Elastocaloric Effect: Stress-Induced Cooling in Smart Alloys

Mechanical Load

Applied strain ε 0.0%

Loading rate 2.0×

Ambient temperature 22 °C

Live State

Stress σ

0 MPa

Martensite ξ

ΔT (elastocaloric)

0.0 °C

Stress–strain hysteresis loop (σ vs ε)

How it works

A superelastic NiTi (Nitinol) wire, held above its austenite-finish temperature, is stress-loaded rather than heated. Applied stress drives a reversible, diffusionless martensitic transformation: cubic austenite (A) shears into monoclinic martensite (M) as strain increases, then reverts to A on unloading — this is what the propagating transformation band on the wire visualises.

Martensite fraction:  ξ(ε) = smoothstep(ε₁,ε₂,ε)   [loading]
                       ξ(ε) = smoothstep(ε₁',ε₂',ε) [unloading, ε₁'<ε₁]
Transformation stress (Clausius–Clapeyron):
                       σₜ(T) = σ₀ + C·(T − T₀),   C ≈ 6–8 MPa/K
Adiabatic temperature swing:
                       dΔT = (L/cₚ)·dξ − k·ΔT·dt

Loading (ε↑, forward A→M) is exothermic — releasing latent heat L, so the wire heats up (ΔT>0).
Unloading (ε↓, reverse M→A) is endothermic — absorbing heat from its surroundings, so the wire cools below ambient (ΔT<0). This reverse-transformation cooling is exactly the effect harnessed in solid-state elastocaloric refrigeration, a leading candidate to replace vapour-compression cooling and its greenhouse-gas refrigerants.
Loading rate controls how fast strain is applied relative to how fast heat can leak to the environment (rate constant k). Fast cycling is closer to adiabatic — the loop above widens and ΔT swings hard both ways; slow cycling lets heat escape continuously, flattening ΔT toward isothermal (~0).
Ambient temperature shifts the plateau stress σₜ via the Clausius–Clapeyron slope — a real Nitinol device must transform at a higher stress on a hot day than a cold one, exactly as plotted here.
The hysteresis loop (loading curve above unloading curve) is the energy dissipated per cycle as heat — the same gap that, in an elastocaloric heat pump, is engineered to pump that heat somewhere useful.

Austenite (parent phase) Martensite (transformed) Cooling below ambient Heating above ambient

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Elastocaloric Effect: Stress-Induced Cooling in a Smart Alloy