# Shape-memory alloys: NiTi switches between austenite and martensite

> A triangle cycle for nickel–titanium: cooled, the cubic austenite turns into twinned martensite without changing shape; loaded, the twins reorient and the wire bends; heated above Af, the alloy returns to austenite and to its remembered shape. Above Af, stress alone does the same round trip (superelasticity), with 6–8 % recoverable strain, transformation temperatures set by composition and uses from actuators to elastocaloric cooling.

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- Updated: 2026-10-14
- Category: Graphical abstracts
- Method: Graphical abstract
- Starting materials: Near-equiatomic nickel–titanium (50 at.% Ni)
- Product: A wire that bends when cold and recovers its shape on heating, or springs back on unloading above Af
- Key result: Reversible strain 6–8 % (Kubášová et al. 2024); transformation between −40 and 100 °C, hysteresis 20–40 °C, about 10 °C per 0.1 at.% Ni (Chekotu et al. 2019); −15.9 K elastocaloric effect after 10 million cycles (Li et al. 2026)
- Figure: 1920 × 1080 px, 353 editable layers (3 generated objects with settings)

## Steps

1. **Austenite (hot)**: Above Af NiTi is austenite, an ordered body-centred cubic B2 lattice (Chekotu et al. 2019; Kubášová et al. 2024)
2. **Cool: twinned martensite**: Below Ms the austenite turns into twinned, monoclinic B19′ martensite by a diffusionless shear, keeping its shape (Kubášová et al. 2024; Chekotu et al. 2019)
3. **Load: detwinning**: Deformation reorients the twins in one preferential direction; the shape changes without permanent damage (Chekotu et al. 2019)
4. **Heat: shape recovered**: Above As the martensite turns back into austenite, and above Af the original shape is fully recovered (Kubášová et al. 2024)
5. **Superelasticity**: Above Af, stress alone makes stress-induced martensite that reverts on unloading; 6–8 % reversible strain, and the hysteresis dissipates energy (Kubášová et al. 2024)

## Uses

- Graphical abstracts on shape-memory alloys and Nitinol
- Talks on martensitic transformations, superelasticity and elastocaloric cooling
- Teaching the shape-memory effect

## References

1. Kubášová et al., “A Review on Additive Manufacturing Methods for NiTi Shape Memory Alloy Production”, Materials 2024. https://doi.org/10.3390/ma17061248 (review)
2. Chekotu, Groarke, O’Toole & Brabazon, “Advances in Selective Laser Melting of Nitinol Shape Memory Alloy Part Production”, Materials 2019. https://doi.org/10.3390/ma12050809 (review)
3. Turkmen, Issa, Awayssa & El-Hassan, “Incorporation of Nitinol (NiTi) Shape Memory Alloy (SMA) in Concrete: A Review”, Materials 2025. https://doi.org/10.3390/ma18194458 (review)
4. Li et al., “Fatigue resistant elastocaloric effect in TiNi via texture-precipitate synergy”, Nat. Commun. 2026. https://doi.org/10.1038/s41467-026-68835-0 (primary)

Credit: Drawn by LabCanvas after Kubášová et al., Materials 2024 (doi:10.3390/ma17061248); Chekotu et al., Materials 2019 (doi:10.3390/ma12050809); Turkmen et al., Materials 2025 (doi:10.3390/ma18194458) and Li et al., Nat. Commun. 2026 (doi:10.1038/s41467-026-68835-0).

## Questions

**What does the “Shape-memory alloys: NiTi switches between austenite and martensite” template show?** A triangle cycle for nickel–titanium: cooled, the cubic austenite turns into twinned martensite without changing shape; loaded, the twins reorient and the wire bends; heated above Af, the alloy returns to austenite and to its remembered shape. Above Af, stress alone does the same round trip (superelasticity), with 6–8 % recoverable strain, transformation temperatures set by composition and uses from actuators to elastocaloric cooling.

**What are the steps shown in “Shape-memory alloys: NiTi switches between austenite and martensite”?** 1. Austenite (hot): Above Af NiTi is austenite, an ordered body-centred cubic B2 lattice (Chekotu et al. 2019; Kubášová et al. 2024). 2. Cool: twinned martensite: Below Ms the austenite turns into twinned, monoclinic B19′ martensite by a diffusionless shear, keeping its shape (Kubášová et al. 2024; Chekotu et al. 2019). 3. Load: detwinning: Deformation reorients the twins in one preferential direction; the shape changes without permanent damage (Chekotu et al. 2019). 4. Heat: shape recovered: Above As the martensite turns back into austenite, and above Af the original shape is fully recovered (Kubášová et al. 2024). 5. Superelasticity: Above Af, stress alone makes stress-induced martensite that reverts on unloading; 6–8 % reversible strain, and the hysteresis dissipates energy (Kubášová et al. 2024).

**What are the starting materials and the product?** Starting materials: Near-equiatomic nickel–titanium (50 at.% Ni). Product: A wire that bends when cold and recovers its shape on heating, or springs back on unloading above Af.

**Which papers is “Shape-memory alloys: NiTi switches between austenite and martensite” based on?** Kubášová et al., “A Review on Additive Manufacturing Methods for NiTi Shape Memory Alloy Production”, Materials 2024 (doi:10.3390/ma17061248); Chekotu, Groarke, O’Toole & Brabazon, “Advances in Selective Laser Melting of Nitinol Shape Memory Alloy Part Production”, Materials 2019 (doi:10.3390/ma12050809); Turkmen, Issa, Awayssa & El-Hassan, “Incorporation of Nitinol (NiTi) Shape Memory Alloy (SMA) in Concrete: A Review”, Materials 2025 (doi:10.3390/ma18194458); Li et al., “Fatigue resistant elastocaloric effect in TiNi via texture-precipitate synergy”, Nat. Commun. 2026 (doi:10.1038/s41467-026-68835-0).

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