Transcript Introduction to Shape Memory Alloys

E3 AEROSPACE ENGINEERING
RESEARCH
SHAPE MEMORY ALLOYS
(SMAS)
¡
E3 Teacher Summer Research Program
Aerospace Engineering
Texas A & M University
By
Moses Z. Horton
Ali A. Jafry
Introduction to Shape Memory Alloys.
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 . 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.
Schematic of a stress-strain-temperature curve showing
the shape memory effect.
Temperature-induced phase transformation of an
SMA without mechanical loading.
Thermally-Induced Transformation with Applied
Mechanical Load
Temperature-induced phase transformation with applied
load.
SMA Stress Strain Temperature Phase Diagram.
Pseudoelasticity
The pseudo elastic behavior of SMAs is
associated with recovery of the
transformation strain upon unloading.
The super elastic behavior is observed
during loading and unloading above
A0S and is associated with stressinduced martensite and reversal to
austenite upon unloading.
Pseudo elastic stress-strain diagram.
Schematic of a thermo mechanical loading path
demonstrating pseudo elastic behavior of SMAs.