Metals - Orthopaedic Biomaterials in Research and Practice - page 173

Heat treatment The ability to regulate mechanical properties without changes in overall

composition is the key to the high desirability of metals in fabrication
of artifacts such as surgical devices. Thus, parts may be formed easily
while the alloy is relatively deformable, and then the yield point and duc-
tility may be adjusted thermally to produce the final required mechani-
cal behavior.
In thermal processing, metallurgists are guided by what are called
time-temperature-transformation or TTT diagrams, which describe
the interrelationship between time, temperature, and transformation
of phase structure for each alloy. In general, the time and temperature
required for a structural change vary inversely, but the exact relationship
may be complex.
The principal applications of heat treatment are as follows:

Annealing. Working alloys at low temperatures above their yield
points produce work hardening and thus loss of toughness and duc-
tility. If large changes in cross section are required by the forming
process, this may result in cracking of the part, since the ultimate
strain may be exceeded locally. This may be prevented by heat-
ing to between one-third and one-half of the melting temperature
followed by controlled cooling. This cycle relieves the cold work,
restoring unworked properties, but results in modest grain growth.
Sometimes, it is used deliberately for this purpose to eliminate so-
called “duplex” structures: ones that contain both large and small
grains rather than a normal distribution around a single average
grain size. Duplex structures are relatively weak; annealing tends
to normalize them by removing the small grains since large grains
grow at the expense of small ones. A related process called tem-
pering is a partial annealing used to slightly toughen very brittle
strong alloys used for cutting edges. Tempering generally requires
extremely rapid cooling ( quenching ).
Aging. Aging is storage for long periods at slightly elevated tempera-
tures (below annealing temperatures) to permit the formation of
intermetallic, carbide, or oxide precipitates. These very small,
very strong crystals internally stress the alloy, raising both yield
and ultimate strengths without affecting moduli. The resulting
alloy is said to be aged or precipitation hardened.
Phase transformation. The great power of modern metallurgy comes
from the discovery that some alloys, particularly many steels,
can have phases of identical compositions but with different
local arrangements of atoms, resulting in different densities and
mechanical properties. The change from one structure to another,
in the solid state, is called a phase transformation. We distinguish
between two general types of phase transformation: those requir-
ing diffusion, as in the formation of interposed lamellae of α +
β from a eutectic composition, and those that do not require dif-
fusion but occur owing to a very local rearrangement of atoms.
Phase transformations may be orchestrated in many alloys by very
careful and complex cycles of heating, storage, and cooling.