Research On Microstructure And Properties Of Nb/Ta Doped Ni₅₂Mn₂₄Ga₂₄ Magnetic Shape Memory Alloy

Ni-Mn-Ga magnetic shape memory alloy has been widely studied because of its high response frequency and large magnetic field-induced strain.It is a kind of intelligent material with good development prospect.However,the alloy belongs to intermetallic compound and has the disadvantage of high brittleness,so its practical application is seriously limited.Some studies have shown that doping other elements can introduce the second phase into the alloy,which can improve the brittleness of the alloy to a certain extent,but the martensitic transformation of the alloy will be affected due to the change of the matrix composition.In this paper,Ni₅₂Mn₂₄Ga₂₄ alloy was doped with Nb or Ta,and the effects of Nb or Ta doping on the microstructure,martensitic transformation behavior and mechanical properties of the alloy were studied in detail.It is found that doping Nb can refine the grain size of the alloy and form Nb-rich second phase.The Nb-rich second phase in the as-cast alloy is continuously distributed along the grain boundary.After solution treatment at 850℃,the second phase changes to short rod-shape and discontinuously distributes along the grain boundary.After aging at 400℃and 600℃,the degree of segregation of the rod-like second phase along the grain boundary increases and the size of the rod-like second phase increases slightly.The XRD results show that the Ni₅₂Mn₂₄Ga₂₄ alloy is mainly composed of cubic austenite and tetragonal martensite at room temperature,and the alloy is mainly composed of cubic austenite and hexagonal second phase after doping Nb element.With the increase of Nb content,the martensitic transformation temperature and Curie transition temperature decrease,and the hysteresis increases.After solution treatment at 850℃,aging treatment at 400℃and 600℃,the martensitic transformation temperature has the changing trend of decreasing first,then increasing and decreasing finally.The microhardness and compressive strength of the alloy increase obviously after doping Nb element,but the plasticity changes little.Similar to Nb doping,the grain size of the alloy is refined and Ta-rich second phase is formed after Ta doping.The Ta-rich second phase in the as-cast alloy presents a spot-shape and is dispersely distributed.After solution treatment at 850℃,the size of the second phase increases and presents a granular shape and intermittent distribution along grain boundaries,and some acicular second phases appear in the grain interior.After aging at 400℃and 600℃,the segregation degree of granular second phase along grain boundary increases,and the number of acicular second phase increases.XRD results show that the alloy is mainly composed of cubic austenite phase and hexagonal second phase after doping Ta element,similar to the above results of doping Nb element.With the increase of Ta content,the martensitic transformation temperature and Curie transition temperature decrease and the hysteresis increases.After solution treatment at 850℃,aging treatment at 400℃and 600℃,the martensitic transformation temperature of the as-cast alloy decreases first,then increases and decreases finally.After doping Ta element,the microhardness and compressive strength of the alloy are obviously improved,and the plasticity changes little.After ball milling,the martensitic transformation and Curie transformation of Ni-Mn-Ga-Ta alloy disappear gradually.After annealing at 800℃,the martensitic transformation and Curie transition of the alloy are recovered due to the increase of atomic ordering.