NiTi: superelastic effect & shape-memory — reversible martensitic transformation
NiTi undergoes a reversible B2 (austenite) ↔ B19' (martensite) transformation. Above Af: superelastic — stress-induced martensite forms and reverts on unloading (up to 8% strain). Below Mf: shape-memory — deformed martensite recovers shape on heating above Af. Both effects are driven by the same crystallographic transformation with different activation path. Clausius-Clapeyron: dσ/dT ≈ −6 to −8 MPa/°C.