Study On Martensitic Transformation,Microstructure And Compressive Properties Of NiFeGaCo And NiMnSnCoGd Alloys

NiFeGa and NiMnSn Heusler alloys exhibit abundant and peculiar physical phenomena,such as magnetic induced strain,giant magnetocaloric effect,magnetoresistance and other physical effects.These potential application prospects have attracted much attention.However,up to now there are still many problems such as unstable martensitic transformation temperature and larger material brittleness in these alloys needed to be solved.In this paper,Ni54-xFe16+xGa27Co3(x=0,1,2)and(Ni45Mn40Sn10Co5)100-xGdx(x=O,0.1,0.2,0.5,2)alloys were produced by electric arc furnace and vacuum sprayed furnace.These alloys have been systematically studied and analyzed by means of OM,DSC,XRD,SEM,TEM,VSM and so on.For Ni54-xFe16+xGa27Co3(x=0,1,2)alloys,the XRD results show that Ni54Fe16Ga27Co3 alloy owns 14M martensite and ? second phase at room temperature;Ni53Fe17Ga27Co3 alloy is composed of NM,14M martensite and ? phase;Furnace-cooling alloy of Ni52Fe18Ga27Co3 is consisted of austenite,NM martensite and ? phase.The water-cooling alloy of Ni52Fe18Ga27Co3 is consisted of NM,14M martensite and ? phase.The DSC data was analyzed and it was found that the martensitic transformation temperature of Ni54-xFe16+xGa27Co3(x=0,1,2)alloys decreases with the increase of Fe content.Specially,the martensitic transformation temperature of Ni52Fe18Ga27Co3 alloy decreases to the room temperature,so this alloy shows the coexistence of martensite and austenite phase.The martensitic transition temperature of water-cooled sample is slightly higher than that of the furnace-cooled sample.Because a large number of second phases precipitate out along with grain boundaries and inside the grains in the furnace-cooled alloys,which results in the best compressive strength and ductility in these alloys.However,the mechanical performance of Ni52Fe18Ga27Co3 in which austenite and moderate second phase exist inside this alloy becomes slightly worse.Final,Ni54Fe16Ga27Co3 alloy is found to be of the lowest compressive strength and ductility.Comparing the fracture morphology of the compressed samples,the fracture mode of the Ni53Fe17Ga27Co3 alloy with the highest compressive strength is transgranular fracture;while the fracture mode of the Ni54Fe16Ga27Co3 and Ni52Fe18Ga27Co3 alloys with the lower compressive strength is mostly intergranular fracture,and these cracks are generated at the grain boundaries firstly.TEM observations conform to the same results as we analysed in XRD experiments,except for the 6M modulated martensite which is not found in XRD analysis because of trace amount of this phase.For the second phase,the small particle ? phase with FCC structure,has a specific K-S orientation relationship with the martensite,that is,(111)?//(110)M,[110]?//[111]M;while the large particle ? phase with L12 structure,always have not a special orientation relationship with the martensite.For(Ni45Mn40Sn10Co5)100-xGdx(X=0,0.1,0.2,0.5,2)alloys,it is found that there exit 10M and 14M martensite phase in the three Ni45Mn40Sn10Co5,(Ni45Mn40Sn10Co5)99.9Gd0.1,(Ni45Mn40Sn10Co5)99.8Gd0.2 alloys at room temperature.(Ni45Mn40Sn10Co5)99.5Gd0.5 and(Ni45Mn40Sn10Co5)98Gd2 is composed of 10M,14M martensite and the second phase.As the additive amount of rare earth Gd is increased,the crystal grains are becoming smaller.Analyzing the DSC data,the martensitic transformation temperature increases with the increase of Gd when X=0?0.2%in(Ni45Mn40Sn10Co5)100-xGdx alloys;When X=0.5?2%,the martensitic transition temperature decreases firstly and then rises again.The doping of Gd has not evident effect on the thermal hysteresis of these alloys.Comparing the compressive strength of the alloys,we found that when the quantity of Gd element is a very few,the compressive strength slightly increases with the increase of rare earth content slightly.When the second phases appeare at the grain boundary in(Ni45Mn40Sn10Co5)99.5Gd0.5 alloy,the compressive strength increases about 200 MPa;Specially,the compressive strength of the(Ni45Mn40Sn10Co5)98Gd2 alloy increases enormously.Comparing the fracture morphology of the compressed samples,it is found that Ni45Mn40Sn10Co5 and(Ni45Mn40Sn10Co5)98Gd2 alloys have intergranular fracture modes,while(Ni45Mn40Sn10Cos)100-xGdx(X=0.1,0.2,0.5)alloys have transgranular fracture ones.According to the TEM analysis,we found that there are two kind of second phases in the(Ni45Mn40Sn10Co5)100-xGdx alloys.The first one is the same as the second phase found in Ni54-xFe16+xGa27Co3 alloys.The second phase which is abound of Mn elements,has a hexagonal structure and its lattice constant is a=0.517 nm,c=0.613 nm.We found different shapes of the hexagonal second phase such as circular,square,hexagonal etc.These kind of second phases have a special oriented with the martensite,(20(?))M//(0002)s,[7(?)1]M//[2(?)(?)0]s.