The Regulation Of Magnetic Phase Transitions And Associated Physical Properties In All-d-metal Ni-Mn Based Heusler Alloys

As a new type of Heusler alloy system discovered in recent years,all-d-metal Ni-Mn-Ti based magnetic phase transition alloys can occur magnet-field or stress-induced martensitic transformation(MT),which is accompanied by abundant magento-functional properties,including magnetic field induced shape memory effect,magnetocaloric effect(MCE),elastocaloric\barocaloric effect,magnetoresistance,magnetic-field induced strains and so on.Accordingly,the all-d-metal alloys have great application potential in many frontier fields,such as the solid-state refrigeration field.However,due to the short discovery time,the research about the structure and physical properties is still not perfect,so it is very important to make an in-depth investigation for all-d-metal Heusler alloy systems.In this thesis,the effect of element substitution or different annealing conditions on the phase transition behavior and the corresponding magnetocaloric effect has been investigated in all-d-metal Mn-Ni-Co-Ti alloy ribbons systems.Besides,based on its excellent mechanical properties,the Ni₄₈Cu₂Mn₃₃Ti₁₇ alloy was prepared by directional solidification technology,and the orientation-dependent of elastocaloric effect and mechanical properties were systematically investigated by combining DIC with IR technology.In the first part of this thesis,the all-d-metal Mn₅₀Ni₃₂Co₈Ti₁₀ alloy ribbons are prepared by the arc melting and melt spunning technologies,the effec of annealing on phase transformations and MCE was systematically investigated.It is found that the annealing leads to the precipitation of?phase and its fraction is increasing evidently with the increase of annealing temperature.So,the MT temperature shifts to lower temperature with the increase of annealing temperature,the MT behavior was suppressed and become slow.The magnetic entropy change(?S_M,_max)of melt spun and annealed ribbons were calculated by the Maxwell relation.Under the magnetic field change of?₀H=0-5 T,the?S_M,_max of melt spun is 16.0 Jkg^-1K^-1,and the?S_M,_max of annealed ribbons at 1053K,1103 K,1153 K are?3.5 Jkg^-1K^-1,?6.4 Jkg^-1K^-1,?7.2 Jkg^-1K^-1 respectively.In the second part of this thesis,based on the previous Mn₅₀Ni₃₂Co₈Ti₁₀ alloy ribbons,the magnetocaloric effect can effectively improve by element substitution.In this paper,Cu element is selected to replace Ni,the effects of Cu-substitution and annealing on MT,and MCE for the alloy ribbons were studied.After Cu-substitution,the feature of the first-order phase transition does not change,but the phase transformation temperatures shift to lower temperatures with the increasing of Cu-substitution.The?S_M,_max gradually increases and the effective refrigeration capacity largely increases in comparison with that Cu-free substitution.After annealing,the phase transformation becomes slow and which results in the decrease of the?S_M,_max,but the effective refrigeration capacity almost unchanged due to the the double increase of refrigeration temperature interval.In the third part of this thesis,the Ni₄₈Cu₂Mn₃₃Ti₁₇alloy was synthesized by the directionally solidified method.The crystalline orientation dependence of elastocaloric effect and the strain evolution in Ni₄₈Cu₂Mn₃₃Ti₁₇ alloy were analyzed by combinating DIC and IR measurements.EBSD results show that the crystal grows along the direction of<001>orientation.The B2 cubic austenite structure is determined by the Transmission Electron Microscopy(TEM)method.The elastocaloric measurement results show that along the<001>texture orientation Ni₄₈Cu₂Mn₃₃Ti₁₇alloy presents a large?T_ad,max value of 20.6 K under 463 MPa,for which?T_max/?_max value is 44.5K/GPa,while along the direction perpendicular to the<001>orientation,?T _ad,max of the sample is only 18.2 K under 765 MPa.The?T_max/?_max value is just 25.8 K/GPa.The difference of orientation dependence of elastocaloric effect would originate from the different volume fractions of stress-induced MT under the same conditions.