Optimizing Ni–Ti-based shape memory alloys for ferroic cooling

Abstract

Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni[Formula: see text]Ti[Formula: see text]Cu5V[Formula: see text] SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti.

NiTi‐Based Elastocaloric Cooling on the Macroscale: From Basic Concepts to Realization

“…Within the framework of the Deutsche Forschungsgemeinschaft (DFG) priority program, various new results on fundamental material aspects, especially on phase transformation behavior, were obtained . These results supported the development of a new pseudoelastic SMA that exhibits superior properties for ferroic cooling . All key findings are presented here in brief.…”

“…In a first step, precise reference data sets on phase transformation temperatures and latent heats were obtained for ternary NiTiA (A=Cr, Co, Cu, Fe, Hf, Pd, V, Y, Zr) and quaternary NiTiCuB (B=Co, Pd) SMAs . The various SMAs were prepared using an optimized melting procedure that yields material states that are characterized by very low deviations from target alloy composition, and by low impurity concentrations.…”

“…Adding Ni, Cr, Co, Fe, and V to stoichiometric NiTi results in similar trends for all different alloy systems (compositions are given in Ref. ). A decrease in A F is always associated with a decrease in latent heat.…”

“… Correlation between latent heats Δ H and austenite finish temperatures A F for various NiTi‐based alloy systems . Alloying Co, Cr, Fe, Ni and V to stoichiometric NiTi results in a decrease of both parameters.…”

“…Elastocaloric materials, especially NiTi, exhibit very large latent heats of up to 30 J g −1 [8] during the diffusionless first‐order phase transformation and require only a small work input leading to material coefficients of performance (COP) of up to 30 . Additionally, high temperature changes up to 30 K have been reported on the micro‐ and macroscale …”