【作者】 余华军；

【导师】 祖小涛；

【作者基本信息】 电子科技大学 ， 凝聚态物理， 2009， 博士

【摘要】 铁磁形状记忆合金是最近几年研究发现的一种集磁控形状记忆和磁场诱发应变于一体的新型智能材料，有望成为传感器和驱动器等关键部件的首选材料。而Ni-Fe-Ga铁磁形状记忆合金具有磁晶各向异性能大和热加工性能优异等优点，其丰富的物理内容主要包括：第一，该合金是铁磁性的金属间化合物，它的高温母相具有L2₁／B2立方结构，而马氏体相通常是具有复杂的调制层结构。第二，其马氏体相变是热弹性的，马氏体相变特性受合金的化学成分和热处理工艺等的影响。第三，该合金具有高饱和磁化强度和大磁晶各向异性常数。因此，具有十分广阔的工程应用前景，并逐步成为各国学者研究的重点材料之一。本文通过电弧炉熔炼和吸铸的方法制备了Ni-Fe-Ga（-X）系列合金，系统地研究了Ni-Fe-Ga（-X）合金的金相组织、微观结构、马氏体相变行为和磁性能，考察了化学成分、第四元素合金化和热处理工艺等对Ni-Fe-Ga合金的影响。改进了制备Ni-Fe-Ga（-X）合金的方法，尝试提出改善合金磁热效应的途径，为Ni-Fe-Ga合金的工程化应用打下基础。研究结果表明，化学成分对Ni-Fe-Ga合金的室温组织和相结构的影响非常显著。室温下当Ni_73-xFe_xGa₂₇系列合金中的Fe含量由18at％提高到20at％时，合金由单一的马氏体相转变成奥斯体相。X射线衍射和TEM选区电子衍射揭示，Ni55Fe₁₈Ga₂₇合金室温下马氏体相为单斜的14M调制结构。同时，也发现Ge、Al、Co和Cu等第四元素合金化对Ni-Fe-Ga合金组织结构有着重要的影响。一定量的Ge元素替代Ni_56．5Fe₁₇ Ga_26．5合金中的部分组元，能使合金在室温下由马氏体相转变成奥斯体相；Co的添加使Ni_56．5Fe₁₇Ga_26．5-zCo_z合金在室温下的显微组织由马氏体相和γ相而组成，随Co含量的增加，γ相数量增多，尺寸增大；Ni_56．5Fe₁₇Ga_26．5-xAl_x和Ni_56．5-zFe₁₇Ga_26．5Cu_z合金中不仅由马氏体或者奥氏体组成，而且存在多种相结构。Ni-Fe-Ga合金的马氏体相变行为对化学成分和第四元素的合金化十分敏感。在Ni_73-xFe_xGa₂₇系列合金中，马氏体相变温度M_s由Ni₅₅Fe₁₈Ga₂₇合金的31．7℃下降到Ni_52．5Fe_20．5Ga₂₇合金的-118．3℃；而在Ni_73．5-yFe_yGa_26．5系列合金中，M_s由Ni_56．5Fe₁₇Ga_26．5合金的108．9℃下降到Ni_54．5Fe₁₉Ga_26．5合金的28．1℃。固定Fe／Ga比例不变时，Ni-Fe-Ga合金的马氏体相变温度随Ni含量的增加而线性升高，具体地说，在Ni_56．5+zFe₁₇Ga_26．5系列合金中，M_s从Ni含量为54．5at％时的39．6℃增加到Ni含量为58．5at％时的139．9℃。发现Ge元素无论是替代合金的Ni或Fe，还是Ga，都明显地降低合金的马氏体相变温度，Ge替代Ni马氏体相变温度下降最快，Ge替代Fe次之，Ge替代Ga最慢：而Co替代Ni-Fe-Ga合金中的Ga原子显著地提高了合金的马氏体相变温度。初步认为应该从价电子浓度、尺寸因素和有序度作用等综合因素考察马氏体相变温度的变化。热处理工艺也是影响Ni-Fe-Ga合金马氏体相变行为的重要因素之一。结果表明，马氏体相变温度比未退火的明显下降，然后随着退火温度的升高而升高；而随退火时间的延长，合金的逆马氏体相变温度逐渐升高，马氏体相变温度首先是升高，直到退火时间为5 h，然后随退火时间的延长而下降。合金母相的有序度、晶体缺陷和内应力等是热处理工艺影响合金马氏体相变温度的可能原因。在强磁场磁化时，Ni-Fe-Ga（-X）合金表现出一共同的特征，即合金的高温奥氏体呈易磁化，易达到饱和；低温马氏体相表现出难磁化，达到饱和缓慢的特性。而在Ni_56．5-xFe₁₇Ga_26．5Ge_x（x=0，0．5，1．0，2．0）合金中，合金的饱和磁化强度和磁晶各向异性常数均随着Ge含量的增加而减小；居里温度是随着Ge原子含量增加而降低的。通过等温磁化曲线，研究了Ni-Fe-Ga（-X）合金的磁热效应。结果显示，Ni₅₅Fe₁₈Ga₂₇合金的磁熵变最大值不是很高，在50 kOe的磁场下，最大值仅为-2．0J／kgK，很难作为磁制冷技术的磁工质。Al和Ge的添加尽管提高了合金的磁熵变最大值，但是仍不能达到磁制冷技术的要求。但是，通过研究发现，改变合金的成分，磁性原子的合金化可能是提高磁热效应的有效方法。室温压缩实验结果表明，Ge元素的添加提高了NiFeGa合金的抗压强度，改善了合金的延伸率。(Ni_56．5Fe₁₇Ga_26．5)_100-xGe_x合金的抗压强度和延伸率随Ge含量的增加而增加。更多还原

【Abstract】 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.更多还原