shape memory alloys- principles and applications

SHAPE MEMORY ALLOYS:

FORMATION, PROPERTIES

AND TECHNOLOGICAL IMPACTMuhammed Labeeb

SMA

▪ Shape memory alloys are metal alloys that “remember” their original shapes and having the ability to return to original shape after being deformed by heating

▪ A class of smart materials

▪ The most effective and widely used alloys are NiTi, CuZnAl, and CuAlNi

▪ SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite

▪ The shape change involves a solid state phase change involving a molecular rearrangement between Martensite and Austenite

▪ SMA also exhibits superelastic (pseudoelastic) behavior

A BRIEF HISTORY

▪ 1932 - A. Ölander discovers the pseudoelastic properties of Au-Cd alloy.

▪ 1949 - Memory effect of Au-Cd reported by Kurdjumov & Kandros.

▪ 1967 – At Naval Ordance Laboratory, Beuhler discovers shape memory effect in nickel titanium alloy, Nitinol (Nickel Titanium Naval Ordance Laboratory), which proved to be a major breakthrough in the field of shape memory alloys.

▪ 1970-1980 – First reports of nickel-titanium implants being used in medical applications.

▪ Mid-1990s – Memory metals start to become widespread in medicine and soon move to other applications.

PRINCIPLE

▪ SMAs have two stable phases :

▪ the high-temperature phase, called austenite and

▪ the low-temperature phase, called martensite.

▪ the martensite can be in one of two forms:

▪ twinned

▪ detwinned

▪ A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs

Athermal reaction with no diffusion.

PRINCIPLE

PRINCIPLE

▪ Upon cooling in the absence of applied load the material transforms from austenite into twinned martensite. (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.

▪ If mechanical load is applied to the material in the state of twinned martensite (at low temperature) it is possible to detwin the martensite.

▪ Upon releasing of the load, the material remains deformed. A subsequent heating of the material to a temperature above the austenite finish temperature (Af) will result in reverse phase transformation (martensite to austenite) and will lead to complete shape recovery.

(Af: temperature at which transformation of martensite to austenite is complete )

PRINCIPLE

TEMPERATURE

ST

RE

SS

Mf Ms As Af

TEMPERATURES

TR

ES

S

Mf Ms As Af

Twinned Martensite (unstressed)

Detwinned Martensite (stressed - deformed)

Detwinned Martensite (stressed - deformed)

Austenite (undeformed)

Twinned Martensite (unstressed)

PRINCIPLE

▪ SMA remembers the shape when it have austenitic structure.

▪ So if we need SMA to remember and regain/recover certain shape, the shape should be formed when structure is austenite

▪ Reheating the material will result in complete shape recovery

PSEUDOELASTIC BEHAVIOR

ST

RE

SS

TEMPERATURE

M f Ms As Aff s s f

Austenite

Detwinned Martensite(stressed)

▪ Occurs when an alloy is completely in the Austenite phase

▪ Is not dependent on temperature

▪ When the load is increased to a point, the alloy transitions from the Austenite phase to the detwinned Martensite phase

▪ Once the load is removed, the alloy returns to the it original Austenite shape

▪ Rubber like effect

APPLICATIONS

▪ Aeronautics

▪ Wings

▪ Alternatives to hydraulic systems

▪ Medical

▪ Optometry

▪ Self-expandable cardiovascular stent

▪ Piping

▪ Couplings

▪ Robotics

▪ Artificial limbs

APPLICATIONS

Robots can be given a more fluid movement in joints and limbs

APPLICATIONS

Plane wings with SMA wires can change shape by inducing voltages in them. This can replace hydraulic and electromechanical actuators.

APPLICATIONS

Wires have the ability to flex the robotic muscles according to electric pulses sent through the wire.

APPLICATIONS

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Use of memory alloys for coupling tubing: A memory alloy coupling is expanded (a) so it fits over the tubing (b). When the coupling is reheated, it shrinks back to its original diameter (c), squeezing the tubing for a tight fit

APPLICATIONS▪ Nanomuscles

▪ Surgical instruments

▪ Tissue Spreader

▪ Stents (angioplasty)

▪ Coronary Probe

▪ Brain Spatula

▪ Endoscopy: miniature zoom device, bending actuator

▪ Force sensor

▪ Smart skin (wing turbulence reduction)

APPLICATIONS

▪ The most common commercial application involves the pseudo-elastic property during it’s high temperature state.

▪ This includes eye-glasses, cell phone antennas, and so on, which are experiencing their high temperature state at room temperature.

ADVANTAGES AND DISADVANTAGES OF SHAPE MEMORY ALLOYS

▪ ADVANTAGES

▪ Bio-compatibility

▪ Diverse field of application

▪ Good mechanical properties

▪ DISADVANTAGES

▪ Expensive

▪ Poor fatigue properties

REFERENCE

▪ http://en.wikipedia.org/wiki/Shape_memory_alloy

▪ http://www.smaterial.com/SMA/sma.html