Plasticization Mechanism Of Nano-eutectic Magnetic Shape Memory Alloy

Magnetic shape memory alloy?MSMA?has been a hot research topic in the field of materials science in recent years.These alloys not only have the characteristic of the traditional shape memory alloys controlled by temperature,but also demonstrate shape memory effect and superelasticity triggered by a magnetic field.In this regard,MSMAs combines the properties of ferroelastic and magnetostrictive materials.These alloys also have novel properties such as large recoverable strain,large output stress,giant magnetocaloric effect and high response frequency.Hence,they are considered as a new generation of actuating and sensing materials following the success of traditional piezoelectric and magnetostrictive materialsHowever,compared with the traditional shape memory alloys and magnetostrictive materials that have been widely used,MSMAs,such as Ni-Mn based Heusler alloys,are still not ready for practical use.The main restriction for application is that the material is extremely brittle.The high brittleness is an intrinsic property of these intermetallics.The reported toughening solution is to dope a fourth element,such as Cr,Co and other elements,to obtain an appropriate amount of face-centered cubic second phase?y phase?at the grain boundary,so that the ductility can be increased.However,the uneven phase distribution and large grain size normally lead to limited property improvement.In the meantime,the phase transformation properties of the Heusler matrix phase was badly controlled in the literature,which leads to the reduction or suppression of the original magnetic functional characteristics.In this thesis,we propose a new grain refinement strategy to improve the mechanical properties of these alloys by eutectic solidification without sacrificing the magnetic functionsNi₄₈Co₄Mn_39-xFe_xSn₉?x=0,1,2,3,4,5,6,7,8?and Ni₄₃Co₅Mn_44-xCr_xSn₈?x=0,1.5,3,5,6,6.5?alloy series were prepared using arc melting technique as demonstrations to achieve grain refinement and mechanical property improvement utilizing eutectic solidification.The effects of Fe and Cr doping on the microstructure,crystal structure,martensitic transformation,magnetic properties and mechanical properties were carefully investigated of Ni-Co-Mn-Sn alloy systems.This thesis attempts to establish the interplay between the microstructure and mechanical properties and reveal the toughening mechanism of Ni-Co-Mn-Sn alloys doped with Fe and Cr elements.The findings in this thesis can provide a new strategy and references for alloy design,to simultaneously achieve good mechanical properties and ideal magnetomartensitic transformation behaviour in Ni-Co-Mn-Sn alloys.For the Ni₄₈Co₄Mn₃₉Sn₉ alloy system doped with Fe element,it is found that the refinement of the grain size and improvement of mechanical properties were achieved via introduction of ultrafine eutectic ? phase.With the gradual increase of Fe doping,the volume fraction of the ? phase gradually increased and the grain size of the alloy was effectively refined.When the nominal Fe content is 5 at%,the matrix phase and the ? phase exhibited a full eutectic structure,which shows a grain size of 250 nm,a compressive strength of 1587 MPa and ductility of 17%.This is mainly attributed to the refinement of the grain structure and the effective restriction on cracking propagation.According to the magnetic measurements,Fe doping has the function of adjusting the magnetic coupling between magnetic elements.With increase of Fe doping,the transformation temperatures were significantly reduced.The eutectic Fe6 alloy showed a martensitic transformation of 231 K being lower than its Curie transition,thus having the characteristic of a metamagnetic transformation.For the Ni₄₃Co₅Mn₄₄Sn₈ alloy system doped with Cr element,results show that doping Cr can significantly refine the microstructure of the alloys.With the increase of the Cr content,the volume fraction of the ? phase gradually increased and the grain size of the alloy decreased.At 5-6 at%,the matrix phase and ? phase exhibited eutectic structure characteristics,with the average grain size of about 380 nm.As a result of grain refinement,high compressive strength of 1498 MPa and ductility of 14%were achieved.According to magnetic measurements,it is found that the Curie transition temperature?TCA?of austenite is not sensitive to Cr doping,while the Curie transition temperature?TcM?of martensite increased and the martensitic transformation temperatures decreased rapidly with the increase of Cr doping.This indicates the transformation between a ferromagnetic austenite to paramagnetic martensitic occurs only in a Cr doping range of 3.0-5.5 at%.