Investigation On The Regulation Of Phase Transition And Magnetocaloric Effects In Ni-Mn-Sn-Fe(Co) Alloys

Ni-Mn-Sn alloy has excellent magnetocaloric properties,but there are some problems such as small martensite/austenite magnetization difference and narrow refrigeration working range.The doping of ferromagnetic element Fe/Co is one of the effective methods to solve this problem.In the present thesis,Ni_50-x0-x Mn₃₈Sn₁₂Fe_x（x=0⁸.5）and Ni_47-x7-x Mn₄₃Sn₁₀Co_x（x=0¹1.0）series alloys were designed for broadening working interval and refrigeration capacity.The effects of Fe/Co doping on the structure,crystal structure,martensitic transformation（MT）,magnetic properties,magnetocaloric effect（MCE）of Ni-Mn-Sn alloy were studied.The effect of low-temperature aging on microstructure,MT,magnetic properties and MCE were analyzed.Finally,Fe/Co doped microwires were prepared by a melt-extraction method,and the heat treatment processes were optimized.The effect of magneto-structural coupling behavior on Ni-Mn-Sn-Co microwires was analyzed.It was found that the microstructure was affected by the substitution of Fe for Ni in Ni_50-x0-x Mn₃₈Sn₁₂Fe_x.When the doped Fe is less than 1.1 at.%,the room-temperature phase is single 4O-modulated martensite.When the doped Fe reaches 2.9 at.%,a small amount of Fe-rich second phase（γphase,face-centered cubic structure）is precipitated in the matrix.With further increase of Fe content,the content and size ofγphase increase gradually.The compressive fracture strength and strain of NMSF4 alloy are 818 MPa and 7.2%respectively due to the dispersion strengthening effect,which is greatly increased comparing with 351MPa and 3.1%in Ni-Mn-Sn ternary alloy.The appearance ofγphase is beneficial to improving the intrinsic brittleness of the alloy.Although Fe-doping in Ni_50-x0-x Mn₃₈Sn₁₂Fe_x has little effect on the Curie temperature of martensite and austenite,it can obviously reduce the martensite transformation temperatures.Therefore,the regulation of magnetic-structural transformation can be realized by adjusting Fe content,and the alloy with partial and complete magnetic-structural coupling can be obtained.Of note is that NMSF4 alloy exhibited a strong metamagnetic transition behavior,i.e.Magnetic-field-induced reverse martensite transformation,which is favorable for the magnetocaloric effects.The magnetic entropy（ΔS_M）of 33.8 J/（kg·K）with 7 K of working temperature range(ΔT_FWHM)was obtained at 5.0 T,reaching144 J/kg of effective refrigeration capacity RCP_eff.When the external magnetic field is 2.0 T,ΔS_M reached 12.2 J/（kg·K）andΔT_FWHM reached 8 K.The inverse and conventional magnetocaloric effect are induced in NMSF1 alloy by creation of a partial overlap between reverse MT and magnetic transition Consequently,inverse magnetocaloric effect associated with a positiveΔS_M of 9.6 J/（kg·K）andΔT_FWHM from 306 to 314 K stretch across reverse MT and T_C^A temperature range.This IMCE is followed by a conventional magnetocaloric effect with a negativeΔS_M of-2.7 J/（kg·K）andΔT_FWHM from 316 to 339 K under an external magnetic field of 5.0 T.By taking advantage of these two successive MCE using a special designed device,large caloric cooling effect with enhanced working temperature interval may be of interesting for practical room-temperature refrigeration applications.The total refrigeration capacity RCP reaches 139 J/kg,which is comparable with NMSF4 alloy.When the content of Co in Ni_47-x7-x Mn₄₃Sn₁₀Co_x alloy reaches 11 at.%（NMSC11-1）,no second phase is precipitated.Co doping decreases the martensite Curie point（T_C^M）and increases the austenite Curie point（T_C^A）,thus broadening the Curie temperature window of the alloy.When Co doping amount is 6 at.%（NMSC6-1）,T_C^M decreases below 150 K.When Co doping increases from 6 at.%to 11 at.%,the T_C^A increases from 387 K to 476 K,thus broadening the temperature window for a high magnetization austenite（13.5,91.7 and 109.1A·m²/kg for x=0,6 and 11,respectively）.Two successive magnetostructural transformations（A→10M and A→10M+6M）occur in the alloy x=6,which are responsible for the giant magnetic entropy changeΔS_M=29.5 J/（kg·K）,wide working temperature spanΔT_FWHM=14 K and large effective refrigeration capacity RCP_eff=314 J/kg under a magnetic field of 5.0 T.ΔS_M reaches 18.9J/（kg·K）andΔT_FWHM reaches 7 K under 2.0 T.There is no second phase and composition segregation for Ni_39.9Mn_43.8Sn_10.3Co_6.0（NMSC6-2）alloy after aging at 473 K.The critical magnetic field for the saturatedΔS_M is 5.5 T.After aging for 600 min,the critical magnetic field decreases to 3.0 T,which is of great significance for the improvement ofΔS_M andΔT_FWHM.Under 2.0 T,ΔS_M reached 31.3 J/（kg·K）after600 min ageing,which is a significant improvement comparing with 22.7 J/（kg·K）before aging.Small-sized materials exhibit large surface to volume ratio and low thermal/magnetic hysteresis during magnetic refrigeration cycling,thus may help in enhancing the refrigeration efficiency.Under external magnetic field of 5.0 T,aΔS_M decreased from 33.8 J/（kg·K）of NMSF4 alloy to 15.2 J/（kg·K）of Ni_45.6Fe_3.6Mn_38.4Sn_12.4（NMSF3.5-W）microwires,whileΔT_FWHM increased from7 K to 12 K,and finally RCP_eff reached 141 J/kg,which was equivalent to NMSF4 bulk alloy(RCP_eff=144 J/kg).ΔS_M obtained by NMSF 4.0-W microwires with different diameters into bundles is only 6.8 J/（kg·K）,butΔT_FWHM reaches 29.5 K,and RCP_eff of 172 J/kg is finally obtained.Under 5.0 T,ΔS_M of 20 J/（kg·K）with a broadΔT_FWHM of 20 K around the martensite transformation were obtained in Ni_41.3Mn_42.4Sn_10.0Co_6.3（NMSC6-W）microwires,and finally the RCP_eff reached 303 J/kg,which was equivalent to NMSC6-1 bulk alloy(RCP_eff=314 J/kg).