Study On Transformation Behaviors And Magnetocaloric Effect Of NiFeGa Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys are new kind of smart materials,which arecapble of exhibiting magnetic field controlled shape memory effect as well as magneticfield induced strain.They have potential application in many fields,such as sensing andactuating.Amongst them,Ni-Fe-Ga alloys have received much attention due to theirhigh magnetocrystalline anisotropy energy and good hot-working ability.These alloyshave several characterizations.First,they are ferromagnetic intermetallic compoundsundergoing a martensitic transformation from a cubic L2₁/B2 structure to a complexmodaluted structure.Second,the martensitic transformation behavior of the alloys,which depends on composition and heat treatment,is thermoelastic and exhibits atwo-way shape-memory effect.Third,they have high saturation magnetization andmagnetocrystalline anisotropy constants.Therefore,Ni-Fe-Ga ferromagnetic shapememory alloys become one of the important materials and are receiving ever-increasingattention.In this dissertation,the effect of compostion,alloying and heat treatment onmicrostructure,behaviors of martensitic transformation and magnetic properties ofNi-Fe-Ga(-X) alloys are systematically investigated with an aim to improve themagnetocarolic effect.The alloys are prepared by arc-melting and suction-casting.The results show that the composition and alloying with other element have animportant effect on microstructure of Ni-Fe-Ga alloys at room temperature.Themicrostructure transforms from single martensitic phase(seven-layer modulated(14M)structure) to austenite phase as Fe content was increase from 18at% to 20at% inNi_73-xFe_xGa₂₇ alloys at room temperature.The phase of the alloys changes fromaustenite to martensite at room temperature as Ge was partly substituted by Ni,Fe andGa of Ni_56.5Fe₁₇Ga_26.5 alloy.The mircostructure of the alloy is composed of martensitephase andγphase at room temperature as Ga was substituted by Co in Ni-Fe-Ga alloy.And the volume fraction ofγphase increases with increasing Co content.The structuresof Ni_56.5Fe₁₇Ga_26.5-xAl_x and Ni_56.5-zFe₁₇Ga_26.5Ct_z alloys show martensite or austenitephase characteristics depending on the content of A1 and Cu. The phase transformation behavior is very sensitive to both stoichiometriccomposition and alloying.The M_s temperature decreases from 31.7℃to-118.3℃as xincreases from 18 to 20.5 in Ni_73-xFe_xGa₂₇ alloys and from 108.9℃to 28.1℃as xfrom 17 to 19 in Ni_73.5-xFe_xGa_26.5 alloys.The martensitic transformation temperaturesincrease with increasing Ni content when keeping the ratio of Fe/Ga.Thetransformation temperatures decrease almost linearly with increasing Ge content in allthe three types.Whereas,the decreases in rate of the martensitic transformationtemperatures are different for the three cases.It is large for Ni substituted by Ge,slowfor Ga and intermediate for Fe.It is suggested that the martensitic transformationtemperatures depend on both the size effect and the degree of parent phase as well asvalence electron concentration e/a.Heat treatment also has notable influence on the martensitic transformationbehavior of Ni-Fe-Ga alloy.The results show that the transformation temperaturesdecrease after annealing at 100℃for 3 h.Subsequently with increasing the annealingtemperature,the transformation temperatures increase.The A_s and A_f temperaturesincrease with the annealing time.While the M_s and M_f temperatures first increase anddecrease subsequently with increasing the annealing time.The effect of heat treatmenton the transformation temperatures should be attributed to the degree of parent phase,crystal defect and internal stress.The magnetization(M-H) curve of the parent phase is more easily saturated thanthat of the martensite phase.In Ni_56.5-xFe₁₇Ga_26.5Ge_x(x=0,0.5,1.0,2.0) alloys,thesaturation magnetization and magnetocrystalline anisotropy constant besides Curietemperature decrease with inceasing Ge content.The magnetocarolic effect is estimatedfrom the magnetization(M-H) curve.The entropy change of Ni₅₅Fe₁₈Ga₂₇ alloy is quitelow and-2.0 J/kgK in the field of 50 kOe.The result implies the alloy cannot be appliedas the magnetic material of magnetic refrigeration.It is suggested that controllingcomposition or alloying is an effective way to improve the magnetocarolic effect.The results of the compression tests show that the compression strength andductility of the alloys containing Ge is much higher than that of the alloy without Geelement.The compression strength and ductility of(Ni_56.5Fe₁₇ Ga_26.5)_100-xGe_x alloyenhance with increasing Ge content.