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

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

10.11 Binary Eutectic Systems • 359

Concept Check 10.5

Below is a portion of the H 2 O–NaCl phase diagram:

10

0

 10

 20

 30

50

40

30

20

0

 10

 20

10

01020

100 90 80 70

30

Composition (wt%)

NaCl

H 2 O

Ice  Salt

Salt



Liquid

(brine)

Ice



Liquid

(brine)

Liquid

(brine)

Temperature (

°F)

Temperature (

°C)

(a)Using this diagram, briefly explain how spreading salt on ice that is at a temper-

ature below 0◦C (32◦F) can cause the ice to melt.

(b)At what temperature is salt no longer useful in causing ice to melt?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

EXAMPLE PROBLEM 10.2

Determination of Phases Present and Computation

of Phase Compositions

For a 40 wt% Sn–60 wt% Pb alloy at 150◦C (300◦F),(a)What phase(s) is (are)

present?(b)What is (are) the composition(s) of the phase(s)?

Solution

(a)Locate this temperature–composition point on the phase diagram (point

Bin Figure 10.9). Inasmuch as it is within theα+βregion, bothαandβ

phases will coexist.

(b)Since two phases are present, it becomes necessary to construct a tie line

across theα+βphase field at 150◦C, as indicated in Figure 10.9. The com-

position of theαphase corresponds to the tie line intersection with the

α/(α+β) solvus phase boundary—about 11 wt% Sn–89 wt% Pb, denoted

asCα. Similarly for theβphase, which will have a composition of approxi-

mately 98 wt% Sn–2 wt% Pb (Cβ).