Page 124 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 4.3 Phase Diagrams |03

solubility limit for sugar in water. Most alloys consist
of two or more solid phases and may be regarded as 5 an Q
mechanical mixtures; such a system with two solid
phases is known as a two-phase system.
A phase is defined as a physically distinct and
homogeneous portion in a material; each phase is a
homogeneous part of the total mass and has its own
(H) (D)
characteristics and properties. For example, consider
a mixture of sand and water as an example of a two-
FIGURE 4.3 (a) Schematic illustration of grains, grain
phase system. These two different components have boundaries, and particles dispersed throughout the structure of
their own distinct structures, characteristics, and a two-phase system, such as a lead-copper alloy. The grains
properties. There is a clear boundary in this mixture represent lead in solid solution in copper, and the particles are
between the water (one phase) and the sand particles lead as a second phase. (b) Schematic illustration of a two-
(the second phase). Another example is ice in water; phase system consisting of two sets of grains: dark and light.
in this case, the two phases are the same chemical The green and white grains have separate compositions and
properties.
compound of exactly the same chemical elements (hy-
drogen and oxygen), even though their properties are very different. (Note that it is
not necessary that one phase be a liquid; for example, sand suspended in ice is also
a two-phase system.)
A typical example of a two-phase system in metals occurs when lead is added
to copper in the molten state. After the mixture solidifies, the structure consists of
two phases: one having a small amount of lead in solid solution in copper, the other
having lead particles (roughly spherical in shape) dispersed throughout the structure
(Fig. 4.3a). The lead particles are analogous to the sand particles in water described
above. This copper-lead alloy has properties that are different from those of either
copper or lead alone. Lead is also added to steels to obtain leaded steels, which have
greatly improved machinability (Section 21.7).
Alloying with finely dispersed particles (second-phase particles) is an impor-
tant method of strengthening alloys and controlling their properties. In two-phase
alloys, the second-phase particles present obstacles to dislocation movement and
thus increase strength. Another example of a two-phase alloy is the aggregate struc-
ture shown in Fig. 4.3b, where there are two sets of grains, each with its own com-
position and properties. The darker grains may have a different structure from the
lighter grains; they may, for example, be brittle, while the lighter grains are ductile.
Defects may appear during metalworking operations such as forging or extrusion
(Chapters 14 and 15 ); such flaws may be due to the lack of ductility of one of the
phases in the alloy. In general, two-phase alloys are stronger and less ductile than
solid solutions.

4.3 Phase Diagrams

Pure metals have clearly defined melting or freezing points, and solidification takes
place at a constant temperature (Fig. 4.4). When the temperature of a molten metal
is reduced to the freezing point, the energy of the latent heat ofsolidi/Qcation is given
off while the temperature remains constant. Eventually, solidification is complete
and the solid metal continues cooling to ambient (room) temperature.
Unlike pure metals, alloys solidify over a range of temperatures (Fig. 4.5).
Solidification begins when the temperature of the molten metal drops below the
liquidus; it is completed when the temperature reaches the solidus. Within this tem-
perature range, the alloy is in a rnus/ay or pasty state; its composition and state are
described by the particular alloy’s phase diagram.