in water - another two-phase system, since there are two distinct structures with interfaces between them.

TEMPERATURE–COMPOSITION DIAGRAMS

Most engineering processes are conducted at

atmospheric pressure, and variations are more likely to

occur in temperature and composition. The most

useful mapping, therefore, is usually a temperature–

composition phase diagram at 1 atmosphere pressure.

The temperature range often

includes only solids and liquids,

since few processes involve

engineering materials in the

gaseous state.

COOLING CURVES

If the cooling history is plotted in the form of a

temperature-versus-time plot, known as a

cooling curve , the transitions in structure will

appear as characteristic points, such as slope

changes or isothermal (constant-temperature) holds.

SOLUBILITY STUDIES

Both pure materials have a distinct melting point, below which they appear as a pure solid. As we move away from a pure material, we often encounter a single-phase solid solution, in which a small amount of one component is dissolved and dispersed throughout the other. If there is a limit to this solubility, there will be a line in the phase diagram, known as a solvus line , denoting the conditions where the single-phase solid solution becomes a two-phase mixture. Figure 4-5 presents the equilibrium phase diagram for the lead– tin system, using the conventional notation in which Greek letters are used to denote the various single-phase solids. The upper portion of the diagram closely resembles the salt–water diagram, but the partial solubility of one material in the other can be observed on both ends of the diagram.

COMPLETE SOLUBILITY IN BOTH LIQUID AND SOLID STATES If two materials are completely soluble in each other in both the liquid and solid states , a rather simple diagram results, like the copper–nickel diagram of Figure 4-6. The upper line is the liquidus line, the lowest temperature for which the material is 100% liquid. Above the liquidus, the two materials form a uniform- chemistry liquid solution. The lower line, denoting the highest temperature at which the material is completely solid, is known as a solidus line. Below the solidus, the materials form a solid-state solution in which the two types of atoms are uniformly distributed throughout a single crystalline lattice. Between the liquidus and solidus is a freezing range, a two-phase region where liquid and solid solutions coexist.

INSOLUBILITY If one or both of the components are totally insoluble in the other, the diagrams will also reflect this phenomenon. Figure 4-7 illustrates the case where component A is completely insoluble in component B in both the liquid and solid states.

SOLIDIFICATION OF

ALLOY X

At temperature t1 the first minute amount of solid forms with the

chemistry of point S. As the temperature drops, more solid forms ,

but the chemistries of both the solid and liquid phases shift to follow

the tie-line endpoints. The chemistry of the liquid follows the liquidus

line, and the chemistry of the solid follows the solidus. Finally, at

temperature t3 solidification is complete , and the composition of

the single-phase solid is now that of alloy X, as required.

THREE-PHASE

REACTIONS

These lines are further characterized by either a V intersecting from above or an inverted-V intersecting from below. The intersection of the V and the line denotes the location of a threephase equilibrium reaction.

One common type of three-phase reaction, known as a eutectic, has already been observed in Figures below

Alloys with the eutectic composition have the lowest melting point of all neighboring alloys and generally possess relatively high strength. For these reasons, they are often used as casting alloys or as filler material in soldering or brazing operations. The eutectic reaction can be written in the general form of:

INTERMETALLIC

COMPOUNDS

If components A and B form a compound, and the compound cannot

tolerate any deviation from its fixed atomic ratio, the product is known

as a stoichiometric intermetallic compound and it appears as a single

vertical line in the diagram. (Note:This will be seen for the Fe3C

iron carbide at 6.67 wt.% carbon in the upcoming iron–carbon

equilibrium diagram.)

If some degree of chemical deviation is tolerable, the vertical line

expands into a single-phase region, and the compound is known as a

nonstoichiometric intermetallic compound.

THE IRON CARBON DIAGRAM

IRON–CARBON EQUILIBRIUM DIAGRAM Steel, composed primarily of iron and carbon, is clearly the most important of the engineering metals. For this reason, the iron– carbon equilibrium diagram assumes special importance. The diagram most frequently encountered, however, is not the full iron–carbon diagram but the iron–iron carbide diagram. The iron-carbon equilibrium phase diagram. Single phases are: ferrite, austenite, delta-ferrite, cementite