Enhancing functional stability of NiTi tube for elastocaloric cooling through overstress training

Abstract

Tubular NiTi is a promising candidate refrigerant for eco-friendly elastocaloric cooling, but its severe functional degradation during cyclic phase transition (PT) is a key concern in the technology development. Here, plastic deformation of 6.7% is applied to NiTi tube by overstress training under 1900 MPa for five cycles to improve the cyclic PT stability without losing cooling efficiency. It is found that after 106 compressive cycles under 1000 MPa, the overstress-trained NiTi tube exhibits small residual strain (0.5%), stable adiabatic temperature drop (ΔT = 11K) and improved coefficient of performance (COP = 55), showing high functional stability and good cooling performance. Transmission electron microscopy (TEM) observations show that the microstructure of the overstress-trained NiTi tube consists of 5–10 nm sized austenite (B2) and martensite (B19′) nanodomains with near-saturated dislocation density (ρ ≈ 5.153 × 1016 m−2). Such dislocation-enriched nanostructure effectively suppresses further formation and motion of dislocation during subsequent cyclic compression, thereby significantly enhancing the cyclic stability of the NiTi tube. The residual B19′ nanodomains and dense dislocations change the PT mode from nucleation-growth of the B19′ phase to direct growth of residual B19′ nanodomains, thereby reducing the dissipation of the PT process and increasing the COP of the NiTi tube. Our study provides an economic and effective method for to enhance the cyclic stability of bulk NiTi tubes for solid-state refrigeration.

Keywords

• NiTi shape memory alloy
• Functional stability
• Elastocaloric effect
• Dislocation
• Residual martensite