Page 398 - Materials Science and Engineering An Introduction

370 • Chapter 10 / Phase Transformations

During a phase transformation, an alloy proceeds toward an equilibrium state that is
characterized by the phase diagram in terms of the product phases and their compositions
and relative amounts. As Section 10.3 notes, most phase transformations require some finite
time to go to completion, and the speed or rate is often important in the relationship between
the heat treatment and the development of microstructure. One limitation of phase diagrams
is their inability to indicate the time period required for the attainment of equilibrium.
The rate of approach to equilibrium for solid systems is so slow that true equilibrium
structures are rarely achieved. When phase transformations are induced by temperature
changes, equilibrium conditions are maintained only if heating or cooling is carried out
at extremely slow and unpractical rates. For other-than-equilibrium cooling, transforma-
tions are shifted to lower temperatures than indicated by the phase diagram; for heat-
supercooling ing, the shift is to higher temperatures. These phenomena are termed supercooling and
superheating, respectively. The degree of each depends on the rate of temperature
superheating
change; the more rapid the cooling or heating, the greater the supercooling or superheat-
ing. For example, for normal cooling rates, the iron–carbon eutectoid reaction is typically
displaced 10 C to 20 C (18 F to 36 F) below the equilibrium transformation temperature. 3
For many technologically important alloys, the preferred state or microstructure is
a metastable one, intermediate between the initial and equilibrium states; on occasion,
a structure far removed from the equilibrium one is desired. It thus becomes imperative
to investigate the influence of time on phase transformations. This kinetic information
is, in many instances, of greater value than knowledge of the final equilibrium state.

Microstructural and Property Changes
in Iron–Carbon Alloys

Some of the basic kinetic principles of solid-state transformations are now extended
and applied specifically to iron–carbon alloys in terms of the relationships among heat
treatment, the development of microstructure, and mechanical properties. This system
has been chosen because it is familiar and because a wide variety of microstructures and
mechanical properties is possible for iron–carbon (or steel) alloys.

10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
Pearlite
Consider again the iron–iron carbide eutectoid reaction
Eutectoid reaction cooling
for the iron–iron g(0.76 wt%C) m a(0.022 wt% C) + Fe 3 C (6.70 wt% C) (10.19)
carbide system heating
which is fundamental to the development of microstructure in steel alloys. Upon cooling,
austenite, having an intermediate carbon concentration, transforms into a ferrite phase,
which has a much lower carbon content, and also cementite, which has a much higher
carbon concentration. Pearlite is one microstructural product of this transformation
(Figure 9.27); the mechanism of pearlite formation was discussed previously (Section
9.19) and demonstrated in Figure 9.28.

3 It is important to note that the treatments relating to the kinetics of phase transformations in Section 10.3 are
constrained to the condition of constant temperature. By way of contrast, the discussion of this section pertains to
phase transformations that occur with changing temperature. This same distinction exists between Sections 10.5
(Isothermal Transformation Diagrams) and 10.6 (Continuous-Cooling Transformation Diagrams).