GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised Pages446 • Chapter 11 / Phase TransformationsNot all alloys that satisfy the aforementioned conditions relative to composi-
tion and phase diagram configuration are amenable to precipitation hardening. In
addition, lattice strains must be established at the precipitate–matrix interface. For
aluminum–copper alloys, there is a distortion of the crystal lattice structure around
and within the vicinity of particles of these transition phases (Figure 11.44b). During
plastic deformation, dislocation motions are effectively impeded as a result of these
distortions, and, consequently, the alloy becomes harder and stronger. As theθphase
forms, the resultant overaging (softening and weakening) is explained by a reduction
in the resistance to slip that is offered by these precipitate particles.
Alloys that experience appreciable precipitation hardening at room tempera-
ture and after relatively short time periods must be quenched to and stored under
refrigerated conditions. Several aluminum alloys that are used for rivets exhibit this
behavior. They are driven while still soft, then allowed to age harden at the normal
natural, artificial ambient temperature. This is termednatural aging; artificial agingis carried out at
aging elevated temperatures.Concept Check 11.8
Is it possible to produce a precipitation hardened 2014 aluminum alloy having a
minimum yield strength of 350 MPa (50,000 psi) and a ductility of at least 18%EL?
If so, specify the precipitation heat treatment. If it is not possible then explain why.[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]11.12 MISCELLANEOUS CONSIDERATIONS
The combined effects of strain hardening and precipitation hardening may be em-
ployed in high-strength alloys. The order of these hardening procedures is important
in the production of alloys having the optimum combination of mechanical proper-
ties. Normally, the alloy is solution heat treated and then quenched. This is followed
by cold working and finally by the precipitation-hardening heat treatment. In the
final treatment, little strength loss is sustained as a result of recrystallization. If the
alloy is precipitation hardened before cold working, more energy must be expended
in its deformation; in addition, cracking may also result because of the reduction in
ductility that accompanies the precipitation hardening.
Most precipitation-hardened alloys are limited in their maximum service tem-
peratures. Exposure to temperatures at which aging occurs may lead to a loss of
strength due to overaging.Crystallization, Melting,
and Glass Transition
Phenomena in Polymers
Phase transformation phenomena are important with respect to the design and pro-
cessing of polymeric materials. In the succeeding sections we discuss three of these
phenomena—viz., crystallization, melting, and the glass transition.