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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 500~700oC 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.

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twinned

Shape Memory Alloy (cont.)

detwinning twinning

· Stress-strain curve for SMA has non-linearlity referred as pseudoelasticity. · Pseudoelastic behavior causes SMA to strain up to 6~8%! · 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.

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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.

Metallurgical factors alloy composition annealing temperature aging temperature Sputtering condition Ar pressure sputtering power substrate temperature film thickness

substrate

· 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).

plasma

target

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