Shape Memory Alloy

1
Shape Memory Alloy

• Shape memory alloys (SMA) form group of metals
  that recovers particular shape when heated above
  their transformation
• temperatures.
• Shape recovery is due to solid-to-solid phase
• transformation from martensite to austenite.
• SMA is typically formed above 500700oC
• where the material exhibits highly ordered
• austenitic crystalline structure. Upon cooling
• below transition temperature, unstrained SMA
  will have twinned martensitic structure.

• SMA deforms easily under stress due to twin
  boundaries propagating throughout the structures
  in
• the direction of stress.
• Austenite crystalline structure is regained upon
  heating, returning to its original shape.

2
Shape Memory Alloy (cont.)

• Stress-strain curve for SMA has non-linearlity
  referred as pseudoelasticity.
• Pseudoelastic behavior causes SMA to strain up
  to 68!
• Mechanisms for pseudoelasticity involves either
  twinning or stressed-induced martensitic
  transformation (superelasticity) .

• Twinning pseudoelasticity caused by formation
  of twins or by motion of twin boundaries.

• Superelasticity SMA in austentite phase
  seemingly shows plastic deformation that is
  fully recoverable when stress is removed.
  Martensite reverts back to the austenite phase
  providing the shape recovery.

3
Thin Film Shape Memory Alloy

• Drive to miniaturization forces attention to
  sputter deposited thin film SMA for MEMS
  applications.
• Sputter deposition is performed in an ultra high
  
• vacuum (UHV) system.
• SMA thin films strongly depend on metallurgical
• factors and sputtering conditions.

• Sputtering involves ejection of particles by
  momentum
• transfer from a SMA target on to a substrate.
• Ejection of target particles is furnished by ion
• bombardment using argon gas flow (causes plasma).