Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16

386 • Chapter 10 / Phase Diagrams

austenite grain. The layered pearlite forms because carbon atoms need diffuse only

minimal distances with the formation of this structure.

Furthermore, subsequent cooling of the pearlite from pointbin Figure 10.30 will

produce relatively insignificant microstructural changes.

Hypoeutectoid Alloys

Microstructures for iron–iron carbide alloys having other than the eutectoid compo-

sition are now explored; these are analogous to the fourth case described in Section

10.12 and illustrated in Figure 10.16 for the eutectic system. Consider a composi-

tionC 0 to the left of the eutectoid, between 0.022 and 0.76 wt% C; this is termed a

hypoeutectoid alloy hypoeutectoid(less than eutectoid)alloy.Cooling an alloy of this composition is

represented by moving down the vertical lineyy′in Figure 10.33. At about 875◦C,

pointc, the microstructure will consist entirely of grains of theγphase, as shown

schematically in the figure. In cooling to pointd, about 775◦C, which is within the

α+γphase region, both these phases will coexist as in the schematic microstructure.

Most of the smallαparticles will form along the originalγgrain boundaries. The

compositions of bothαandγphases may be determined using the appropriate tie

line; these compositions correspond, respectively, to about 0.020 and 0.40 wt% C.

While cooling an alloy through theα+γphase region, the composition of the

ferrite phase changes with temperature along theα–(α+γ) phase boundary, line

MN, becoming slightly richer in carbon. On the other hand, the change in composition

of the austenite is more dramatic, proceeding along the (α+γ)–γboundary, line

MO, as the temperature is reduced.

C 0

O

 

 

c

d

e

f

Composition (wt % C)









+ Fe 3 C

+ Fe 3 C

Fe 3 C

400

500

600

700

800

900

1000

1100

0 1.0 2.0

y

y

Temperature (

°C)

 





Te

M

N





Proeutectoid

Eutectoid

Pearlite

 

 

Figure 10.33 Schematic

representations of the

microstructures for an

iron–carbon alloy of

hypoeutectoid compositionC 0

(containing less than 0.76 wt%

C) as it is cooled from within

the austenite phase region to

below the eutectoid

temperature.