Elastocaloric Effect In Pd-In-Fe Magnetic Shape Memory Alloys

With the increasingly serious energy crisis and environmental problems,traditional vapor compression refrigeration cannot meet the requirements of green energy saving due to many disadvantages such as low efficiency,environmental pollution and noise.Solid state refrigeration technology has attracted more and more attention from the researchers because of advantages of no pollution,high efficiency and low energy consumption.Elastocaloric cooling based on stress-induced martensite phase transition is one of the hot spots in solid state refrigeration.Elastocaloric cooling has the advantages of large adiabatic temperature change,high energy efficiency and simple stress-driven mode.In this paper,the stress-induced transformation behavior of Pd-In-Fe magnetic shape memory alloys has been studied.The microstructure,crystal structure,martensitic transformation,superelasticity and elasticcaloric effect has been investigated in Pd-In-Fe alloys prepared by different methods including arc melting,highly undercooling quickly solidification and optical floating-zone technology.In this thesis,firstly we have investigated the microstructure,martensitic transformation,and elastocaloric effect of Pd-In-Fe polycrystalline alloys.For this new magnetic shape memory alloy,both phase constitution and morphology highly depend on the Fe content.Exhibiting a single austenite phase at room temperature,Pd_59.3In_23.2Fe_17.5 alloy has a large superelastic cooling effect of-5.4 K with a low critical stress of 80 MPa to trigger the martensitic transformation.Interestingly,the onset of superelastic stress can be drastically reduced by mechanical training.Secondly,a large degree of undercooling of 182 K has been achieved in Pd_59.3In_23.2Fe_17.5 shape memory alloy through the glass fluxing combined with superheating cycling.An adiabatic temperature rise of 3 K was measured at 347 K under a mechanical loading of 154 MPa.The undercooled Pd_59.3In_23.2Fe_17.5 exhibits a linear superelasticity with a small hysteresis energy loss of 0.35 MJ/m³.A Pd_56.6In_25.6Fe_17.8 single crystal with the orientation near<101>was prepared by optical float-zone-melting technique.After high-temperature annealing,the chemical composition of the single crystal were relatively homogeneous.The sensitivity of the critical stress for martensite transformation to transformation temperature was calculated as 3.38 MPa/K by the low-temperature mechanical experiments.An adiabatic temperature change of 2.7 K was obtained upon releasing a uniaxial stress of300 MPa at 204 K in the Pd_56.6In_25.6Fe_17.8 single crystal.