Shape Memory Alloy — Superelasticity & Phase Transformation

Interactively explore austenite ↔ martensite transitions, superelastic hysteresis, and thermal shape recovery in smart materials like Nitinol.

Phase: Austenite Cubic structure — high stiffness, shape-restoring
Crystal Lattice
Stress-Strain Curve
50
Temp (°C)
0.0
Stress (MPa)
0.00
Strain (%)
100%
Austenite fraction
75
Stiffness (GPa)
Presets:
Nitinol (NiTi) — the most widely used SMA. Transformation temperatures: Ms=−10°C, Mf=−30°C, As=10°C, Af=40°C. Superelastic plateau ~500 MPa with ~8% recoverable strain. Used in medical stents, orthodontic wires, and robotic actuators.
About Shape Memory Alloys

Shape memory alloys (SMAs) exploit a reversible solid-to-solid phase transformation between a high-symmetry austenite phase and a lower-symmetry martensite phase.

Shape Memory Effect: Cooling into martensite, deforming, then heating above Af recovers the original shape. The austenite phase is the "remembered" configuration.

Superelasticity: At T > Af, applied stress can induce martensite. Upon unloading the alloy transforms back to austenite recovering up to ~8% strain — a "rubber-like" metal.

Transformation temperatures (Nitinol): Martensite start Ms, martensite finish Mf, austenite start As, austenite finish Af. These are tunable by composition (Ni:Ti ratio).

Applications: Cardiovascular stents, guidewires, orthodontic archwires, actuators, vibration dampers, aerospace morphing structures.

About this simulation

This simulation visualises the solid-to-solid phase transformation at the heart of shape memory alloys like Nitinol: switching between a high-symmetry austenite crystal lattice and a lower-symmetry martensite lattice. Both a live crystal lattice animation and a stress-strain curve update together as you change temperature and applied stress, showing exactly how the material "remembers" its original shape or absorbs strain elastically.

🔬 What it shows

Two related behaviors: superelasticity, where stress alone (above the austenite finish temperature Af) induces martensite that snaps back to austenite on unloading, recovering up to ~8% strain along a hysteresis loop; and the shape-memory effect, where cooling into martensite, deforming, then reheating past Af restores the original austenite shape. Four transition temperatures — martensite start/finish (Ms/Mf) and austenite start/finish (As/Af) — govern where each transformation occurs.

🎮 How to use

Drag Temperature to move through the Ms/Mf/As/Af transition points and watch the phase chip switch between Austenite and Martensite. Adjust Applied Stress to induce stress-driven martensite in the superelastic regime, and switch Loading Mode between Superelastic and Thermal to compare the two recovery mechanisms. Live stats track stress, strain, phase fraction, and elastic modulus E.

💡 Did you know?

Nitinol's transformation temperatures are tunable simply by adjusting its nickel-to-titanium ratio, which is why the same base alloy can be engineered for very different products — from body-temperature-activated medical stents that spring open once implanted, to eyeglass frames that flex back into shape after being bent.

Frequently asked questions

What's the difference between austenite and martensite?

Austenite is the high-symmetry, high-temperature crystal structure; martensite is a lower-symmetry structure that forms below a certain temperature or under applied stress. The transformation between them is diffusionless — atoms shift cooperatively into new lattice positions rather than migrating individually, which is why it can happen almost instantly and reversibly.

What is superelasticity, and why does it recover so much strain?

Above Af, applying stress induces martensite formation even though temperature alone would keep the alloy in austenite. This stress-induced martensite is unstable once the stress is removed, so the material snaps back to austenite, recovering strains of up to ~8% — far beyond the roughly 0.2% elastic limit of ordinary metals like steel.

What do Ms, Mf, As and Af actually mean?

Ms and Mf are the martensite start and finish temperatures on cooling; As and Af are the austenite start and finish temperatures on heating. Because these four temperatures don't coincide, the transformation traces a hysteresis loop — the material takes a different path forming martensite than it does reverting to austenite.

How does the shape-memory effect differ from superelasticity?

The (thermal) shape-memory effect requires cooling the alloy below Mf first, deforming it while it's soft martensite, and then reheating above Af to recover the original shape. Superelasticity instead works entirely above Af through stress alone, with no heating step needed to recover the shape — it snaps back the instant the stress is released.

Why is Nitinol used for medical stents and eyeglass frames?

Its transition temperatures can be set near body temperature so a compressed stent expands to its remembered shape once inside a warm body, while its superelastic range lets eyeglass frames bend far beyond what rigid metal could survive and spring straight back — both applications exploit the same underlying austenite-martensite transformation this simulation visualises.