Study On The Elastocaloric Properties Of Ni-Mn-based And Ti-Ni-Cu-Co Shape Memory Alloys

Refrigeration has become a vital technology for modern society,which is increasingly demanded for industry,transportation and households,and it accounts for?30 percent of the global electricity consumption.However,the currently used traditional vapor-compression cooling technology employs gaseous refrigerants which have high global warming potential,and the leakage of the refrigerants harms the environment.Recently,solid-state refrigeration based on elastocaloric effect has attracted great interests and has been regarded as an environment-friendly alternative to the widely used traditional vapor-compression cooling technology.In particular,the elastocaloric refrigeration is considered to be the most promising non-vapor-compression cooling technology by the US Department of Energy,and it shows remarkable application prospects in solving energy crisis and environmental protection issues.Elastocaloric refrigeration relies on high-performance elastocaloric materials,and thus increasing efforts have been devoted to developing materials with outstanding elastocaloric performance.In this dissertation,we studied the elastocaloric properties of Ni-Mn-based and Ti-Ni-Cu-Co shape memory alloys.The Ni-Mn-based magnetic shape memory alloys exhibit fascinating multicaloric effects(including elastocaloric,magnetocaloric and barocaloric effects)have attracted increasing attention during the past decade,but they are intrinsically brittle and their elastocaloric effect shows poor cyclability,which remains a major challenge for exploiting more efficient multicaloric refrigeration.Here we demonstrate that microalloying with boron is very effective in enhancing the mechanical properties and cyclic stability of elastocaloric effect in Ni-Mn-In intermetallic magnetic shape memory alloys.By employing a novel strategy of strengthening grain boundary,the cyclability of elastocaloric effect in the prototype Ni-Mn-based magnetic shape memory alloys is strikingly enhanced by two orders of magnitude.The elastocaloric effect of the boron-free Ni51.5Mn33In15.5 alloy degrades rapidly after only?20 cycles.In contrast,high cyclic stability of a large elastocaloric effect is achieved in the boron-microalloyed(Ni51Mn33In14Fe2)99.4B0.6 alloy:the high adiabatic temperature change of?5.6 K remains stable for 2700 cycles of loading and unloading.This high cyclability of elastocaloric effect far exceeds that reported in other polycrystalline magnetic shape memory alloys.The possible mechanism for the grain boundary strengthening by boron microalloying was also investigated.Furthermore,the same strategy was demonstrated to be effective in Ni-Mn-Sn-based magnetic shape memory alloys.In addition,the effect of carbon addition on the elastocaloric effect and mechanical properties for Ni-Mn-Ti all-d-metal Heusler shape memory alloy has been systematically investigated.The carbon addition is very effective in enhancing the superelastic cyclic stability in Ni-Mn-Ti alloys.The carbon-free Ni50Mn32Ti18 alloy is fractured only after 8 compression cycles;in contrast,the carbon-doped(Ni50Mn2Ti18)99.2C0.8 alloy exhibits repeatable superelastic performance over 1000 cycles.The present strategy has been proved to be very effective in enhancing the mechanical properties in a variety of Ni-Mn-based shape memory alloys,which is useful for overcoming the cyclability issues in the ubiquitous brittle intermetallic phase-transforming materials.The Ti-Ni-based shape memory alloys are most studied and considered as the most promising elastocaloric materials for practical applications owing to their superb mechanical properties,large elastocaloric effect,superior corrosion resistance,and commercial availability.However,the binary Ti-Ni shape memory alloys usually exhibit a large hysteresis which leads to large energy dissipation.Here we have developed a bulk polycrystalline Ti-Ni-Cu-Co shape memory alloy exhibiting large elastocaloric effect,low stress hysteresis and room-temperature working temperature,all of which are of great importance to and urgently demanded for high-efficiency room-temperature elastocaloric refrigeration.This newly developed(Ti50Ni42.5Cu7.5)99Co1 alloy shows a large room-temperature elastocaloric effect with directly measured adiabatic temperature change up to 14.4 K during unloading.The stress hysteresis of the isothermal superelastic stress-strain curve is as low as 60 MPa when the maximum tensile strain is 2.7%.Owing to the large elastocaloric effect and low stress hysteresis,a very high coefficient of performance up to 19 is achieved on the material level.Advanced in-situ synchrotron high-energy X-ray diffraction technique was employed to reveal the phase transformation sequence and to accurately determine the crystal structure of different phases,based on which the lattice compatibility between the transforming phases was evaluated and the phase transformation strain was predicted,providing in-depth fundamental understanding of the martensitic transformation in this newly developed alloy.This work could be useful for designing high-performance elastocaloric materials for solid-state cooling applications.