Elastocaloric Effect And Microstructure In Directionally Solidified Ni-Mn-based Magnetic Shape Memory Alloys

Global warming poses a serious challenge to modern society,but the greater demand for refrigeration is still increasing.Current hydrofluorocarbons(HFCs)refrigerants significantly contribute to greenhouse effects in the most of refrigerators.Therefore,it is a matter of some urgency to find an alternative cooling technologies which do not produce harmful substances for environment to get rid of this vicious circle.Recently,solid-state cooling technologies based on the caloric effect of the phase transformation materials has attracted considerable interest.Caloric effects refer to the thermal response of a solid under the application/removal of an external field.Ni-Mn-based Heusler metamagnetic shape memory alloys(MSMA)possess large isothermal entropy change,low hysteresis,and magneto-structure coupling,which have became a hot topic of research.Nevertheless,conventional Ni-Mn-based alloys also present a major drawback of inherent brittleness,which is a mainly challenge in cooling applications.It is essential to solve inherent brittleness and improve fatigue life.In the thesis,using a combination of in situ non-touch digital image correlation(DIC)and infrared(IR)thermography technique to monitor the evolution of the two-dimensional temperature and strain field upon uniaxial stress,the relation mechanism of microstructure and martensitic transformation(MT)behaviors was investigated.In the thesis,firstly we have studied the relied relation of the elastocaloric effect(eCE)and crystalline orientation in MSMA Ni₄₅Mn₄₄Sn₁₁ by a combination of DIC and IR technique.A polycrystalline alloy containing a<111>orientation grain and an off-<111>orientation grain was grown by directional solidification method.In the same total strain,the<111>orientation grain which has a smaller transformation strain exhibited a larger adiabatic temperature change(?T_ad)of 11.5 K.And average?T_ad of-9.5 K was observed upon stress releasing.Moreover,large magnetic entropy changes of 25.8 J kg^-1 K^-1 for magnetic field changes of 50 k Oe were obtained.In order to obtain the better mechanical properties,in Ni-Mn-Sn alloy,the?-phase(fcc structure)was introduced by doping excess Co element,and the MT temperature and microstructure in Ni₃₂Co₁₁Mn₅₀Sn₇ alloy were regulated by the cooling rates.Ni-Co-Mn-Sn double-phase alloys with straight?phase significantly improved the mechanical performance,whose compressive strength reached 1162 MPa.As the cooling rate increases,the content of?phase in the sample decreases,and eventually the single-phase alloy was achieved whose MT temperature moves to a high temperature.In addition,it was found that heterogeneous strain distribution reduced the shape memory effect by DIC technique in non-textured polycrystal of single-phase specimen.We also have systematically studied the microstructural evolution,MT behaviors,and eCE of directionally solidified Ni_35.5Co_14.5Mn₃₅Ti₁₅ all-d-metal Heusler MSMA.Electron backscatter diffraction(EBSD)analysis revealed that the specific crystallographic inherited relation is from the<001>orientation of austenite to the<105>orientation of 5M martensite.Besides,the multi-modulated martensite with seven-layer-modulated(7M)and eight-layer-modulated(8M)structures was identified by transmission electron microscopy(TEM).Using a unique technique of in situ digital image correlation(DIC)with the combination of infrared thermography,MT and eCE behaviors were studied.Ni_35.5Co_14.5Mn₃₅Ti₁₅ alloy yielded a large?T_ad of 11.5 K with a low?_cr of 38 MPa and a moderate stress hysteresis of 54 MPa.The adiabatic temperature change normalized to a unit of critical stress achieved 0.31 K MPa^-1 in the alloy.This improved eCE was attributed to the enhanced compatibility in the<001>oriented polycrystalline alloy and the high mobility of the low-energy twin boundary in the multi-modulated martensite.