[Paper Review] Functional Degradation and Self-enhanced Elastocaloric Cooling Performance of NiTi Tubes under Cyclic Compression

Dingshan Liang, Peng Hua+3 more|arXiv (Cornell University)|Jul 23, 2021

Shape Memory Alloy Transformations52 references4 citations

TL;DR

This study reveals that functional degradation in nanocrystalline NiTi tubes under cyclic compression paradoxically enhances their elastocaloric cooling performance. Residual strain accumulation from phase-transition-induced dislocations and dislocation-pinned martensite reduces hysteresis loop area (D) and forward transformation stress, leading to a self-enhanced material coefficient of performance (COPmater) by 40–104%, primarily due to decreased D.

ABSTRACT

Superelastic NiTi tubes are promising candidates for eco-friendly elastocaloric cooling, but their cyclic stability suffers severely from functional degradation. Herein, we investigate the functional degradation of nanocrystalline NiTi tubes via in-depth analysis and find out that it is beneficial to elastocaloric cooling performance. The results show that the functional degradation accompanies with progressive accumulation of residual strain and significant reduction in both hysteresis loop area (D) and forward transformation stress. The accumulation of residual strain arises from phase-transition-induced dislocations and dislocation-pinned residual martensite. The former separates the original austenite grains to much smaller nanodomains (equivalent to grain size effect) and contributes to the strain hardening during phase transition, leading to the significant reduction of D. The latter induces compressive residual stress in the austenite phase and thus gives rise to the evolutive reduction of forward transformation stress. Consequently, the material coefficient of performance (COPmater) was self-enhanced for 40~104 %. The beneficial effect is mainly because of the cyclically-decreased D. The study might provide a processing route to tailor COPmater and stabilize the mechanical response of NiTi by cyclic compression.

Motivation & Objective

• To investigate the mechanisms of functional degradation in superelastic nanocrystalline NiTi tubes under cyclic compression.
• To determine the impact of residual strain and microstructural evolution on elastocaloric performance.
• To evaluate whether functional degradation can be leveraged to enhance cooling efficiency in shape memory alloy-based cooling systems.
• To explore the potential of cyclic compression as a processing route to stabilize mechanical response and improve COPmater in NiTi tubes.

Proposed method

• In-depth microstructural and mechanical analysis of NiTi tubes after repeated cyclic compression testing.
• Use of in situ X-ray diffraction and electron backscatter diffraction (EBSD) to track phase transformation behavior and residual strain accumulation.
• Quantification of hysteresis loop area (D) and forward transformation stress evolution across cycles.
• Correlation of microstructural features—such as nanodomains and dislocation density—with mechanical and caloric response.
• Calculation of material coefficient of performance (COPmater) to assess cooling performance enhancement.
• Analysis of residual stress states via X-ray diffraction and modeling of their effect on transformation stress.

Experimental results

Research questions

• RQ1 How does cyclic compression induce functional degradation in nanocrystalline NiTi tubes?
• RQ2 What microstructural changes are responsible for the observed reduction in hysteresis loop area (D) and forward transformation stress?
• RQ3 To what extent does functional degradation improve the material coefficient of performance (COPmater) in elastocaloric cooling?
• RQ4 Can residual strain accumulation from phase transitions be harnessed to enhance elastocaloric performance?
• RQ5 What role do dislocation-pinned martensite and grain refinement play in the self-enhancement of COPmater?

Key findings

• Residual strain accumulates progressively during cyclic compression due to phase-transition-induced dislocations and dislocation-pinned martensite.
• The hysteresis loop area (D) decreases significantly due to strain hardening from nanodomain formation, which mimics a grain size effect.
• Forward transformation stress reduces over cycles due to compressive residual stress in the austenite phase induced by dislocation-pinned martensite.
• The material coefficient of performance (COPmater) increases by 40–104% due to the cyclically decreased D, indicating self-enhanced cooling performance.
• The beneficial effect on COPmater is primarily attributed to the reduction in hysteresis loss, not to changes in transformation entropy or temperature span.
• Cyclic compression can be used as a processing route to tailor COPmater and stabilize mechanical response in NiTi tubes.