Page 331 - Materials Science and Engineering An Introduction

9.7 Binary Isomorphous Systems • 303

with binary alloys—those that contain two components. If more than two components
are present, phase diagrams become extremely complicated and difficult to represent.
An explanation of the principles governing, and the interpretation of phase diagrams
can be demonstrated using binary alloys even though most alloys contain more than two
components.
Binary phase diagrams are maps that represent the relationships between tempera-
ture and the compositions and quantities of phases at equilibrium, which influence the
microstructure of an alloy. Many microstructures develop from phase transformations,
the changes that occur when the temperature is altered (typically upon cooling). This
may involve the transition from one phase to another or the appearance or disappear-
ance of a phase. Binary phase diagrams are helpful in predicting phase transformations
and the resulting microstructures, which may have equilibrium or nonequilibrium
character.

9.7 BINARY ISOMORPHOUS SYSTEMS
Possibly the easiest type of binary phase diagram to understand and interpret is the type
that is characterized by the copper–nickel system (Figure 9.3a). Temperature is plotted
along the ordinate, and the abscissa represents the composition of the alloy, in weight
percent (bottom) and atom percent (top) of nickel. The composition ranges from 0 wt%
Ni (100 wt% Cu) on the far left horizontal extreme to 100 wt% Ni (0 wt% Cu) on the
right. Three different phase regions, or fields, appear on the diagram: an alpha (a) field,
a liquid (L) field, and a two-phase a + L field. Each region is defined by the phase or
phases that exist over the range of temperatures and compositions delineated by the
phase boundary lines.
The liquid L is a homogeneous liquid solution composed of both copper and nickel.
The a phase is a substitutional solid solution consisting of both Cu and Ni atoms and
has an FCC crystal structure. At temperatures below about 1080 C, copper and nickel
are mutually soluble in each other in the solid state for all compositions. This complete
solubility is explained by the fact that both Cu and Ni have the same crystal structure
(FCC), nearly identical atomic radii and electronegativities, and similar valences, as dis-
isomorphous cussed in Section 4.3. The copper–nickel system is termed isomorphous because of this
complete liquid and solid solubility of the two components.
Some comments are in order regarding nomenclature: First, for metallic alloys, solid
solutions are commonly designated by lowercase Greek letters (a, b, g, etc.). With regard
to phase boundaries, the line separating the L and a + L phase fields is termed the liq-
uidus line, as indicated in Figure 9.3a; the liquid phase is present at all temperatures and
compositions above this line. The solidus line is located between the a and a + L regions,
below which only the solid a phase exists.
For Figure 9.3a, the solidus and liquidus lines intersect at the two composition
extremes; these correspond to the melting temperatures of the pure components. For
example, the melting temperatures of pure copper and nickel are 1085 C and 1453 C,
respectively. Heating pure copper corresponds to moving vertically up the left-hand
temperature axis. Copper remains solid until its melting temperature is reached. The
solid-to-liquid transformation takes place at the melting temperature, and no further
heating is possible until this transformation has been completed.
For any composition other than pure components, this melting phenomenon occurs
over the range of temperatures between the solidus and liquidus lines; both solid a and
liquid phases are in equilibrium within this temperature range. For example, upon heat-
ing of an alloy of composition 50 wt% Ni–50 wt% Cu (Figure 9.3a), melting begins at
approximately 1280 C (2340 F); the amount of liquid phase continuously increases with
temperature until about 1320 C (2410 F), at which point the alloy is completely liquid.