Regulation Of Transformation,Microstructure And Associated Physical Properties In Magnetic Alloys

Magnetic materials are often accompanied by miscellaneous physical effects during the phase transition,indicating important application values in many engineering fields including solid-state refrigeration,magnetic sensor,magnetic actuator,magnetic storage and artificial intelligence.However,most of the currently reported magnetic-phase-transition materials possess poor mechanical properties,which hinders their practical applications.Therefore,it is vital to find out new magnetic-phase-transition materials with both excellent functional properties and mechanical properties.Based on the literature research,studied in this paper are Heusler all-d-metal Ni-Co-Mn-Ti,Fe-doped Co-V-Ga and Laves-phase Tb_0.95Mn_2-xCo_x alloys.The regulation of transformation,microstructure and physical properties as well as the special magnetic behaviors at low temperatures and relevant mechanisms are investigated.In addition,their possible applications are explored in the perspective of mechanical properties.Both multiple magnetic functional properties near the martensitic transformation and excellent mechanical properties are shown in the Heusler-type all-d-metal Ni-Co-Mn-Ti alloys.Experimental results indicate that the transformation temperature of the Ni_36.5Co_13.5Mn₃₅Ti₁₅ ribbon increases significantly after annealing.The thermoelastic intermartensitic transformation between the 5-layer modulated martensite and unmodulated martensite is observed in the annealed ribbons.The magnetic-field-driven metamagnetic inverse martensitic transformation is obtained in annealed ribbons,and the critical field to drive metamagnetic transformation is lower than 1.81 T,thus excellent multiple magneto-functional properties are realized:large magnetic resistance,giant magnetocaloric effect and magneto-strain.By adjusting the annealing temperature,the kinetic de-arrest of martensitic transformation is realized in Ni_36.5Co_13.5Mn₃₅Ti₁₅ ribbons.In addition,the giant exchange bias effect of more than 100 mT is observed in the Ni_38.8Co_2.9Mn_37.9Ti_20.4 alloy thin film,and we have confirmed that the exchange bias effect is originated from the interfacial exchange interaction between the ferromagnetic cluster and the antiferromagnetic main phase.These results suggest that Ni-Co-Mn-Ti alloy ribbons or films have important application value in many fields those involve magnetic-field-induced effects.In addition,the reports about the magnetic-field-driven reverse martensite transformation in Co-based Heusler alloys are less.In this work,the influence of Fe on the martensitic transformation and associated physical properties is studied.The increase of Fe content reduces the martensitic transformation temperature,and enhances the ferromagnetic ordering of the austenite phase.The difference of magnetization across the martensite transformation increases by three orders of magnitude,which provides a larger driving force for the magnetic-field-driven reverse martensite transformation.Therefore,the magnetic-field-driven reverse martensite transformation is achieved with multiple magnetoresponsive effects?magnetocaloric effect,magnetostrictive strain and magnetoresistance?in Co₄₆Fe₅V₃₄Ga₁₅ alloy.More importantly,Co₄₆Fe₅V₃₄Ga₁₅ alloy shows high Vickers hardness?518 HV?and compressive strength?1423 MPa?.Furthermore,based on the composition optimization and entropy analysis,giant barocaloric effect in Co₅₁Fe₁V₃₃Ga₁₅alloy and large elastocaloric properties in the oriented Co₄₉Fe₃V₃₃Ga₁₅ alloy are obtained.The research and application fields of Co-based Heusler alloys are expanded.Laves phase family has the largest number,but they are used less.TbMn₂-based Laves phase shows very rich magnetic behaviors.Their magnetic and structural behaviors have not been fully understood so far even though they have been studied for a long time.Therefore,it is valuable and scientifically significant to study the TbMn₂-based alloys.In order to increase the Curie temperature and accordingly expand the application temperature range of Tb_0.95Mn₂ alloy,Co was used to replace Mn according to the concept of iso-structure alloying.The magnetocaloric effect and magnetic deflagration phenomenon at low temperature have been studied.The substitution of Co for Mn does not change the crystal structure,but significantly increases the Curie temperature of Tb_0.95Mn_2-xCo_x alloys.Co-substitution also improves the magnetocaloric effect:the refrigeration capacity values for x=0.5 and 1 samples are 327.0 Jkg^-1 and 291.6 Jkg^-1 under the magnetic field change of 0-5 T.Conclusively,all these results indicate that the Tb_0.95Mn_2-xCo_x alloys are promising in the applications such as magnetic refrigeration.