Metals - Orthopaedic Biomaterials in Research and Practice - page 165

ratios. Finally, since all of these relationships are dependent on the bond-
ing or binding energy between atoms, they are subject to change with
temperature.
Phase diagrams
All of the information about the chemical and structural relationships
between metallic elements is difficult to tabulate numerically, so a
graphic format was developed by metallurgists to present it in a simple
manner. The resulting diagram is called a constitution or phase dia-
gram . A phase is a homogeneous combination of the constituent ele-
ments that has constant structure and properties. A phase diagram is a
plot of composition versus temperature for a mixture of elements, with
regions of constant properties at equilibrium, defined as phases, sepa-
rated by lines or surfaces, called phase boundaries.
Phase diagrams may be two- or three-dimensional or be made for
two or three elements in the presence of a constant amount of one, two,
or three additional elements, and so on. The simplest is the binary phase
diagram, which shows the equilibrium phases possible for mixtures,
that is, alloys, of two metals for compositions between 100% of one and
100% of the other.
It is unfortunate that the phase diagrams for alloys of orthopaedic
interest are quite complex. Thus, for purposes of illustration of the gen-
eral properties and use of phase diagrams, Figure 7.2 is an imaginary
diagram, constructed for two imaginary metals, aronium and borium
(A and B). This diagram obeys the thermodynamic principles of phase
relationships for two metallic elements that have limited solubility in
each other but possess no intermetallic compounds of fixed composi-
tion. It will be used in the subsequent discussions to illustrate the uses
of phase diagrams.
This phase diagram has several important regions, labeled with
Arabic or Greek letters:
L. In this range of compositions and temperatures, only a homoge-
neous liquid is found. Note that while aronium melts at 1000°C
and borium at 900°C, intermediate compositions can stay fully
molten down to temperatures as low as 700°C. This type of behav-
ior is not uncommon and is related to the presence of a eutectic
composition in the system.
E. The eutectic point. This composition, 65:35,* is called a eutectic
since it melts abruptly (at 700°C) as if it were a pure metal.
S + L . These two regions represent mixtures of two phases, a solid
and a liquid. However, the solid phases are different in each region,
being alpha (α) in the left or aronium-rich region and beta (β) in the
* The notation 65:35 means 65 weight percent (w/o) A, 35 w/o B including the relative
structures of α and β phases.