Elastocaloric effect with a broad temperature window and low energy loss in a nanograin Ti-44Ni-5Cu-1Al $({\mathrm{at}}_{·}\%)$ shape memory alloy

Superelastic Ti-Ni-based shape memory alloys are promising elastocaloric materials for solid-state refrigeration. In this paper, we studied the mechanical properties and elastocaloric effect of a severely cold rolled Ti-44Ni-5Cu-1Al (at.%) alloy composed of nanograins $(\sim 5\text{–}16\mathrm{nm})$. The alloy exhibits partial stress-induced martensitic transformation with a large quasilinear reversible strain of $\sim 3.4\%$ and a small energy dissipation of $\sim 1.6\mathrm{MJ}/{\mathrm{m}}^3$ when a maximum stress of 1 GPa is applied. The temperature decrease induced by adiabatic removal of a stress of 1 GPa is of about −5 K, which is mainly due to the reverse transition occurring during unloading. The effective working temperature window of elastocaloric effect larger than 200 K. The coefficient of performance of this alloy reaches ∼13, which is higher than values reported so far for Ti-Ni alloys.

Figure 1

(a) Bright-field image, (b) dark-field image and corresponding diffraction pattern for the nanograin Ti-44Ni-5Cu-1Al (at.%) alloy. The arrow indicates the rolling direction, and the circle indicates the position of the objective aperture with which the dark-field image is taken.

Figure 2

(a) DSC cooling and heating curves measured at a rate of 10 K/min, (b) temperature dependence of the electrical resistivity (ρ) for the nanograins (black line) and coarse grain (blue line) Ti-44Ni-5Cu-1Al (at.%) alloys, (c) in situ x-ray diffraction profiles at different tensile strains during the loading process for the nanograin Ti-44Ni-5Cu-1Al (at.%) alloy.

Figure 3

(a) Stress-strain curves with strain rate of $0.005{\mathrm{s}}^{-1}$ (red line) or $0.2{\mathrm{s}}^{-1}$ (blue line), (b) stress-strain curves at different testing temperatures with a $0.005\text{−}{\mathrm{s}}^{-1}$ strain rate the maximum strain (${\varepsilon }_{\max }$) is defined in the figure, (c) temperature dependences of the mechanical energy dissipation calculated by integrating the areas within the loops in (b), and (d) temperature dependences of the maximum strain (${\varepsilon }_{\max }$), transformation strain $\left({\varepsilon }_{\mathrm{tr}}\right)$, anelastic strain (${\varepsilon }_{\mathrm{ae}}$), and elastic strain (${\varepsilon }_{\mathrm{el}}$) for the nanograin Ti-44Ni-5Cu-1Al (at.%) alloy.

Figure 4

(a) Time dependence of the temperature of the specimen measured in the loading, holding, unloading, and holding processes with a fast loading/unloading rate of $0.2{\mathrm{s}}^{-1}$, (b) testing temperature dependence of the experimental adiabatic temperature change ${\mathrm{\Delta }\mathrm{T}}_{\mathrm{adi}}^{\exp }$ during the stress fast removing process $\left(0.2{\mathrm{s}}^{-1}\right)$ from 1 GPa. The solid line is the calculated adiabatic temperature change ${\mathrm{\Delta }\mathrm{T}}_{\mathrm{adi}}^{\mathrm{cal}}$ calculated from isothermal stress-strain curves using the Maxwell relation for the nanograin Ti-44Ni-5Cu-1Al (at.%) alloy.