of a Shape Memory Alloy
Shape Memory Alloys (SMAs) are a unique class of
metal alloys that can recover apparent permanent strains when
they are heated above a certain temperature. The SMAs have two
stable phases - the high-temperature phase, called austenite
and the low-temperature phase, called martensite. In
addition, the martensite can be in one of two forms: twinned
and detwinned, as shown in Figure 1. A phase transformation
which occurs between these two phases upon heating/cooling is
the basis for the unique properties of the SMAs. The key effects
of SMAs associated with the phase transformation are pseudoelasticity
and shape memory effect.
Figure 1. Different phases of an SMA.
Upon cooling in the absence of applied load the
material transforms from austenite into twinned (self-accommodated)
martensite. As a result of this phase transformation no observable
macroscopic shape change occurs. Upon heating the material in
the martensitic phase, a reverse phase transformation takes place
and as a result the material transforms to austenite. The above
process is shown in Figure 2. Four characteristic temperatures
are defined in Figure 2: martensitic start temperature (M0s)
which is the temperature at which the material starts transforming
from austenite to martensite; martensitic finish temperature (M0f),
at which the transformation is complete and the material is fully
in the martensitic phase; austenite start temperature (Aos)
at which the reverse transformation (austenite to martensite)
initiates; and austenite finish temperature (Aof)
at which the reverse phase transformation is completed and the
material is the austenitic phase.
Figure 2. Temperature-induced phase transformation
of an SMA without mechanical loading.