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376 • Chapter 10 / Phase Transformations

c

a
a
Figure 10.20 The body-
centered tetragonal unit
10 m
cell for martensitic steel
Figure 10.19 Photomicrograph of a steel showing iron atoms
having a spheroidite microstructure. The small (circles) and sites that may
particles are cementite; the continuous phase is be occupied by carbon
a-ferrite. 1000 . atoms ( s). For this
(Copyright 1971 by United States Steel Corporation.) tetragonal unit cell, c a.

Concept Check 10.1 Which is more stable, the pearlitic or the spheroiditic microstruc-
ture? Why?
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

Martensite
martensite Yet another microconstituent or phase called martensite is formed when austenitized
iron–carbon alloys are rapidly cooled (or quenched) to a relatively low temperature
(in the vicinity of the ambient). Martensite is a nonequilibrium single-phase structure
that results from a diffusionless transformation of austenite. It may be thought of as a
transformation product that is competitive with pearlite and bainite. The martensitic
transformation occurs when the quenching rate is rapid enough to prevent carbon diffu-
sion. Any diffusion whatsoever results in the formation of ferrite and cementite phases.
The martensitic transformation is not well understood. However, large numbers of
atoms experience cooperative movements, in that there is only a slight displacement of
each atom relative to its neighbors. This occurs in such a way that the FCC austenite
experiences a polymorphic transformation to a body-centered tetragonal (BCT) marten-
site. A unit cell of this crystal structure (Figure 10.20) is simply a body-centered cube that
has been elongated along one of its dimensions; this structure is distinctly different from
that for BCC ferrite. All the carbon atoms remain as interstitial impurities in martensite;
as such, they constitute a supersaturated solid solution that is capable of rapidly trans-
forming to other structures if heated to temperatures at which diffusion rates become