GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59

2nd Revised Pages

Chapter 11 Phase Transformations

Top: A Boeing 767 airplane in flight. Bottom: A transmission electron micrograph showing the microstructure of the aluminum

alloy that is used for the upper wing skins, parts of the internal wing structures, and selected areas of the fuselage of the Boeing

767 above. This is a 7150–T651 alloy (6.2Zn, 2.3Cu, 2.3Mg, 0.12Zr, the balance Al) that has been precipitation hardened. The

light matrix phase in the micrograph is an aluminum solid solution. The majority of the small plate-shaped dark precipitate

particles are a transitionη′phase, the remainder being the equilibriumη(MgZn 2 ) phase. Note that grain boundaries are

“decorated” by some of these particles. 80,475×. (Top photograph courtesy of the Boeing Commercial Airplane Company.

Electron micrograph courtesy of G. H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

WHY STUDYPhase Transformations?

The development of a set of desirable mechanical

characteristics for a material often results from a phase

transformation that is wrought by a heat treatment. The

time and temperature dependencies of some phase

transformations are conveniently represented on

modified phase diagrams. It is important to know how to

use these diagrams in order to design a heat treatment

for some alloy that will yield the desired

room-temperature mechanical properties. For example,

the tensile strength of an iron–carbon alloy of eutectoid

composition (0.76 wt% C) can be varied between

approximately 700 MPa (100,000 psi) and 2000 MPa

(300,000 psi) depending on the heat treatment

employed.

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