| Ferromagnetic shape memory alloys is a new kind of smart materials,which is an ideal type of actuating materials and sensing materials in temperature or magnetic field due to their giant magnetic shape memory effect and fast dynamic response,which is because their martensitic transformation occurs accompanied with magnetic transition.The preparation methods of ferromagnetic shape memory alloys determines their microstructure and the various physical properties.It is necessary to understand the related physical mechanism and the foundation for the practical application of the materials by studying the preparation technology and microstructure of the alloy.By means of optical microscope,XRD,DSC,SEM as well as TEM,we study the influence of composition and heat-treatment on the microstructure and thermally-induced martensitic transformation of Ni50-xMn41Sn9Cox(x=0,2,5,7)and Ni48Mn39.5Sn12.5-xAlx(x=0.5,3)and Ni53xFe17+xGa27Co3(x=0,1)All of the Ni50-xMn41Sn9Cox at-cast alloy show cellular dendrite at room temperature.In the cooling process,the dual solidification has traces of a peritectic process where D03 is the primary solidified phase with dendritic growth,followed by the peritectic reaction of this phase with the residual liquid to form the austenite L21.Most of the L21 austenite transform to martensite in cooling process,so all the specimens remain D03,L21 and martensite coexisting structures except the as-casted Ni43Mn41Sn9Co7 samples,which has y-phase precipitates along the grain boundaries and formed D03,L21 and y-phase coexisting structures.After heat treated at 950? for 4h in vacuum followed by quenching into water for homogenization,the majority of D03 austenite turn into L21 austenite in alloy,remanent D03 and L21 coexist in the alloy at room temperature.When the Co content is low,the alloy remain NM and 40 martensite coexisting,the martensite transformation temperature gradually decreased with the increase of Co content,but keep above room temperature,as well as the NM transform to 40 martensite.As for the Ni48Mn39.5Sn12.5-xAlx(x=0.5,3)alloy,the as-cast sample remain as cellular dendrite structure which is composed of D03 and L21 austenite without precipitation of ? phase.A little D03 austenite phase and 40 martensite phase coexist in the alloy after annealing.With the increase content of Al,the martensite laths become wider the martensitic transformation temperature decreases as well as the thermal hysteresis of martensite transformation.As for the samples of Ni53Fe17Ga27Co3 and Ni52Fei8Ga27Co3 alloy,the microstructure of Ni53Fe17Ga27Co3 as-cast samples remain non-modulated tetragonal martensite and a few austenite at room temperature.After annealing,some non-modulated tetragonal martensite will transform to 14M modulated martensite,with the precipitation of y phase.The y phase of Ni53Fe17Ga27Co3 annealed sample which is furnace cooling,remains long strip and exists in grain boundary mainly,wrapped by a thin ?' layer and austenite,incidates the austenite become more after annealing.Ni52Fe18Ga27Co3 as-cast samples show NM martensite and a little austenite,14M modulated martensite and remnant y phase coexisting.The y phase of Ni53Fe17Ga27Co3 annealed sample which is water cooling showed elongated and ellipsoidal shape due to the different precipitation sites,which indicated that the cooling mode and composition change had obvious effect on the microstructure of the samples. |
PDF Full Text Request