Study On The Crystal Structure And Magnetocaloric Properties In Novel Ni-mn Based Ferromagnetic Shape Memory Alloys

In the past 20 years,researchers found that Ni-Mn based ferromagnetic shape memory alloys show very good multifunctional characteristics,including magnetic field-inducedstrain,shapememoryeffect,magnetocaloriceffectand magnetoresistance.Thus,it is expected to be a new generation of sensors,actuators,magnetic refrigeration and transducers used in aerospace,biomedical engineering and daily life.In the present paper,three kinds of Ni-Mn based ferromagnetic shape memory alloys were focused.The chemical composition was designed and tuned reasonably and that the martensitic transformation and magnetocaloric properties were systematically studied.Furthermore,the precise crystal structure was determined by advanced characterization techniques,such as synchrotron high-energy X-ray diffraction and neutron diffaction.The main results are as follows:The Curie transition temperature,martensitic transformation temperature and intermediate martensitic transformation temperature of Ni_57-xMn_21+xGa₂₂?x=0,2,4,5.5,7,8??at.%?alloys were determined by differential scanning calorimetry and physical property measurement system.The structural response of coexisting multiple martensites to stress field in Ni_51.5Mn_26.5Ga₂₂ alloy was investigated by the in-situ high-energy X-ray diffraction technique.Stress-induced transformation between coexisting multiple martensites was observed at 110 K,at which five-layered modulated?5M?,seven-layered modulated?7M?and non-modulated?NM?martensites coexist.We found that a tiny stress of as low as 0.5 MPa could trigger the transformation from 5M and 7M martensites to NM martensite and this transformation is partly reversible.Besides the transformation between coexisting multiple martensites,rearrangement of martensite variants also occurs during loading,at least at high stress levels.The present study is instructive for designing advanced multifunctional alloys with easy actuation.We have substituted Co element for Ni element in Ni_50-xCo_xMn₄₀Sn₁₀?x=6,7,8,9,10?alloys.It is found that,with the increase of the content of Co atoms,the magnetization of parent phase has been significantly improved,and the magnetization of martensitic phase only slight variations,thus giving rise to evident increase of?M between both phases.The giant effective magnetic refrigeration capacity in a Ni₄₀Co₁₀Mn₄₀Sn₁₀ multifunctional alloy has been investigated.With a large magnetization difference between austenite and martensite,this alloy shows a strong magneticfielddependenceoftransformationtemperatures.Complete magnetic-field-induced structural transformation and a considerable magnetic entropy change are observed in a broad operating temperature window of 33K near room temperature.Consequently,an effective magnetic refrigeration capacity of 251 J/kg for 5 T is achieved,which is the largest value for Ni-Mn-based Heusler alloys and comparable to that of the high-performance Gd-Si-Ge and La-Fe-Si magnetocaloric materials.Incorporating the advantages of low cost and non-toxicity,this alloy shows very promising prospects for room-temperature magnetic refrigeration.By replacing the Sn element with the Al element,the thermal hysteresis of Ni₄₀Co₁₀Mn₄₀Sn₉Al₁ is significantly reduced to 13 K,compared with that of Ni₄₀Co₁₀Mn₄₀Sn₁₀ alloy?33 K?.The crystal structure of Ni₄₀Co₁₀Mn₄₀Sn₉Al₁ alloy was studied by synchrotron high-energy X-ray diffraction technique.The high temperature phase was Heusler L2₁ structure,and the low temperature phase was 6 layered modulated martensite.By means of TEM diffraction patterns and high resolution graphs,non-modulated martensite structure and other multi-layered modulated martensite were also observed in addition to the 6 layered modulated martensite structure.Under the 5T external magnetic field,the magnetic entropy change of 30.5J/?K kg?was achieved through magnetic-field-induced martensitic transformation.Reversibility of the magnetocaloric effect in materials with first-order magnetostructural transformation is of vital significance for practical magnetic refrigeration applications.The regulation and design of Ni_51-xCo_xMn_33.5In_15.5?x=0,1,1.2,2,3?alloys was carried out.The large reversible magnetocaloric effect in a Ni_49.8Co_1.2Mn_33.5In_15.5 magnetic shape memory alloy was found.A large reversible magnetic entropy change of 14.6 J/?kg K?and a broad operating temperature window of 18 K under 5 T were simultaneously achieved,correlated with the low thermal hysteresis?8K?and large magnetic-field-induced shift of transformation temperatures?4.9 K/T?that lead to a narrow magnetic hysteresis?1.1 T?and small average magnetic hysteresis loss?48.4 J/kg under 5 T?as well.Furthermore,a large reversible effective refrigeration capacity?76.6 J/kg under 5 T?was obtained,as a result of the large reversible magnetic entropy change,broad operating temperature window,and small magnetic hysteresis loss.The large reversible magnetic entropy change and large reversible effective refrigeration capacity are important for improving the magnetocaloric performance,and the small magnetic hysteresis loss is beneficial to reducing energy dissipation during magnetic field cycle.