A phase is a physically distinct and homogeneous portion of a materiel. The phrase "physically distinct" means means that a phase is a distinct in structure and/or composition. The solid, liquid and gas phases are well known and often considered by many to be the three possible states of matter, i.e. "phases of matter." However, these phases are distinct in structure only. The most common example of those three phases can be found in water, e.g. solid ice, liquid water and gaseous vapor. The material does not change composition when changing from one structure to the other. A less common (or less recognized) phase distinction based on structure is that of iron. Iron in the solid state can exist in three crystal structures. These are: (1) a body-centered-cubic arrangement of atoms which exists at temperatures below 916° C, (2) a face-centered-cubic arrangement which exists at temperatures from 916 °C to 1294° C, and (3) a second body-centered-cubic form, of different lattice parameter than the low temperature b.c.c. phase, from 1294 °C to the melting point. Thus a solid may exist in more than one phase, and in fact many metals e.g. tin, cobalt, titanium, zirconium, uranium, possess this characteristic, defined as polymorphism, i.e. many structures.
With respect to the phase distinction based on composition, an example is the two face-centered-cubic metals copper and silver. Even though their structures are identical, i.e. both are f.c.c. their compositions are distinct. Another case is the Cu-Zn system. Here copper, a face-center-cubic structure, can dissolve up to approximately 38% zinc without changing crystal structure. This phase is referred to as "alpha brass" and includes pure copper; we can think of copper as an a-brass with zero zinc content. Yet, if the 38% zinc solubility limit is exceeded, a second Cu-Zn phase forms which can accommodate additional zinc by re-arranging its structure to a body-centered-cubic structure called "beta brass." Now we have the situation where the phases are distinguishable by both composition and structure.
B. Phase Diagrams
Phase diagrams are graphical representations of what phases exist with
respect to some variables, typically temperature, composition and pressure.
Pressure only has significant effect when dealing with gaseous phases. Metallic
phase diagrams usually deal only with temperature and composition, ignoring
pressure. In alloy systems it is basically a map, that depicts the phases that
can exist in equilibrium at any combination of temperatures and composition. A
familiar phase diagram with one variable is that of the phases of water versus
temperature (Fig. 1). To account for the effects of adding salt, a two-variable
phase diagram would be required (Fig. 2). Someone working on sea water
desalination by distillation might use a three-variable phase diagram showing
the effects of pressure as well, since the gas phase is involved..
Binary metallurgical phase diagrams indicate the phases present at equilibrium for alloys of two metals as a function of alloy composition and temperature. Different solid phases are usually identified by Greek letters, commencing with alpha on the left and proceeding right, with respect to composition. (This convention may not apply when more than one solid phase exists with respect to temperature.) Two of the lines indicating phase boundaries are given specific names; the liquidus above which there is nothing but a homogeneous liquid phase; and the solidus, below which there are nothing but solid phases.
C. Cooling Curves and Phase Diagrams
All pure crystalline solids, including metals, have the property of melting
and freezing at a single temperature. If a pure metal is melted and allowed to
cool slowly, its cooling curve, i.e. temperature vs. time behavior will resemble
those labeled A and D in Fig. 3. The metal's temperature decreases due to the
thermal conduction of the metal (both liquid and solid forms) and the difference
in temperature between the metal and the environment.* At the
melting/freezing point the heat loss continues, but the temperature stops
dropping; the heat of fusion released by the solidifying metal exactly balancing
the heat lost to the environment. This "thermal arrest" in the metal's cooling
curve is exactly analogous to an ice bath; as long as there is metal in both the
solid and liquid phases the temperature will remain constant at the metal's
melting/freezing point. As soon as the last bit of liquid solidifies the
temperature drop will resume, although at a somewhat slower rate due to the
lower thermal gradient and conductivity of the metal.
Figure 3 shows a phase diagram of two metals that are completely soluble in the solid state, in this case, palladium and silver.** Above the liquidus curve there exists a homogeneous liquid solution phase, and below the solidus there exists a homogeneous solid solution. If one cools a liquid alloy solution of 30 weight percent silver (see cooling curve B), solidification will commence at point M and be completed at point N. Thus, from the cooling curves of various compositions the liquidus and solidus curves can be determined, which is all that is required for a complete phase diagram for alloys which exhibit complete solid solubility.
Some combinations of metals have an eutectic composition (Greek for "easy melting"), the eutectic being the alloy composition that has the lowest melting point (minimum in the liquidus curve), and the property of melting/freezing at a single temperature like a pure metal. Thus, the cooling curve of an alloy of eutectic composition will look like that of a pure metal. It is also characteristic of an eutectic that it transforms from a single liquid phase to as many solid
* Newton's law of cooling; the rate of heat loss is proportional to the difference in temperature.
**Both the palladium and silver has a face-centered-cubic arrangement of atom
sites. A solid solution means that some of these sites are occupied by palladium
atoms while the others are occupied by silver atoms.
phases as there are metallic components in the alloy. In the case of
binary alloys, such as copper-silver, there will be two solid phases upon
complete solidification of the eutectic composition. This typically
results in a finely laminated "eutectic structure." (A eutectic should not
be confused with an eutectoid; the eutectoid transforms from a single
solid phase to as many different solid phases (distinct from the initial phase)
as there are metalic components in the alloy and typically forms a finely
laminated structure,* but does so entirely in the solid state, starting and
finishing as a solid.)
*The best known eutectoid and eutectoid structure is the 0.80% carbon alloy of steel and the pearlite that it forms. The initial state in this reaction is g-Fe, austenite, and the final state is a-Fe + Fe3C, i.e., ferrite and cementite. The ferrite and cementite lamellar microstructure is also known as pearlite.
The construction of an eutectic phase diagram from cooling curves is
depicted for the copper-silver alloy system in Figure 4. The pure metal
compositions, A and F, and the eutectic, D, have cooling curves with a "thermal
arrest" only, i.e. no other break in the curve. In alloy compositions other than
the eutectic and the pure metal, different phases of the alloy melt/freeze at
different temperatures, resulting in a "mushy" melting zone that spans a range
of temperatures. Although the slope of the cooling curve decreases due to the
heat released from the solidifying metal, there is no thermal arrest in this
zone since there is no pure metal or eutectic to cause one (Fig. 4, curves B, C
& E. As one phase of the alloy solidifies out preferentially, the
composition of the solid and liquid change continuously with temperature. The
equilibrium composition of the solid and liquid for any particular alloy and
temperature can be determined by drawing a horizontal line through the
alloy/temperature point, e.g. point O, Figure 4, on the equilibrium phase
diagram; vertical lines dropped from the liquidus (point N) and solidus (point
M) intersects indicate the composition of the liquid and solid respectively.
When the last bit of liquid has solidified the cooling curve becomes
steeper since there is no longer any heat of fusion being released. The slope of
the curve after solidification will be somewhat less steep than before
solidification since the solid metal has a lower thermal conductivity and
difference in temperature from the environment.
For some alloys of an eutectic forming alloy system, the compositional shifts that occur during solidification will cause the last bit of liquid to solidify to reach eutectic composition. This liquid, being of eutectic composition, will behave like a eutectic and will cause a thermal arrest at the eutectic temperature until all of it has solidified (Fig. 4, curves C and E). The cooling curve will then steepen and continue as with other alloys.
The eutectic phase diagram consists of six regions as follows:
One region of the "mushy" two phase region of alpha solid solution plus liquid.
One region of the "mushy" two phase region of beta solid solution plus liquid.
One region of solid single phase, i.e. all alpha single phase solid solutions.
One region of solid phase, i.e. all beta single phase solid solutions.
One region of two phase alpha plus beta solid solutions.
D. Microstructures
If one were to examine the alloys discussed above under the microscope after
solidification, one would see the results of the sequence of solidification as
depicted by the phase diagram. The pure metal "A" and "F" compositions would
show a single phase structure which under the microscope shows only grain
boundaries. The composition "B" would show only the single phase alpha solid
solution of silver in copper, which again under the microscope would show only
grain boundaries. (Note: If the alloy cooled at a very slow rate, the beta phase
would commence to precipitate out when the alpha solid solution boundary is
crossed at about 650 °C. Under typical cooling rates in most operations the beta
phase would not have time to form.) The formation consisting of alternate layers
of alpha and beta. The microstructure for the "C" composition would show large
chunks of solid alpha, often called primary or proeutectic alpha, which
precipitates out of the liquid as it cools through the "mushy" range, surrounded
by alternate layers of the laminated alpha and beta eutectic formation which
solidifies at constant temperature during the terminal arrest. A similar
microstructure would be observed for composition "E" except we would see chunks
of primary beta rather than alpha. The micrograph below shows a typical
eutectic structure with a primary or proeutectic phase.
LEAD TIN PHASE DIAGRAM:
A. Introduction
The equilibrium phase diagram of the lead-tin system is similar to that of the copper-silver system shown in Figure 4. Due to the low melting point of these alloys, often used as solders, they can be easily melted and cooled in the laboratory.
Lead has a face-centered-cubic (FCC) crystal structure, and tin a body-centered-tetragonal crystal structure (BCT). These two metals form a simple eutectic system, as shown in Figure 5.
The Pb-Sn eutectic consists of two solid phases, an alpha phase (FCC structure rich in Pb) and a beta phase (BCT structure rich in Sn). If a sample of pure Pb or pure Sn is solidified, a thermal arrest will occur, which means no variation in temperature will take place over the time span required for complete solidification.* This is schematically represented by the cooling curve for 100% Sn shown in Figure 5(D). If a sample of lead with a small amount of tin (<19.%) is cooled, solidification will take place over a range of temperatures, and the resulting solid lead-rich primary phase which is formed is called alpha. The cooling curve will resemble those of Figure 3(B and C) since in all these compositions we are dealing with complete solid solubility compositions. A similar result will occur when tin containing a small amount of lead is solidified; this solid tin-rich primary phase is called beta.
The one other place where only a thermal arrest will occur during solidification is at the eutectic composition (61.9% Sn, 38.1% Pb), when at the eutectic temperature (183 °C), as shown by the cooling curve for this composition in Figure 5(B). Both alpha and beta phase solidify simultaneously, forming a laminated eutectic structure consisting of alternate layers of alpha and beta.
*A small amount of supercooling may occur just before solidification has a
chance to begin.
For Pb-Sn compositions between 19.% Sn and 97.5% Sn (maximum solubility limits), a primary phase will form first upon cooling. A change in slope of the cooling curve occurs at the start of formation of the primary phase. On the left of the eutectic the primary phase is alpha, and a sample cooling curve is shown in Figure 5(A). On the right of the eutectic the primary phase is beta, and a sample cooling curve is shown in Figure 5(C).
For the 30% Sn composition, the alpha phase starts solidifying at 258 °C. Increasing amounts of alpha form as cooling progresses from 258° to 183 °C. At 183° the alpha phase contains 19.2% Sn, and the remaining liquid contains 61.9% Sn (eutectic composition). Upon further extraction of heat a thermal arrest will occur as the remaining liquid solidifies, forming alpha and beta simultaneously. After complete solidification the microstructure contains primary alpha and the eutectic structure of alternate layers of alpha and beta.
For the 90% Sn composition, Figure 5(C), the beta phase starts solidifying at 218 °C, and continues to solidify as the temperature decreases to 183 °C. At 183 °C the beta phase contains 97.5% Sn, and the remaining liquid contains 61.9% Sn (eutectic composition). As solidification continues a thermal arrest now occurs as the eutectic structure is formed. The resultant microstructure contains primary beta and eutectic structure.
The primary phase and the eutectic structure can easily be identified through a microscopic examination. Compositions near the eutectic will contain only a small amount of primary phase. Compositions of nearly pure lead or tin will not contain any eutectic structure.
A eutectic composition of solder (about 60 Sn, 40 Pb) is usually used due to
its narrow freezing range which, therefore, will not require a solder joint to
be held very long before it is solidified and "soldered." A solder which is used
for "body" solder or wiping solder would require a non-eutectic (i.e. 40 Sn, 60
Pb) solder in order to have a "mush" range and not solidify two quickly.