Shape Memory Alloys (SMAs)：Definition, Properties & Key Concepts
Core Definition
Shape memory alloys (SMAs) are metals “remembering” original shapes. They showcase two unique properties: pseudo – elasticity and the shape memory effect. Among them, Ni – Ti based alloys dominate commercial applications due to their optimal material property combination.
Unique Properties & Mechanisms
Shape Memory Effect
- Process: When cold (below transformation temperature), SMAs have low yield strength, deform easily, and retain new shapes. Heating above the transformation temperature triggers a crystal structure change, making them revert to the original shape. If resisted during transformation, they generate large forces for remote actuation.
- Key Alloys: Nitinol (nickel – titanium alloy) is the most common, boasting good electrical/mechanical properties, long fatigue life, and high corrosion resistance.
Pseudo – Elasticity
A nonlinear recoverable deformation behavior of Ni – Ti SMAs above the Austenite Finish (Af) temperature. It arises from stress – induced martensite formation during loading and spontaneous reversion to austenite when unloading.
Critical Temperature Terms
| Term | Definition |
| Af (Austenite Finish) | Temperature where Martensite → Austenite transformation completes (single – stage). |
| As (Austenite Start) | Temperature where Martensite → Austenite transformation begins on heating (single – stage). |
| Mf (Martensitic Finish) | Temperature where Austenite → Martensite transformation completes on cooling (single – stage). |
| Ms (Martensite Start) | Temperature where Austenite → Martensite transformation begins on cooling (single – stage). |
| Md (Martensite Desist) | Temperature above Af, where stress – induced martensite no longer forms (≈80°C above functional Af). |
| TTR (Transformation Temperature Range) | Specific temperature range for phase change (e.g., martensite ↔ austenite in Ni – Ti), adjustable for desired traits. |
Microstructural Phases
- Austenite: High – temperature phase of Ni – Ti. Stiff, elastic, and returns to shape immediately after bending (B2 body – centered cubic crystal structure).
- Martensite: Low – temperature phase of Ni – Ti. Soft, bends easily, and stays in this state until heated above TTR. Becomes softer when chilled (B19 monoclinic crystal structure).
Key Mechanical & Functional Terms
| Term | Definition |
| Activation Range | Deformation range (stretch distance/bend angle) without permanent deformation; working range of springs/wires. |
| Anneal | Heat treatment to remove cold work/aging effects. |
| Permanent Deformation | Unrecovered angle/percentage after straining (wire/component doesn’t return to original shape). |
| Resiliency | Material’s ability to resist permanent deformation when bent. |
| Stress – Induced Martensite | Spontaneous Austenite → Martensite phase change under stress (above Af). Reverts to Austenite when stress is removed (unless permanent deformation occurs). |
| Superelasticity | Nonlinear recoverable deformation of Ni – Ti SMAs above Af (driven by stress – induced martensite ↔ austenite transition). |
Testing & Analysis Methods
- BFR Test (Bend and Free Recovery Test): Analyzes bend angle vs. temperature to determine As/Af temperatures of Ni – Ti wires.
- DSC Analysis (Differential Scanning Calorimetry): Identifies thermally induced transformations of alloys in the lab.
Spring Types & Applications
| Spring Type | Structure & Application |
| Compression Spring | Open – spaced helixes; used to provide “pushing” force when compressed. |
| Extension Spring | Closed coils (tightly wound helixes); used to provide pulling force in tensile applications. |
Application Considerations
SMAs aren’t universal. For actuator design, consider:
- Forces, displacements, temperature conditions, and cycle rates.
- Material limitations (e.g., performance trade – offs in extreme environments).