Elastocaloric cooling of additive manufactured shape memory alloys with large latent heat

Abstract

The stress-induced martensitic phase transformation of shape memory alloys (SMAs) is the basis for elastocaloric cooling. Here we employ additive manufacturing to fabricate TiNi SMAs, and demonstrate compressive elastocaloric cooling in the TiNi rods with transformation latent heat as large as 20 J g −1 . Adiabatic compression on as-fabricated TiNi displays cooling DT as high as −7.5 °C with recoverable superelastic strain up to 5 %. Unlike conventional SMAs, additive manufactured TiNi SMAs exhibit linear superelasticity with narrow hysteresis in stress-strain curves under both adiabatic and isothermal conditions. Microstructurally, we find that there are Ti 2 Ni precipitates typically one micron in size with a large aspect ratio enclosing the TiNi matrix. A stress transfer mechanism between reversible phase transformation in the TiNi matrix and mechanical deformation in Ti 2 Ni precipitates is believed to be the origin of the unique superelasticity behavior. Abstract:The stress-induced martensitic phase transformation of shape memory alloys (SMAs) is the basis for elastocaloric cooling. Here we employ additive manufacturing to fabricate TiNi SMAs, and demonstrate compressive elastocaloric cooling in the TiNi rods with transformation latent heat as large as 20 J g −1 . Adiabatic compression on as-fabricated TiNi displays cooling ∆T as high as −7.5 °C with recoverable superelastic strain up to 5 %. Unlike conventional SMAs, additive manufactured TiNi SMAs exhibit linear superelasticity with narrow hysteresis in stress-strain curves under both adiabatic and isothermal conditions. Microstructurally, we find that there are Ti2Ni precipitates typically one micron in size with a large aspect ratio enclosing the TiNi matrix.A stress transfer mechanism between reversible phase transformation in the TiNi matrix and mechanical deformation in Ti2Ni precipitates is believed to be the origin of the unique superelasticity behavior.

The influence of texture on the reversible elastocaloric effect of a polycrystalline Ni50Mn32In16Cr2 alloy

“…Both DS iso and DT ad are related to DH and DS transformation , which limits the maximum response due to an external stimuli; latent heat in TiNi alloys can be as large as 30 J g À1 , and meanwhile, in ferromagnetic alloys, it is up to 10 J g À1 . 12 With this in mind, extensive studies that correlate the latent heat (and transformation entropy) with the transformation temperatures are of deep interest. 13,14 The application of an external field, named the magnetic field or the mechanical field (hydrostatic pressure or uniaxial stress), to ferromagnetic shape memory alloys produces a modification of the equilibrium conditions in the material, giving thermodynamic stability to one phase over the other.…”

Elastocaloric Cooling: State-of-the-art and Future Challenges in Designing Regenerative Elastocaloric Devices

“…Nonetheless, there is still room for further improvements by investigating new techniques, chemical compositions, process parameters, heat treatments etc. [57] and [59] to [62].…”

Overcoming fatigue through compression for advanced elastocaloric cooling

“…The exploration of elastocaloric materials has now been expanded from Ni-Ti-based alloys to Cu-based alloys and magnetic alloys. 7 At the same time, elastocaloric cooling system prototypes have also been demonstrated based on tensile, 8 compressive, 9 and bending modes. 10 Elastocaloric cooling is based on shape memory alloys (SMAs).…”

“…The elastocaloric cooling takes place when the latent heat is absorbed back in the material during the reverse transformation from martensite to austenite in the unloading part of superelasticity. 5,7 Continuous operation of elastocaloric cooling requires repetitive application of stress to SMAs, and the large number of mechanical loading cycles can potentially lead to onset of fatigue. Fatigue refers to deterioration of mechanical properties of a material and the eventual failure under cyclic application of mechanical loads.…”

“…Concept of work recovery, and the implemented work-recovery design in an elastocaloric system. (a) Schematic of the typical superelastic behavior in SMAs 7. (b) Concept of work recovery achieved by linearly stacking two SMA tube bundles 9.…”