| Ni-Mn-Ga magnetic shape memory alloys have been studied extensively due to the merits of high response frequency and large magnetic field-induced strain.However,this kind of alloy belongs to intermetallic compound,which results in the inevitable disadvantage of brittleness that is unfavorable for its practical applications.Thus,we proposed the idea of preparing Ni-Mn-Ga/metal composites,aiming to improve the mechanical properties by ductile aluminium and copper matrix and provide functional properties using magnetic shape memory alloy particles.In this thesis,the Ni-Mn-Ga bulk was prepared by arc-melting and then ball milled to micro-sized particles.The as-milled Ni-Mn-Ga particles were annealed at high temperature and then mixed with aluminum powders and copper powders respectively.The mixed powders of Ni-Mn-Ga and Cu were sintered by pressureless sintering and spark plasma sintering techniques to obtain Ni-Mn-Ga/Al composites,respectively.The mixed powders of Ni-Mn-Ga and Cu were sintered by pressureless sintering techniques to obtain Ni-Mn-Ga/Cu composites.The results showed that ball milling was effective in crushing the Ni-Mn-Ga bulk to particles and the transformation behavior of ball milled particles disappeared.The martensitic transformation behavior of the particles was recovered by 800oC annealing in vacuum.No integrated Ni-Mn-Ga particle was discovered in pressureless sintered aluminum-matrix composites.The Al3Ni,Al3Ni2 phases were formed successively that resulted in the disappearance of transformation behavior.Compared with pure Al,the mechanical property of composites was not improved obviously.As compared with composites with Al powders with larger particles,the composites with smaller Al particles showed less defect and better mechanical properties.In spark plasma sintering aluminum-matrix composites,the Ni-Mn-Ga particles were mostly dispersed in the interior of the grains and react with the matrix.The composites with 50wt%and 60wt%particles exhibited apparent martensitic transformation behavior.With the increase of the Ni-Mn-Ga particles content,the mechanical properties of the composites were enhanced gradually,which was attributed to the strengthening effect of particles on the matrix.As compared with pressureless sintering,spark plasma sintering leads to less defect and a better transformation and mechanical properties.In pressureless sintering copper-matrix composites,the Ni-Mn-Ga particles showed banded shape and were widely dispersed in the grain boundaries of Cu matrix.With the increase of Ni-Mn-Ga particle content,the matrix grains were refined significantly.The composite with 60wt%content particles exhibited the best martensitic transformation behavior and transformation-induced strain.With the increase of the particles content,the mechanical properties of the composites were gradually enhanced by the combination effect of grain boundary strengthening and precipitates strengthening.The fracture strength of 60wt%composite reached up to 1400MPa,and the fracture strain was close to 20%.As compared with the pressureless sintering aluminum-matrix composites,the copper-matrix composites showed less defect,better martensitic transformation and mechanical properties. |
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