GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised PagesPrecipitation Hardening • 439MATERIALS OF IMPORTANCE
Shape-Memory Alloys
A
relatively new group of metals that exhibit
an interesting (and practical) phenomenon
are theshape-memory alloys(orSMAs). One of
these materials, after having been deformed, has
the ability to return to its pre-deformed size and
shape upon being subjected to an appropriate heat
treatment—that is, the material “remembers” its
previous size/shape. Deformation normally is car-
ried out at a relatively low temperature, whereas
shape memory occurs upon heating.^5 Materials
that have been found to be capable of recover-
ing significant amounts of deformation (i.e., strain)
are nickel–titanium alloys (Nitinol^6 is their trade-
name) and some copper-base alloys (viz. Cu–Zn–
Al and Cu–Al–Ni alloys).
A shape-memory alloy is polymorphic (Sec-
tion 3.10)—that is, it may have two crystal struc-
tures (or phases), and the shape-memory effect in-
volves phase transformations between them. One
phase (termed an austenite phase) has a body-
centered cubic structure that exists at elevated tem-
peratures; its structure is represented schemati-
cally in the inset shown at stage 1 of Figure 11.38.
Upon cooling, the austenite transforms sponta-
neously to a martensite phase, which is similar to
the martensitic transformation for the iron–carbon
system (Section 11.5)—that is, it is diffusionless, in-
volves an orderly shift of large groups of atoms,
occurs very rapidly, and the degree of transforma-
tion is dependent on temperature; temperatures
at which the transformation begins and ends are
indicated by “Ms” and “Mf” labels on the left ver-
tical axis of Figure 11.38. In addition, this marten-
site is heavily twinned,^7 as represented schemat-
ically in the stage 2 inset, Figure 11.38. Under
the influence of an applied stress, deformation of
martensite (i.e., the passage from stage 2 to stage 3,
Figure 11.38) occurs by the migration of twin
boundaries—some twinned regions grow while
others shrink; this deformed martensitic structure
is represented by the stage 3 inset. Furthermore,Time-lapse photograph that demonstrates the
shape-memory effect. A wire of a shape-memory alloy
(Nitinol) has been bent and treated so that its memory
shape spells the word “Nitinol”. The wire is then
deformed and, upon heating (by passage of an electric
current), springs back to its pre-deformed shape; this
shape recovery process is recorded on the photograph.
[Photograph courtesy the Naval Surface Warfare
Center (previously the Naval Ordnance Laboratory).]when the stress is removed, the deformed shape
is retained at this temperature. And, finally, upon
subsequent heating to the initial temperature, the
material reverts back to (i.e., “remembers”) its
original size and shape (stage 4). This stage 3–stage
4 process is accompanied by a phase transforma-
tion from the deformed martensite to the orig-
inal high-temperature austenite phase. For these
shape-memory alloys, the martensite-to-austenite
transformation occurs over a temperature range,
between temperatures denoted by “As” (austen-
ite start) and “Af” (austenite finish) labels on
the right vertical axis of Figure 11.38. Of course,
this deformation–transformation cycle may be re-
peated for the shape-memory material.(^5) Alloys that demonstrate this phenomenon only upon heating are said to have aone-wayshape memory. Some
shape-memory materials experience size/shape changes on both heating and cooling; these are termedtwo-way
shape memory alloys. In this presentation, we discuss the mechanism for only the one-way shape-memory.
(^6) “Nitinol” is really an acronym fornickel-titaniumNavalOrdnanceLaboratory, where this alloy was discovered.
(^7) The phenomenon of twinning is described in Section 8.8.