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Study on improving fatigue life of Nitinol alloy material

Elastic-thermal refrigeration is a new solid-state refrigeration technology that utilizes latent heat of the reversible phase transition of niti shape memory alloy under axial stretching, compression and torsion. Compared with the traditional vapor compression refrigerant, the elasto-thermal refrigerant such as niti shape memory alloy has no greenhouse gas effect and has the advantage of environmental protection. In addition, the elastic-thermal effect energy density is significant, ranking first among 17 non-vapor compression refrigeration technologies evaluated by the U.S. department of energy in 2014.At present, the fatigue life of elastothermal material is the primary factor restricting its engineering application, and the efficiency of elastothermal working medium and elastothermal refrigerator still needs to be improved.

For this, the university of Maryland, Ames laboratory, university of Colorado mining, xi 'an jiaotong university, Iowa state university cooperation, promote efficiency and fatigue life of the two key problems, using powder laser directed energy deposition technique, the nickel titanium alloy material was prepared with nanometer composite structure, can be directly forming columnar, tube and honeycomb structure of the thermal cooling can be used to play back. Different from the traditional shape memory alloy, the Nitinol alloy produced by the additive was found to have quasi-linear stress-strain response, hysteresis independent of loading rate and significantly reduced hysteresis. Advanced in-situ loading synchronous X-ray diffraction and electron microscopy characterization indicate that the cause of low phase transition hysteresis is the interfacial dislocation generated by nanometer Ni3Ti and NiTi grain boundaries, which can be used as nucleation point in phase transition process, effectively reducing the potential barrier to be overcome in phase transition process and reducing the friction loss at phase interface. The above characteristics can improve the fatigue life and make the elastothermal effect remain unchanged after millions of loading and unloading cycles.
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